Text

Enclosure 9: Technical Report TR-HBIP-2002-01, Seismic Hazard Assessment for the Humboldt Bay ISFSI Project. Section 9.0 to Appendix 2A, Page 2A-9
	ML14045A390
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Site:	Humboldt Bay; (DPR-007)
Issue date:	12/27/2002
From:	Swan F; Pacific Gas & Electric Co
To:	; Document Control Desk, NRC/FSME
Shared Package
ML14045A387	List: HBL-14-008, Enclosure 9: Technical Report TR-HBIP-2002-01, Seismic Hazard Assessment for the Humboldt Bay ISFSI Project. Appendix 9A-42 to End. Enclosures 10 & 11: Humboldt Bay Power Plant Tritium Evaluation and DECON-POS-HOS-H011, Groundwater Invest; ML14045A329; ML14045A367; ML14045A368; ML14045A389; ML14045A390; ML14045A394;
References
	HBL-14-008 TR-HBIP-2002-01
	Download: ML14045A390 (126)
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[JJ] Moderate IS8&I Fair to poor Figure 9-14 Detailed stratigraphy of core LC-2 from the Lagoon Creek marsh. Diagrams show typical marsh, sand deposits and diatoms near the coast.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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PACIFIC OCEAN Figure 9-15 Townsite of Orekw (Oreck) and location of cores in Orick marsh. Map shows the village Orekw, the site ofTskerkr's oral history, A Flood (Kroeber, 1976; Carver and Carver, 1996). Also shown is Ida's house site, where floodwaters came to "the front door." Both stories document flooding to about 66 and 69 feet elevation (MLLW). The cores from the Orick marsh record the "Y," as well as earlier tsunami intrusions, and one later tsunami.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

EXPLANATION CJ Active sand and developed areas CJ South Spit pre-jetty dunes CJ South Spit beach sequence c=J Intermediate dune sequence CJ Marsh x 40 Elevation (feet)

PACIFIC OCEAN South Bay Arcata Bay

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Figure 9-16 Geomorphology of the North and South spits of Humboldt Bay.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

PACIFIC OCEAN

@Eureka Arcata Bay Arcata Jacoby Creek Marsh EXPLANATION Site having evidence of tsunamis

[] Site having no evidence of tsunamis

HBNWR Humbolt Bay National Wildlife Refuge 0

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Skm Figure 9-17 Map of the North Spit site, and South Bay and other Humboldt Bay marsh sites.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Figure 9-18 Correlation of tsunami sands in selected cores across the South Bay marsh.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 SBVB-10 M: loose, gray-black M: loose,.black Diatom Samples TF/C-a TF/C = ndat Flat/Channel LM==Low Marsh HM =High Marsh Contacts Abrupt (~I mm)

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Radiocarbon ages reported in calibrated years before AD 1950 BP dates refer to calibrated radiocarbon years before present.

(From Carver and others, 1998)

Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Figure 9-20 Schematic diagram showing estimated tsunami runup heights at the Humboldt Bay ISFSI site.

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Figure 9-22 The 1806 map of Humboldt Bay (Bay of Rezanov) made by Russian explorers.

Soundings are in sazhens (Dr. Lydia Black, personal communication, 2001) (1 sazhen is about 7 feet); numbers in parentheses in the entrance channel clarify the original sounding. Original map in Golovnin, Vasili, undated, Voyage of Kamchatka and maps which accompany-Russian ed., Alaska State Historical Library, Juneau, Alaska m

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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0 Photo 9-1 North Spit, Humboldt Bay (foreground), and Arcata Bay (background). View is to the north from above the bay entrance. The Mad River Slough is located in the marshland on the north side of Arcata Bay. The Humboldt Bay Power Plant is just out of the picture to the lower right.

rJ Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-2 Gouge coring at Crescent City marsh. Hans Abramson, collecting a gouge core in the marsh, is employing a typical technique for obtaining a reconnaissance core.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Ui Photo 9-3 Typical gouge core. The finger points to a thin layer of fine-grained tsunami sand, which is interbedded with marsh peat.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-4 Drilling using the Vibracore at Lagoon Creek. The 3-inch-diameter core tube is shown in position before being driven into marsh sediments.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-5 Typical drive cores. The two split -sample tubes show tsunami sands (arrows) in cores from the

iii Orick marsh.

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Photo 9-6 Crescent City, View to west.

Crescent City Marsh Site Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Photo 9-7 Crescent City marsh.

Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-8 The Lagoon Creek pond and marsh. View is to the south, showing the beach ridge, pond, and marsh in this narrow valley. Tsunami sand layers were found in the marsh sediments inland to the upper end of the marsh visible in this photograph.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

S:\\51 OOs\\5117\\5117.009\\task_13\\02_0724_s9\\_photo_09_09{54,55).ai Photo 9-9 Wilson Creek and Lagoon Creek. Wilson creek during the Pleistocene flowed around where the sea bluffs are today and down Lagoon Creek to Redwood Creek. Sea erosion has cut off Lagoon Creek from Wilson Creek, leaving Lagoon Creek as a stable site undisturbed by stream erosion.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-10 Beach berm at Lagoon Creek 23-feet above MLLW. View is to the north from the northern part of the Lagoon Creek marsh.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Photo 9-11 Townsite of Orick and the Orick marsh at the mouth of Redwood Creek (on left side of photo).

The town was built on the hillslope above the beach and marsh.

Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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m Humboldt Bay ISFSI Project Technical Report

& TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

S:\\5100s\\5117\\5117.009\\task_13\\02_0724_s9\\_photo_09-13(50).ai Tompkins Hill Hookton Slough Site South Bay Site Photo 9-13 Mouth of Humboldt Bay, and the South Bay Hookton Slough sites. South Bay is separated from the Eel River Valley by Tompkins Hill and Table Bluff.

m Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Photo 9-14 Lag pebbles at elevation of 27 feet (MLLW) on the sand dunes on the North Spit believed to be deposited by a tsunami that inundated the dunes.

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 South Bay Site Photo 9-15 South Bay Site. Table Bluff in the middle of the photo separates the Eel River Valley in the middle distance from South Bay on the left.

Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

9.2 DEFINITION OF TSUNAMIS A tsunami is a gravitational sea wave or swell, or train of waves or swells produced by any large-scale, short-duration, disturbance of the ocean floor. Such disturbances are often due to sudden vertical displacement of the sea floor during an earthquake, but are also caused by submarine landslides (Figure 9-1) or, less commonly, by explosive volcanic activity or subaerial landslides (Bates and Jackson, 1987). A local tsunami is one that originated at or near the coast that is inundated by the tsunami. A distant tsunami is one that originates beyond the felt limits of the causative earthquake, sometimes from across the ocean. In the deep open ocean tsunami waves are characterized by great speed of propagation (more than 400 mi./hr), long wave length (more than 500 mi.), long period (varying from 5 minutes to a few hours, but typically 10 to 60 minutes), and low observable wave height or amplitude. When tsunamis enter shallow coastal waters the wave slows, wave length decreases, and amplitude increases. Due to the long length of the waves in the open ocean, the sea surface slopes so gently the waves can pass unnoticed, then appear as destructively high waves in shallow coastal waters (Gonzalez, 1999). The waves are not sinusoidal, but have irregular shapes, best illustrated by the detailed record of the 1964 tsunami at Women's Bay, Kodiak Island, Alaska. There the highest wave was the fifth to arrive; it rose to 19 feet above the post earthquake datum (MLL W), but receded to approximately 30 feet below the datum (Kachadoorian and Plafker, 1967).

As indicated in the definition, tsunamis have several origins. A seismic sea wave is a long-period tsunami that is caused by displacement of the sea floor during an earthquake; it may propagate hundreds to thousands of miles across oceans. A tsunami genic earthquake is any earthquake that generates a tsunami. A landslide-generated tsunami is caused by a submarine landslide or a coastal landslide, commonly triggered by an earthquake, entering the ocean.

Typically, landslide-generated tsunamis have locally high run ups on the nearby shores, higher than the runups elsewhere from the associated seismic sea wave.

Tsunami runup or wave inundation is the horizontal component of the slope distance traveled by the onshore surge, as measured from the shoreline (Figure 9-2). Tsunami runup height at a particular location is the vertical component of the slope distance traveled by the surge. The runup height from tsunamis is dependent upon several factors: the characteristics of the tsunami Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-2 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

itself, the submarine topography offshore, the topography onshore, coseismic elevation changes of the shoreline, and the tidal stage at the time of tsunami arrival. Run up height may be referenced to mean sea level (MSL ), which is the reference "0" elevation for most surface maps, including the US Geological Survey maps. However, field investigations following tsunami events commonly use field characteristics that are then related to elevation. For example, mean sea level is recognizable in the intertidal zone because it approximately corresponds to the upper limit of barnacle and seaweed growth. Another measure is the "extreme high tide shoreline",

which is a marked by the highest tidal debris and the seaward limit of terrestrial vegetation. This shoreline is a little higher than the common tidal measure of mean higher high water (MHHW) that is the mean of the higher of the two daily high tides, as measured over a period of time (usually 19 years). Similarly the tidal measure of mean lower low water (MLLW) is the mean of the lower of the two daily low tides. These means are not the highest nor the lowest tides recorded at a site.

Mean lower low water historically has been the reference elevation for bathymetry and topography at the Humboldt Bay Power Plant and, hence is selected for use in the analysis of the ISFSI site. For consistency, unless otherwise noted, all elevation measurements in this section are referenced to mean lower low water, which is set at 0 in this report. At the Coast Guard Station on Humboldt Bay at the southern end of the North Spit (40°46.0 N; 124°13.0 W) mean sea level is at 3.7 feet and the tidal range between MLL Wand MHHW is 6.9 feet (NOAA report of August 22, 1984, 941-8767). The highest reported tide above mean lower low water measured in the plant site vicinity since 1920 is 12.5 feet (PG&E, 1985b).

Waves from tectonic sea-floor displacements have produced runups as high as 50 feet or so (Appendix 9A), and can cause much damage along the exposed coast near the source. Wave runup heights from some submarine landslides have been much greater, for example, 170 feet in Prince William Sound, Alaska in 1964 (Appendix 9A), but have been restricted to relatively short reaches of the coast near their origin. Lander and others (1993) conclude that most local tsunamis following earthquakes along the west coast of the United States since 1812 have involved submarine landsliding. Similarly, Platker (Appendix 9A) considers that landslide-generated waves are more common than generally is recognized in studies of historic tsunamis, particularly many of the older ones that were not investigated in detail.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-3 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Based on eyewitness reports and tide-gauge recordings at the coast, large tectonically-generated tsunamis commonly begin with gradual withdrawal of the sea, which exposes the seabed in front of the shore, in some places for a thousand feet or more. Some tsunamis have been observed to begin with a rise of water level followed by drawdown of the sea (Figure 9-3). The arrival of a tsunami wave crest at the coast is usually preceded by a steady rise of the sea, like a flood tide, but much more rapid. As the wave crest approaches the coast, it may or may not form a breaking wave front as it spills across the beach and floods inland. When the crest reaches the coast the water level commonly remains high for some minutes before receding. Tsunamis characteristically consist of wave trains that produce several successive episodes of rising water level that inundate the coast, with intervening withdrawal that exposes the sea floor. The first wave is often smaller than later waves.

Tsunamis can transport near-shore and beach sand, gravel, woody debris, marine life, vegetation, and man-made objects well inland from the normal high-tide line. They can alter natural geologic conditions significantly, and can damage or destroy man-made structures. In remote areas, and in the case of prehistoric tsunamis, the damage and deposits of sand, gravel, and woody debris may be the only evidence of a tsunami inundation.

9.3 HISTORICAL TSUNAMIS IN THE HUMBOLDT BAY AREA A detailed catalog of tsunamis affecting the west coast of the United States since 1806 was prepared by Lander and others (1993), and we consider it the most current and most detailed compilation available. The authors of this catalog used earlier national and international compilations, including the 1965 study by Professor R. L. Wiegel for PG&E (in PG&E, 1966),

which was the basis of the PG&E (1966) report to the Atomic Energy Commission. Lander and others (1993) augmented the data base by review of contemporary newspapers and other original and derivative literature, including tide-gauge records.

Lander and others (1993) evaluated the validity of each of the tsunami reports using a scale ranging from "0- not a valid tsunami report," to "3 -probably a valid report," and "4-certainly a valid report." They noted that some waves reported as tsunamis might have been caused by Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-4 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

meteorological conditions (very low air pressure during a storm) or unusually high astronomical tides. They felt validity 3 and 4 tsunami reports could be used "with a fair degree of confidence" (p. 12).

The catalog is separated into local tsunamis and distant tsunamis. For the U.S. west coast, Lander and others (1993) rate 18 local tsunamis as validity 3 or 4. Only one of these reports was a tsunami whose origin was from the coast between Cape Mendocino and the Oregon border; it is associated with the April 25, 1992, Petrolia earthquake, a shallow thrust event of magnitude 7.1 2, interpreted to be a subduction zone earthquake (Figure 2-6) (Oppenheimer and others, 1993; Section 2.0). The earthquake caused uplift of up to 4.6 feet at the adjacent coast (Carver and others, 1994); this uplift of the sea-floor is interpreted to have generated the small tsunami observed as far away as Crescent City, where the tidal gauge measured the maximum wave height at 3.9 feet. A maximum run up height of 3 feet was reported north of Humboldt Bay at Trinidad. At the Coast Guard Station inside Humboldt Bay at the North Spit (Photo 9-1 ), the maximum-recorded wave height was 3.1 feet (Lander and others, 1993).

We do not know of any historic accounts of storm surges (a tide-like rise of water driven ashore by strong sustained wind and usually accompanied by a rising tide) across the Humboldt Bay spits, but storm waves have over-topped the South Spit on various occasions. These include the 1997-98 El Nino winter, which had very large storm waves that accompanied about a 1.5 foot increase in sea level relative to normal. These overtopping waves did not modify the overall spit morphology or lower its height, but rather drained down the bay side of the spit locally, as passive, gravity-driven sheet wash, or as weakly channeled flow where the spit crest was already relatively low. These overtopping waves may have transported some sand into the bay, to form local sand deposits along the shoreline on the bay side of the spit. These deposits are not similar to the widespread sand sheets on eroded marsh surfaces found in the paleo-marsh stratigraphy at the southwest corner of the bay.

Lander and others (1993) list 47 distant tsunamis having validity 3 or 4, based on tide-gauge data. Only 15 of these were directly observed on the west coast, suggesting the other 32 were rather small. Only seven distant tsunamis caused damage; the 1964 Alaska earthquake tsunami Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-5 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

was the most severe. Of the seven, only events in 1964 and 1960 were observed at Humboldt Bay. The 1946 event was reported at Crescent City, but not at Humboldt Bay. Information on the 1964, 1960 and 1946 events follows.

28 March 1964, Prince William Sound, Alaska, magnitude 9.2-The tsunami from this earthquake arrived at Humboldt Bay at high tide (Lander and others, 1993). Although there was no tide gauge in the bay at the time, the U.S. Army, Engineer District, San Francisco, compiled visual observations of maximum wave elevations at three points inside the bay and at four locations along the northern California coast.

PG&E personnel made the two observations at the Humboldt Bay Power Plant. These data are presented in Table 9-1, taken from PG&E (1966). Sites within Humboldt Bay were somewhat protected from tsunami effects compared to points on the open coast. Nonetheless, "The Eureka Boat Basin suffered little damage, but the water rose over the 10-foot sea wall and flowed 8 feet into the street at the height of the rise. The tide was 6 feet. The bay was filled with logs and debris. Half of the sea and channel markers were moved off their stations by the surge" (Lander and others, 1993, p. 1 07). The tsunami in Humboldt Bay was attenuated by a factor of 3 to 5 compared to Crescent City.

22 May 1960, South-central Chile, magnitude 9.5-Noted by Lander and others (1993) as the most damaging tsunami recorded anywhere in the world, the effects in Humboldt Bay were limited to reported strong currents at the bay entrance and the Eureka small-boat harbor. No damage was observed. For comparison, the run-up height of 12.5 feet (7.4 feet above the predicted tide) was reported at Crescent City, where extensive flooding and some damage occurred.

l April1946, Aleutian Islands, magnitude 7.4-This earthquake resulted in the creation of the Pacific Tsunami Warning Service, due to its spectacular destructiveness in Hilo, Hawaii.

Although a 3-foot wave was reported in Crescent City, and some damage was reported at Fort Bragg and to the south, the Humboldt County coast was "virtually untouched" (Lander and others, 1993).

2 Earthquake magnitudes are moment magnitudes (M), unless stated otherwise.

Humboldt Bay ISFSI Project Technical Report 9-6 TR-HBIP-2002-0 1 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

There are no other reports of known tsunami effects in Humboldt Bay mentioned either by Professor Wiegel (in PG&E, 1966) or Lander and others (1993). Other major Pacific Rim earthquakes (magnitude 8.0 or larger) occurred in Peru, Chile, Japan, Kamchatka, and the Aleutian Islands during the past 150 years of reporting. Run ups above tide level ranging from about 4 inches to as much as 4.7 feet (from the November 4, 1952, Kamchatka earthquake and tsunami) were reported at Crescent City due to these earthquakes, but there were no reports of these tsunamis at Humboldt Bay.

As Professor Wiegel noted (in PG&E, 1966, page 6), "There is little information available on tsunamis within Humboldt Bay." This is the case because distant tsunamis from great Pacific Rim earthquakes usually are too small at the Humboldt Bay coast to create noticeable effects inside the bay. Lander and others (1993, p. 23) note that the orientation of the eastern Aleutian source region and its optimal propagation direction to northern California and the Pacific Northwest suggest the 1964 Alaska earthquake "probably represents the maximum possible" distant tsunami to affect this region.

9.4 GEOLOGIC STUDY OF PAST TSUNAMIS ALONG THE NORTHERN CALIFORNIA COAST As noted in Section 2.0, the geologic record of tsunamis for the past several thousand years along the coasts of Oregon and Washington has contributed to the identification of the earthquake potential of the Cascadia subduction zone (Atwater and others, 1995). Beginning in 1996, PG&E initiated a program of similar investigations along the northern California coast opposite the southern end of the Cascadia subduction zone (Figure 9-4). This project was carried out by Gary A. Carver, Professor Emeritus of Geology at Humboldt State University, and three of his graduate students, and was completed in late 1998 (Carver and others, 1998; Abramson, 1998; Garrison-Laney, 1998; Leroy, 1999). The results of this work are summarized in this section.

The PG&E-supported tsunami investigation had several objectives:

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-7 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Evaluate the Cascadia subduction zone and other possible sources that could generate local tsunamis large enough to potentially cause damage along the coast near Humboldt Bay Document the timing of prehistoric tsunamis along the coast of northern California Estimate the heights of the run ups of past tsunamis Evaluate whether or not past tsunamis entered Humboldt Bay, and if they did, whether they reached the proposed ISFSI site 9.4.1 Characteristics of Tsunami Deposits Although geologic evidence of tsunami inundation can take many forms, the most enduring evidence is stratigraphic. Hence, the studies for PG&E focused on stratigraphic, sedimentologic, and paleontologic evidence of tsunamis preserved in intertidal bay marshes (Figure 9-5) and coastal freshwater marshes and ponds (Figure 9-6) and stratigraphic evidence in coastal sand dunes. Shallow freshwater marshes and ponds are very low energy depositional environments that accumulate organic-rich sediments at relatively slow rates. Continuously submerged beneath shallow, anoxic water, these sediments are typically undisturbed, and even fine details of the stratigraphy are commonly well preserved. Intertidal marshes are low-to moderate-energy depositional environments where finely bedded muddy and peaty sediments are preserved -

relatively undisturbed.

In contrast tsunami deposits are high-energy, clastic sediments, typified as layers of sand and deposits of mixed mud, peat, and exotic debris. Where coastal marshes and ponds are in the run up zone of a tsunami, they are excellent traps for accumulating and preserving these high-energy sandy sediments transported by the landward surge of marine water. Tsunami deposits are easily recognized in the otherwise quiet-water, peaty, and muddy marsh sediments. Where tsunamis inundate freshwater ponds or marshes, post-tsunami deposits often reflect tsunami disturbance to the adjacent area that drains into the pond or marsh as a change from predominately peaty sediments below the tsunami layer to muddy deposits immediately above the tsunami horizon. Sedimentary features that reflect turbulence associated with the rapid landward surge of the tsunami, such as rip-up clasts, vegetation "flopovers", and erosional Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-8 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

unconformities, followed by quiet water deposition when the wave crest reaches the shoreline, typified by graded bedding, also characterize tsunami deposits (Figures 9-5, 9-6, 9-1 0).

Sedimentologic and stratigraphic evidence of tsunami run up has been described for such deposits in freshwater marshes and ponds in Washington and Oregon and on western Vancouver Island (Atwater, 1987; Atwater and others, 1995; Clague and Bobrowsky, 1994a, 1994b; Nelson and others, 1996a, 1996b ). These authors have interpreted these deposits to have originated from large tsunamis caused by earthquakes on the Cascadia subduction zone. The PG&E investigations found similar deposits in the freshwater marshes and ponds on the northern California coast. The characteristics of these deposits are used to assess whether they were deposited by tsunamis or by other processes.

Sand layers can be introduced into marshes and ponds by several processes other than tsunamis, including storm waves and storm surges, wind, streams, and unchanneled surface runoff.

However, sedimentologic characteristics of the sand layers generated by tsunamis differ from sand layers deposited by other processes. The following characteristics were used to establish a tsunami origin for sand layers interbedded with marsh deposits in Northern California:

Landward thinning and fining of the sand layers indicate that the water flow transporting the sand lost energy in the landward direction, and was therefore moving inland. Landward transport of coarse, angular particles derived from local sources, such as minor landslides into the marsh, shows the surges were moving in the inland direction, as well.

The presence of marine diatoms in onshore sand layers shows they were deposited from a flow of ocean water. A diatom is a microscopic, single-celled plant that grows in both marine and fresh water. Diatoms secrete shells of silica, calledfrustules, which are deposited and preserved in sediments. Frustules from diatoms living in freshwater marshes, salt-water marshes, tidal flats, and the offshore ocean are easily differentiated; thus, the diatoms contained in sediment are indicative of the source of water that transported and deposited the sediment.

An energetic depositional mode is manifested by several physical characteristics of the sand layers. Eroded basal contacts and rip-up fragments of local marsh sediments incorporated in the sand demonstrate the inundating surges had enough energy to pluck chunks of peat and Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-9 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

mud from the substrate. Large numbers of broken diatom frustules suggest a high degree of turbulence, with grains crashing against each other.

Where the coastal topography is low lying, tsunami sand layers commonly extend far inland, more than a kilometer at some places in northern California. The sand layers gradually thin landward and are very well sorted and normally graded indicating the sand was deposited from suspension. This implies the water maintained enough velocity to carry sand-sized particles in suspension far inland and deposition occurred as the tsunami surge stopped. The absence of cross bedding indicates the sand was not continually reworked by wave action, as would be expected for sand deposited by storm waves. Grain-size distributions, diatoms, and sand-layer lithologies that match those found on the adjacent beach demonstrate the source of the sand was the beach.

Multiple, normally graded beds within the same sand sheet show that the disturbance event included multiple distinct surges that each deposited a separate normally graded bed of sand from suspension. In some places, the stems or leaves of fragile marsh plants growing on the depositional surface are preserved entombed upright in sand deposited during the earliest one or two pulses, only to be "flopped over" and buried by sand from a stronger later pulse. Silt partings between layers provide evidence for quiet-water deposition from very turbid water between episodes of high-energy sand transport and deposition from suspension. These interpretations are consistent with inundation during multiple surges typical of a local tsunami.

The presence of woody debris and forest litter capping sand layers suggests the surges flowed into forests on the fringes of the marsh and forest debris was carried back to the marsh as the water receded. This detrital material is typically concentrated in a "trash layer" with mixed mud, peat, and sand at the top of the sand layers. Woody debris is a common component of historic tsunami deposits (Clague and others, 1994).

Coincidence of characteristic sand layers with other evidence of local earthquakes also forms a strong argument for tsunami deposition. Subsidence during subduction zone earthquakes can leave distinctive traces in coastal marsh stratigraphy (Atwater and others, 1995). Salt-marsh peat and subaerial soil form near and above the high tide level and after they have rapidly subsided into the intertidal zone during subduction earthquakes, they are commonly Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-10 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

preserved as buried peat layers having gradational lower contacts and sharp upper contacts with intertidal mud. The presence of high-marsh or freshwater diatoms in peat or soils underlying mud containing intertidal diatoms strengthens the argument for coseismic subsidence. A sand layer having sedimentologic characteristics of tsunami deposition that mantles the buried marsh surface is most likely a tsunami deposit. A sand layer that directly mantles a landslide deposit in a coastal marsh sequence suggests the sand was deposited by a tsunami, and that the landslide was triggered by the same earthquake that caused the tsunami.

In marshes that do not have evidence of subsidence, frequently the sediments immediately above the sand layer commonly contain detrital wood and higher mud content. This "post-tsunami disturbance pattern" is interpreted to represent the increased clastic sedimentation from the disturbed area around the marsh following the tsunami.

9.4.2 Methods Used for Tsunami Investigation The methods used for the PG&E tsunami investigation included gouge coring, the collection of vibracores, radiocarbon dating, diatom identification, and grain-size analysis.

Gouge Coring - Gouge cores were collected from marsh and bay sediments to assess the stratigraphy of sites likely to contain sedimentary evidence of past tsunamis along the northern California coast. Gouge coring was the technique used for reconnaissance investigations, because this type of sample collection can be done rapidly with easily used, highly portable equipment. A gouge corer is a l-inch-diameter, half-cylinder steel core barrel 1 meter long attached to threaded rod sections. The core barrel is pushed into the subsurface by hand (Photo 9-2). The resulting sample is slightly less than 1 inch in diameter and usually is slightly disturbed, but it is adequate to assess the stratigraphy and allow identification of sand layers that may represent tsunami deposits (Photo 9-3). The gouge coring technique has the advantages of allowing examination of the sample in the field, and the rapid collection and assessment of many samples in a short time. The disadvantages of using this type of corer are a very small sample volume, some distortion and disturbance of the sample during collection, and a maximum sample length of about 1 meter, requiring multiple overlapping cores to sample stratigraphic sections deeper than 1 meter.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-11 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Vibracores - Vibracores were collected at sites where the gouge coring showed that detailed studies ofpaleotsunami deposits would be fruitful. Vibracores are 3-inch-diameter, nearly undisturbed samples of sediment collected in full-cylinder core tubes. The core tube (thin-walled aluminum pipe) is driven into the sediments using the vibratory motion of an attached power head (Photo 9-4). The cores are continuous to lengths of up to about 5 meters, and of relatively large volume, providing larger samples for sedimentologic and radiometric analysis.

Forty-four vibracores were collected and analyzed from three sites: 13 from Crescent City, 21 from Lagoon Creek, and 10 from South Bay. In addition at Crescent City and Orick, drive cores (vibracore sampling tubes driven into the sediments using a sledgehammer) were collected and analyzed, one from Crescent City, and two from Orick (Photo 9-5). Both vibracores and drive cores produce high-qu~lity sediment samples. The disadvantages of vibracoring include the large size and weight of the apparatus, which make it difficult to use, and restricts the number of cores that can be collected. Additionally the aluminum casing used to collect a vibracore sample must be cut with a saw to expose the sample, making it impractical to examine the cores in the field.

Radiocarbon Dating - Radiocarbon ages for tsunami deposits were obtained for 46 samples from 5 sites along the northern California coast. Forty-two of the samples were from three sites:

Crescent City (13 samples), Lagoon Creek (19 samples), and South Bay (10 samples). Carbon-14 ages for three samples from the dune complex on the North Spit of Humboldt Bay and one sample from the Orick marsh also were obtained during this study. At Lagoon Creek, the presence of the well-dated Little Glass Mountain tephra in the marsh stratigraphy provided additional age control for the tsunami record.

Most (38) samples were analyzed using accelerator mass spectrometry methods on carefully selected small twigs and herbaceous plant parts, and small pieces of detrital wood, charcoal, and spruce cones that were in stratigraphic context with the tsunami sands. Most of the wood and charcoal samples were from tsunami sand layers or trash layers capping sands. Peat samples and herbaceous plant parts were collected from the uppermost peat layers immediately below the sand layers. Seven large wood and peat samples were analyzed by standard radiometric methods.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-12 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Radiocarbon analyses for this study were performed by Beta Analytic, Incorporated. The laboratory's carbon-14 ages were calibrated using a carbon-13 correction, and the calibration program of Stuiver and Reimer (1993), using a lab error multiplier of 1.6. The ages reflect the 95-percent confidence level (2 sigma), and are reported as calibrated radiocarbon years before present, which, by convention, are calendar years before AD 1950.

Diatom Identification - Diatom samples were processed by treating the sediment with 35-percent hydrogen peroxide solution to remove organic material. The sample was rinsed, and a slurry was prepared using distilled water. A two-drop sample of the slurry was settled onto a coverslip, and the sample on the coverslip air-dried. Permanent slides were prepared by mounting the sample with a fixative on microscope slides. These slides were examined using a deep-field zoom microscope at 400X to 1 OOOX, as necessary for identifications.

Grain-size Analysis-Sand samples were analyzed for grain-size distribution to characterize the textures and measure the grain size for modeling the flow parameters during deposition. Sand samples were wet-sieved in a 0.062-millimeter microsieve. The sand fraction was treated with a 30-percent hydrogen peroxide solution to remove organic material, and oven-dried. The dried samples were mechanically screened through selected phi-size microsieve columns, and the size fractions weighed.

Run up Heights-At most locations in northern California where field evidence of paleotsunami inundation was found no surveyed elevation datum was available for elevation reference. Field measurements of heights of marshes, beach berms, pebble layers in the dunes, tsunami sand layers, and other indicators used to indicate or calculate run up height were made by measuring the vertical distance to the tsunami indicators from debris deposited by previous high tides in sheltered locations on the adjacent shoreline. The high tide line debris is assumed to approximate mean higher high water (MHHW). These values in this report are adjusted to mean lower low water (MLL W) by adding 7 feet (the mean tide range for the northern California coast) to the field measurements.

9.4.3 Evidence for Past Tsunamis in Northern California Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-13 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Stratigraphic evidence of past tsunamis-thin sand sheets and associated marine diatoms and other diagnostic features-is abundant in bays, lakes, and marshes along the Pacific Coast in Oregon, Washington, and on western Vancouver Island. The current investigations have focused on finding and assessing similar geologic evidence of large historical and prehistoric tsunamis in the coastal wetlands between Cape Mendocino and the Oregon/California border. The tsunami investigation included:

Surveying the wetlands along the northern California coast to identify sites that could act as sediment traps that could potentially record past tsunami inundation Evaluating the marsh stratigraphy near Crescent City, where tsunamis from distant sources are known to have inundated the coast in 1960 and 1964, to establish diagnostic characteristics of these tsunami events, and to search for evidence of previous tsunami inundation Investigating the stratigraphy in freshwater marshes and ponds at Crescent City and Lagoon Creek, where the most complete stratigraphic record of tsunamis appeared to be preserved.

At these sites, detailed stratigraphic, sedimentologic, paleontologic, and geochronologic analyses were conducted to assess the late Holocene history of tsunami inundation Investigating the sediments in Humboldt Bay for evidence of past tsunamis. One site, a marsh along the South Spit in southwestern Humboldt Bay (South Bay), contained stratigraphic evidence of past tsunamis and was investigated in detail. Subsequent investigation of southeastern Humboldt Bay by Patton and others (2002) has identified a second site at Hookton Slough that has stratigraphic evidence of paleotsunamis. This site has also been extensively investigated.

Mapping the spits and dune fields that serve as barriers and partial barriers to tsunamis entering Humboldt Bay to find erosional features and sediments left by past tsunamis. The paleomorphology of the spits was also assessed because it is important to the interpretation of paleo tsunami evidence in the bay Assessing the height of past tsunami waves from the evidence of past tsunami run up and the sediment characteristics of the tsunami deposits Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-14 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Studying the local Native American oral histories, which include stories interpreted to describe ancient earthquakes and flooding by tsunamis on the northern coast of California prior to the arrival of white settlers in the 1850s Reviewing the empirical data on tsunami runups elsewhere in the world and correlating the data with earthquake source parameters, triggered landslides, and secondary faulting (Appendix 9A)

Evidence for tsunami inundation was found in the stratigraphy at nine of the sixteen sites surveyed (Figure 9-4): eight of the fifteen coastal marsh sites (Table 9-2) and one site in the dunes of the North Spit. The marsh sites at Crescent City and Lagoon Creek provide the most compelling evidence for tsunami inundation and were studied in greatest detail. Strong evidence of paleotsunamis was also found at Orick and in the South Bay; these sites were also carefully studied. Evidence of for past tsunamis was also was found at marsh sites at East Creek, Major Creek, and Big Lagoon but initial investigation indicated the information from these sites was limited and they were not studied in detail. A summary of the types of evidence of past tsunami inundation at each of the coastal marsh sites is presented in Table 9-2. Evidence of paleotsunamis is also evident in the sand dunes on the North Spit. No evidence of past tsunami inundation was found at High Prairie Creek, or at six sites investigated around the north and east sides of Humboldt Bay.

The stratigraphic position and radiocarbon ages for the major tsunami layers in the northern California marshes are very similar in stratigraphic position and radiocarbon ages to Cascadia subduction zone earthquakes derived from coastal deposits from British Colombia to California (Atwater and Hemphill-Haley, 1997; Atwater and others, 1995; Nelson and others, 1996b; Kelsey and others, 2002; Witter and others, in review; Hughes and others, 2002). Therefore, we assumed one-to-one correlation, and used the event nomenclature of Atwater and Hemphill-Haley (1997) for the tsunami horizons at northern California marshes (Figure 9-7; Table 9-3).

Crescent City-The marsh at Crescent City, which lies behind a wide beach and low beach berm, was investigated for historic and paleotsunami deposits by Carver and others (1998)

(Figure 9-8; Photos 9-6 and 9-7). Radiocarbon ages for samples at the base of the marsh Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-15 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

sediments at Crescent City show the marsh was formed about 3,000 years ago. Two types of sand layers are present: thin sand layers limited to the seaward part of the marsh and interpreted as deposits from distant-source tsunamis (or from small regional tsunamis generated by sources other than a long rupture of the Cascadia subduction zone); and thick sand layers that can be traced across the marsh to the inland edge, interpreted to have been deposited by local tsunamis generated by long ruptures of the Cascadia subduction zone. Sand deposits from overtopping storm waves were not encountered at the Crescent City marsh site.

The stratigraphy at the Crescent City marsh (Figure 9-9) contains at least seven sand layers that have the characteristics of far-traveled tsunamis. The 1960 tsunami that originated from the Chilean earthquake and the 1964 tsunami from the Alaskan earthquake deposited the upper two of these sand layers. These tsunamis were generated by the two largest earthquakes of the past century. The earthquakes were near maximum for the Pacific region, and the 1964 earthquake had an optimum wave path to northern California; thus, they probably represent the maximum size for far-traveled tsunamis (Lander and others, 1993). This is the only place along the southern Cascadia margin where sand deposited by these well-documented modern tsunamis can be compared with paleotsunami sands at the same site, thus allowing direct comparison of geologic deposits and historical observations. At least five similar prehistoric sands were found in the seaward part of the marsh stratigraphy. These also are interpreted to represent the stratigraphic evidence of tsunamis from distant sources, similar to the Chilean and Alaskan tsunamis. It is likely that many more far-traveled tsunamis reached this site but the evidence is not preserved.

At least five thick, extensive, sand layers are preserved in the stratigraphy at Crescent City.

These possess most of the characteristics of tsunami deposition and are interpreted to have originated from local Cascadia subduction zone events (Figure 9-9). Each of these sand layers unconformably overlies older marsh sediments and reflects tsunami erosion of the seaward part of the marsh. Several show - excellent examples of multiple graded beds from repeated wave inundation indicative of a typical local tsunami wave train (Figure 9-1 0).

Lagoon Creek - The Lagoon Creek marsh (Figure 9-11; Photos 9-8 and 9-9) is a freshwater pond and marsh situated 17 feet above mean lower low water in a narrow valley behind a 23-Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-16 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

foot-high beach berm (Photo 9-1 0).

During the past 3,000 years the Lagoon Creek site has been relatively stable when compared to sea level. The site appears to have undergone only a small amount of long-term uplift (Carver and others, 1998; Garrison-Laney, 1998) that appears to have matched the eustatic rise in sea level during the late Holocene. The geomorphology of the region around the site suggests the late Holocene tectonic elevation change has been minimal: there are no raised terraces, older dunes or prograding beaches indicating uplift at or near the site. In addition, the beach berm that impounds the shallow pond, which gives the name to Lagoon Creek, appears to have been relatively stable for the past 3000 years because the freshwater marsh sediments have radiocarbon ages for samples near the base of the marsh sediments of that age; this indicates no late Holocene subsidence at the site. Hence, the Lagoon Creek site has experienced rather constant relative sea level during the late Holocene.

During investigations of this favorably situated marsh, several paleo tsunami deposits were discovered (Carver and others, 1998; Garrison-Laney, 1998; Abramson, 1998). Freshwater lacustrine and wetland sediments underlying the marsh include at least eight interbeds of coastal sand that have characteristics of a tsunami origin (Figures 9-12,9-13, 9-14). Six ofthe sand layers are thick and widespread; several extend about a mile inland. Most core samples of these sand layers are normally graded, and some of the sand layers consist of several fining-upward sequences. In the seaward part of the marsh, the sands contain ripped-up clasts of marsh sediments. The sands also contain marine diatoms, including many that are broken, but well preserved. Marine diatoms were found in all the sand layers and, in one layer, the diatoms were traceable in the marsh stratigraphy inland for about one-quarter mile beyond the landward extent of the sand. All the sand layers thin and fine landward and the mineral composition of the sand in the lagoon and marsh stratigraphy is similar to that of the modern beach. Combined, these characteristics indicate the sands at Lagoon Creek were transported from offshore and carried inland across the marsh by landward surges of seawater that eroded the seaward part of the marsh. The sands were rapidly deposited from suspension as a sheet on the marsh surface.

Calibrated radiocarbon ages indicate a separate sand layer correlates with each of the most recent six Cascadia subduction zone events based on the earthquake history documented by Atwater and Hemphill-Haley (1997) (Table 9-3). These sand layers are interpreted to be deposits from large local tsunamis produced during ruptures on the Cascadia subduction zone.

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Two additional thin sand layer~, which also appear to be tsunami deposits, were found in several cores from the seaward part of the Lagoon Creek marsh. The upper of the two thin sand layers is above the "Y" layer, and thus less than about 300 years old. The other is between the "Y" and "W" sands. Both are traceable inland only about 1,300 feet from the modern beach berm. These two sand layers could have been deposited by far-traveled tsunamis, or tsunamis generated by local faulting or submarine landsliding offshore, or nearby ruptures on short segments of the Cascadia subduction zone, such as the Eel River segment. These do not have the characteristics of sand deposits from storm waves or storm surges that over topped the berm (Carver and others, 1998).

East Creek, Major Creek, Orick, and Big Lagoon - East Creek and Major Creek are adjacent watersheds in the Gold Bluffs Beach State Park (Figure 9-4). Their coastal marshes contain sand layers having some characteristics indicative of tsunami deposition (Carver and others, 1998).

East Creek contains at least two sand layers similar in grain size to the adjacent beach; they have sharp basal and upper contacts and are capped by woody debris. One of these two layers contains marine diatoms, whereas the underlying peat contains only freshwater diatoms. Major Creek contains at least five thin sand layers, one of which possesses the characteristics of a tsunami sand, and is normally graded.

Orick and Big Lagoon are specifically mentioned as having been flooded by the sea in detailed Native American oral histories that depict a great earthquake followed by a tsunami one night long before the arrival of white settlers (Kroeber, 1976, Carver and Carver, 1996). The Yurok story, The Flood, describes a flood from the ocean inundating a small village near Orick (Orekw) (Figure 9-15; Photo 9-11). The stratigraphy in a marsh immediately downhill from the Orekw village site contains three, separate sand layers, two of which lie atop peat containing freshwater diatoms and are overlain by mud having intertidal diatom species (Carver and others, 1998). This lithologic and diatom evidence for subsidence, along with several additional sand-layer characteristics (Table 9-2) strongly suggests deposition by a local tsunami at this site. The middle layer contains a triplet of fining upward sand layers and has a Carbon 14 date of 180 +/-40 years BP (AD 1665-1950) and is interpreted to be the "Y" event in 1700 AD.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-18 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Two thin ( ~ 1 inch) undated sand layers in gouge cores taken from a freshwater marsh approximately 600 to 1300 feet from the beach at the south end of Big Lagoon may be tsunami deposits as well. This site is also identified as having been flooded in the Yurok story, The Flood (Kroeber, 1976, Carver and Carver, 1996).

North and South Spits-The North and South spits of Humboldt Bay act as barriers that partially block tsunamis from entering the bay (Figure 9-16; Photos 1-1, 9-1, 9-12, 9-13). The spits are covered by extensive dune fields composed exclusively of well-sorted aeolian sand derived from the beaches on the western side of the spits. In addition to sand, sediments in the active littoral and beach zones also include significant components of gravel, pebbles, and cobbles. The cobbles and pebbles extend to an elevation of 38 feet above mean lower low water (Carver, 2002b). Carver (2002b) interprets the scattered cobbles and pebbles mantling eroded dunes on the seaward face of the North Spit to have been deposited by landward directed high-velocity surges of water that rose above the level of modem high storm tides (Photo 9-14 ).

Stratigraphic position indicates these cobbles and pebbles were deposited after the formation of the oldest dunes about 1,100 years ago and before the formation of the intermediate-age dunes that formed less than 300 years ago.

The dunes on the northern part of the North Spit range from 53 to 72 feet above mean lower low water. These old dunes are tree covered and have a soil with an incipient A/C horizon developed on them. This soil is uneroded above the limit of pebble scatter, which indicates the spit has not been overtopped or eroded by tsunamis or storm waves (Leroy, 1999). In addition, no tsunami deposits are present in marshes at the north end of Humboldt Bay and along the Mad River Slough behind this part of the spit (Section 9.4.4). These dune features place an upper limit of past tsunami runup height at the coast at Humboldt Bay in the late Holocene.

The height, width, and length of the South Spit (Photo 9-12) are considerably less than the North Spit. Geologic evidence suggests the South Spit has been in approximately the same position and at about the same height for the past 1,000 years. Leroy (1999) reports that the average height of the South Spit dunes is about 18 feet above mean lower low water and the maximum south spit dune height is 23 feet above mean lower low water. Morphology of the dunes and Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-19 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

degree of soil development on the South Spit are similar to those on the youngest parts of the North Spit, which have been dated at about 300 years old, or younger.

South Bay-One site in the southwestern part of Humboldt Bay, referred to as the South Bay site, is just east of the south end of the South Spit (Figure 9-17; Photo 9-12, 9-13, 9-15). The stratigraphy beneath a weakly developed salt marsh at this site contains distinct evidence for local tsunamis (Carver and others, 1998) (Figure 9-18). At least two buried peat units capped by sand layers record earthquake-induced subsidence immediately followed by tsunami inundation that eroded the marsh and deposited the sand sheets containing rip-up clasts of marsh peat.

Radiocarbon dates indicate the sands were deposited about 1,200 and 300 years ago. These tsunami deposits indicate the South Spit must have been substantially overtopped during these events. One, and possibly two, additional sand layers have several characteristics typical of tsunami deposits, but they are not associated with buried marsh soils. Cores from this marsh recovered strata that only date back about 2,000 years. Unconformities within the bay stratigraphy and the presence of sandy tidal channel deposits make correlation of sand layers between cores more difficult at this site than at the other more protected sites.

Non-tsunami sand layers also are present in the sediments at the South Bay site (Carver and others, 1998). These sands, deposited as small sandbars on the tidal flats and tidal channel deposits, have cross-bedding, wide ranges in particle size, and other sedimentologic and compositional characteristics that are not typical of tsunami deposits. They are interpreted to be deposited either by storm waves that overtopped the South Spit and washed sand from the spit into the bay margin or by erosion and re-deposition of sandy channel deposits by currents within the bay. No sand deposits that have the characteristics of storm surges were found at the other marsh sites, including Crescent City.

Hookton Slough - The Hookton Slough marsh is at the southeast margin of South Bay about 6 kilometers from South Spit (Figure 9-17; Photo 9-13). At this site four buried marsh soils are interpreted to record abrupt subsidence from large earthquakes on the nearby Little Salmon fault, the southern Cascadia subduction zone, or both (Patton and others, 2002; Witter and others, 2002). Diatoms from the buried soils and overlying mud confirm the abrupt subsidence. The three older buried marsh soils are mantled by sand sheets that exhibit characteristics of tsunami Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-20 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

origin including multiple normally graded beds and textures similar to the beach and dune sands at the coast. Coseismic subsidence of these soils occurred about 1600, 2150 and 3500 years BP based on radiocarbon ages from delicate detrital plant fossils (Witter, and others, 2002). Of these the event at 1600 years and possibly the event at 3500 years correlate to Cascadia events "S" and "L". The event at 2150 is a separate event thought to be associated with displacement on the nearby Little Salmon fault. The event also may correlate with a subsidence and tsunami event about 2000 years ago recorded at Sixes Rivers (Kelsey and others, 2002).

9.4.4 Humboldt Bay Sites Having No Evidence of Past Tsunamis Sand layers interpreted to be deposits from past tsunamis in Humboldt Bay are restricted to the southwestern and southeastern margins of the South Bay (South Bay and Hookton Slough sites) as described above. No evidence of past tsunamis was found at other sites examined during the investigation of the bay (Figure 9-4), including the eastern side of the South Bay, the area near the Humboldt Bay Power Plant, and Arcata Bay including the Mad River Slough (Photo 9-1 ).

On the eastern side of the bay, the stratigraphy at four marshes, the Jacoby Creek marsh, the Eureka Slough marsh, the Railroad site, and marshes in the Humboldt Bay National Wildlife Refuge near the College of the Redwoods (Carver and others, 1998) (Figure 9-17), were found to contain a buried and subsided soil interpreted to be stratigraphic evidence of the most recent large Cascadia subduction zone earthquake (event "Y"). This buried soil horizon provided a guide to the stratigraphic position of potential evidence of the most recent tsunami generated by a Cascadia earthquake, and allowed assessment of the presence or absence of tsunami genic sediments correlative with those found at open-ocean sites on the northern California coast. At all four of these sites, the buried soil was directly overlain by well-stratified intertidal mud. The soil/mud contacts were sharp, and had no evidence of scour or erosion, as would be expected if the subsidence had been followed by rapidly flowing water from tsunami inundation. In addition, no sand was observed at the soil/mud contact at any of these sites. Sand might be expected on the contact between the subsided soil and the overlying mud if significant tsunami inundation by water carrying suspended sand had occurred.

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Mad River Slough-Evidence for the absence of tsunami-deposited sand is particularly strong at the northern end of Arcata Bay in the area of the Mad River Slough where the marsh stratigraphy has been extensively studied in great detail (Vick, 1989; Jacoby and others, 1995). The uppermost buried soil and its overlying sediment are remarkably well preserved, and at many places include entombed marsh plant assemblages that were growing on the marsh surface at the time the marsh subsided. The above-ground stems and leaves of the fragile marsh plants are buried in their original upright growth position in overlying intertidal mud. The mud is very fine grained, micro-laminated, and the contact between the overlying mud and the underlying peaty marsh soil is very sharp. In outcrop, the contact can be resolved to within a few millimeters. No sand or other evidence of tsunami disturbance has been found along this contact at the many locations where it has been studied. The most likely interpretation of these contact relationships is that no significant tsunami inundation by water carrying suspended sand or with erosive flow occurred in the Mad River Slough area of Arcata Bay following the most recent Cascadia subduction zone earthquake (event "Y"). Contacts of bay mud over salt marsh peat that have carbon-14 dates correlative to Cascadia events "W," "U," and "S" also are present in the Mad River Slough area, and these contacts also do not have tsunami sand layers (Jacoby and others, 1995).

Humboldt Bay Power Plant Area - Several sites whose geologic setting make them suitable for assessing the presence or absence of tsunami evidence were investigated near the Humboldt Bay Power Plant and the ISFSI site (Carver and others, 1998). These included relict tidal channels and low-lying marshlands a few hundred meters north, east, and southeast of the ISFSI site.

Several cores from the tidal channels contained thin sandy layers composed of poorly sorted sandy mud and muddy sand having abundant shell fragments. The sandy layers are thinly laminated. The poor sorting, macrofossil fragment content, and laminated structure of these sandy layers is in contrast to the well-sorted, normally graded and shell-fragment-free character of tsunami-generated sand layers. We interpret the sandy layers found in the relict tidal channels as normal channel lag sediments resulting from tidal current scour and deposition in the channel bottoms. No tsunami sand layers were found in the marsh sediments near the ISFSI site.

Although the absence of diagnostic tsunamigenic sediments along the north and east margins of Humboldt Bay, including the area near the Humboldt Bay ISFSI site, does not prove Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-22 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

conclusively that tsunami inundation has not occurred in these areas, the absence of evidence of tsunamis indicates the areas were not significantly affected by the tsunamis that produced the sedimentary record of inundation at open-ocean sites in northern California and along the extreme southern end of the South Bay.

9.4.5 Correlation of Tsunami Deposits Comparison of tsunami histories from the Crescent City, Lagoon Creek, Orick, South Bay, and Hookton Slough marshes shows that no single site contains a complete record of all the tsunamis that have inundated the northern California coast during the past several thousand years (dated events are shown on Figure 9-7) (Carver and others, 1998). The very low lying and exposed Crescent City marsh is the most sensitive of the sites we evaluated; it records the relatively low waves from distant-source tsunamis, including sands from the 7.4-and 16-foot-high runups (above predicted tidal level, Table 9-1) from the 1960 Chile and 1964 Alaska waves, respectively. However, several large tsunamis that left stratigraphic records at Lagoon Creek and South Bay are not as well preserved as sand sheets in the Crescent City marsh. Instead, erosion during subsequent tsunamis has likely removed these deposits from the marsh stratigraphy (Carver and others, 1998). In contrast, Lagoon Creek contains the most complete record of large tsunami inundations, but has only limited evidence from smaller, distant tsunamis. Presumably the higher berm fronting the Lagoon Creek site prevented small runup height waves from entering the marsh, and reduced the flow of the large local tsunamis sufficiently to limit erosional reworking of earlier deposits to the seaward most part of the marsh, allowing better preservation of evidence from large local tsunamis.

The combined tsunami record from Crescent City and Lagoon Creek indicates the presence of an additional major tsunami deposit that is not evident in the Cascadia subduction zone sequence in Washington and northern Oregon (Table 9-3). Although some of the calibrated 2-sigma ages of the "W" layer at Crescent City and the "W" layer at Lagoon Creek both overlap (Figure 9-7) the "W" event age from Washington, the California layers probably do not correlate with the Washington event. The "W" layer at Crescent City appears to be deposited by a local or distant tsunami about 850 years ago (the mean of the four most likely dates for this deposit) that strongly affected the Crescent City marsh, eroding much of the previous record, including most Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-23 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

of the deposits from the~ 1,150-year-old "W" layer preserved in the Lagoon Creek stratigraphy.

The deposit from the Crescent City "W" event in the Lagoon Creek sediments may appear as the small sand deposit in the seaward portion of the Lagoon Creek marsh that lies stratigraphically above the "W" layer and below the "Y" layer there. This deposit could have been caused by an unusually robust far-traveled tsunami, or by submarine landsliding, but it was more likely caused by a tsunami generated by local faulting offshore on the southern part of the Cascadia subduction zone. A tsunami with similar ages is also known from the Sixes River site in southern Oregon (Kelsey and others, 2002) and elsewhere in northern California: possibly from the cores at Orick, from sand sheet overlying a subsided marsh soil in the Eel River Estuary (Li, 1992a; 1992b ), and may correlate with a similarly dated displacement event on the Little Salmon fault at the Little Salmon site (Carver and Burke, 1988, Clarke and Carver, 1992) and the Swiss Hall site east of South Bay (Witter and others, 2002). It appears that this event only ruptured the southern part (Eel River segment) of the Cascadia subduction zone, or alternatively was the result of a large event on the Little Salmon fault system.

The stratigraphic signature of coseismic subsidence associated with a tsunami deposit is particularly well developed for the "Y" event at Orick, and for the horizons identified in South Bay (Carver and others, 1998) and Hookton Slough sites (Patton and others, 2002). Coseismic subsidence coincident with the deposition of tsunami sands is indicated by the stratigraphy, sedimentology, diatoms, and macrofossils found at these sites. At these sites tsunami sand dated to the last approximately 300 years and subsequent mud containing lower intertidal marine or brackish-water diatoms overlie the salt marsh and freshwater peaty sediments. Other studies identified subsidence horizons that are correlated to earthquakes on the Cascadia subduction zone at Mad River Slough (Vick, 1989; Clarke and Carver, 1992; Jacoby and others, 1995), the South Bay (Valentine and others, 1992) and possibly coincident with faulting on the Little Salmon fault at the Swiss Hall site east of South Bay (Witter and others, 2002). The association of tsunami evidence with evidence for coseismic subsidence indicates the tsunamis were locally generated.

At the Lagoon Creek marsh, a small local landslide deposit derived from distinctive Franciscan Formation lithologies is interbedded in the marsh stratigraphy about one-half mile inland from the coast. Several gouge cores and three vibracores (LC-9, LC-10, and LC-13) sampled the Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-24 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

leading edge of this landslide, which came from a conspicuous scar on the steep slope adjacent to the marsh (Carver and others, 1998; Abramson, 1998). The landslide appears to have been active at least twice, each time a distinctive diamicton was deposited across the marsh surface:

once immediately before deposition of sand layer "U," and once immediately before sand layer "S." Each of these sand layers lies directly on the diamicton with no evidence of marsh vegetation or fine grained marsh sediments at the sharp basal contact of the overlying sands. Up-valley both sand layers contain abundant angular clasts of landslide lithologies that were entrained and transported only landward. No other landslide that could have been the source for the distinctive Franciscan material was found in the marsh. We believe seismic shaking triggered the landslides. The entrainment and up-valley redeposition of the slide material in the tsunami sand layers is interpreted to represent the reworking of the slide by rapid up-valley tsunami flow immediately after the slide debris was deposited on the marsh surface. This evidence of strong shaking coincident with tsunami generation re-enforces the interpretation that the large tsunamis found in the paleotsunami record in northern California were locally generated by slip on the subduction zone.

Small sand dikes and sand tubes are intruded into the marsh sediments at Crescent City, Lagoon Creek, and South Bay sites (Carver and others, 1998). Most of these intrusive sand bodies have grain sizes and compositions similar to the tsunami sands. We interpret them to be derived from the tsunami sand layers deeper in the marsh sequence and to have been injected into the overlying marsh deposits when the sand liquefied and vented to the surface due to strong seismic shaking. Many of the dikes and tubes terminate at the top of a buried marsh and are overlain by a separate tsunami sand layer. Strong shaking that produced the sand dikes and tubes indicates that a local earthquake generated the tsunamis.

We also interpret several types of micro-sedimentary structures and stratigraphic characteristics of the more extensive tsunami sands in northern California marshes at Crescent City, Lagoon Creek, Orick, South Bay, and Hookton Slough to be the result of locally generated tsunamis. In particular, the multiple fining-upward sand sequences, separated by marsh grass "flopovers," and a trash layer capping the sequence are interpreted to be caused by several high-runup inundations that were closely spaced in time. These features are rare or absent from deposits of far-traveled Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-25 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

tsunamis. Such multiple waves are known to be characteristic of large tsunamis on coasts adjacent to tsunami sources elsewhere in the world.

Radiocarbon ages3 for large tsunami events in northern California are the same, within the precision range of the carbon-14 ages, as the estimates of the rupture chronology for the last six major events on the Cascadia subduction zone recognized on the southern Washington coast

("Y," "W," "U," "S", "N", and "L") (Atwater and Hemphill-Haley, 1997) (Table 9-3; Figure 9-7). In particular the radiocarbon ages at Lagoon Creek for each of the sand layers correlate, at the 2-sigma confidence level, with each of these Cascadia subduction zone events (Carver and others, 1998; Garrison-Laney, 1998).

The most recent major earthquake on the Cascadia subduction zone was the "Y" event that occurred about 300 years ago, as dated by high precision tree-ring series and radiocarbon analysis in Washington that shows the earthquake occurred in the winter of 1699-1700 (Yamaguchi and others, 1997). Whether several segments ruptured or this was a single very large event was not known until the records of tsunamis in Japan were examined. Historical documents record only one tsunami wave sequence in Japan for the time interval indicated by the carbon-14 and tree-ring analyses. The wave sequence had to have come from a distant source, because it was not associated with an earthquake in Japan. Modeling tsunami propagation from Kamchatka and the Aleutians indicates these are unlikely sources for the wave recorded in Japan. People who would have recorded such an event locally inhabited other circum-Pacific sources but none is reported. Back-calculating from the arrival times recorded for the wave at five locations in Japan, the earthquake that caused the wave occurred about 9:00PM on January 26, 1700 (Satake and others, 1996). This is supported by the Yurok oral history from Orick, and at least six other traditional stories from coastal Indians in northern California and several from Washington and Vancouver Island (Carver and Carver, 1996), that describe the earthquake as happening at night. The reported damage in Japan indicates the tsunami was large.

Model studies (Satake and others, 1996, 2002) of segmented and long-rupture Cascadia sources show 3 Dates in this report are all expressed as calibrated radiocarbon years BP (before 1950, considered "present").

Calibrated radiocarbon date is one that indicates that the date is the result of radiocarbon calibration using tree ring data. These values should correspond exactly to normal historical years BC and AD. The term means the number of years before 1950 and can be directly compared to calendar years.

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that only long ruptures generate tsunamis large enough to have produced the damage-causing run-ups observed and recorded in Japan. The interpretation of a single long rupture for the 1700 AD event is supported by the "Y" paleo tsunami record in marsh deposits from Vancouver Island to northern California. These marshes have only one sand sheet deposited during the "Y" time interval.

Similarly the tsunami sands that record the five previous major subduction earthquakes from Cascadia have single event deposits (one tsunami sand) indicating that a single robust tsunami was generated by each Cascadia event from Canada to northern California. If Cascadia ruptured as segments closely spaced in time (a cycle of events as each segment ruptured a few tens of years apart) then the paleotsunami record for late Holocene events would record several less robust tsunamis for each cycle, and some or perhaps most sites where tsunami sands were deposited would exhibit several closely spaced sand sheets of similar age. However, only single sand sheets have been found for each of these major events.

Nonetheless, some sites on the coast between Vancouver Island and Eureka record other, local paleotsunamis in addition to the major event tsunami sands. As discussed earlier the tsunami sand at Crescent City that dates to 800 to 1000 years BP (mean 850 years) is not correlative with the chronology of major events on Cascadia. This event appears also be recorded at Lagoon Creek by a thin sand that is between the thick well dated and well developed tsunami sand layers "Y" and "W" and in the Orick marsh as a thin sand below the ~300 year horizon. This event is permissively the same event dated about 800 years ago on the Little Salmon fault from the Little Salmon Creek site (Carver and Burke, 1987), the Swiss Hall site (Witter and others, 2002) and also may correlate with a subsidence horizon and tsunami sand found in the Eel River Valley (Li, 1992a, 1992b) and is about the same time as an event in the Sixes River area in southern Oregon (Figure 9-7).

Kelsey and others (2002) propose another southern Cascadia segment rupture from stratigraphic evidence in southern Oregon that is dated at about 2010 to 2300 years BP. This age for an event has no known equivalent as a subduction zone earthquake or tsunami anywhere else in Cascadia; however, a possible slip event on Little Salmon fault in this same time range is suggested from the paleoseismic studies at the margin of South Bay at the Swiss Hall site (Witter and others, 2002) and in the marsh stratigraphy at the Hookton Slough site (Patton and others, 2002).

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The average recurrence interval of large, near-source tsunamis from Cascadia over the ~3,000-year record is less than 500 years. However, the ages for individual events show the recurrence rate for the most recent six events is not uniform (Section 2.4, Table 2-1; Table 9-3) (Atwater and Hemphill-Haley, 1997). Intervals between events "W," "U," and "S" are less than 300 years, whereas the intervals between "Y" and "W" and between "S" and "N" are 700 years or more, possibly reflecting "earthquake clustering" that is known on other seismic sources in the world. More than 300 years have elapsed since the most recent event.

9.4.6 Run up Estimates for Past Tsunamis We estimated the height of run ups and inundation distances for several past Cascadia subduction zone tsunamis in northern California using several different approaches: comparison with historical tsunamis at Crescent City (Carver and others, 1998); analysis of the distribution of sand, diatoms, and particle-size of sand layers at Lagoon Creek (Carver and others, 1998; Garrison-Laney, 1998; Abramson, 1998; Garrison-Laney and others, 2002); consideration of Native American oral histories at Orick (Carver and Carver, 1996 and Carver and others, 1998);

and analysis of pebble distribution and erosion on dunes and spits at Humboldt Bay (Carver and others, 1998; Leroy, 1999, Carver 2002).

Crescent City - At Crescent City, a direct comparison of the extent, thickness, and structure of paleo tsunami sand layers deposited by local tsunamis with the characteristics of the sand layers deposited by the far-traveled 1960 and 1964 tsunamis (Carver and others, 1998) provides a basis for estimating the elevation for larger wave runup heights from a local source. The Crescent City marsh is about 13 feet above mean lower low water ( 6 feet above MHHW), and Highway 101 that is built on the beach berm is several feet above that. The 1964 tsunami coincided with a high spring tide (Wilson and Torum, 1968), so the runup was higher than it would have been if the tide had been low. Tidal records show wave height as 20 to 21 feet (MLLW at Crescent City (Table 9-1 ), but local runup heights, as recorded by damage to structures on and near the beach at Sand Mine Road, were 7 to 10 feet above Highway 101 (10 to 13 feet above the marsh). Thus, at Sand Mine Road the wave run up height in 1964 was about 23 to 26 feet above mean lower low water (16 to 19 feet above MHHW). The sand layers from the 1960 and 1964 tsunamis in Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-28 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

the Crescent City marsh near Sand Mine Road are just under one-half inch thick in the most seaward part of the marsh; neither sand layer extends more than 600 feet inland from the beach.

No marsh erosion occurred during these events.

By contrast, thicker sand layers (> 3.5 in. thick) lower in the stratigraphic section at the Crescent City marsh extend 1300 feet inland, to the back edge of the marsh (Carver and others, 1998). The inland extent of flooding during these tsunamis probably was significantly greater than the 1300-foot distance from the beach to the back edge of the marsh where sand deposits are preserved, but further inland no marsh existed to collect and preserve sand. These thicker sand layers commonly contain multiple fining-upward sequences, reflecting repeated inundation by successive surges of the near-field tsunami wave train. This evidence is especially well developed in sand on the seaward side of the marsh. Only at the back edge of the marsh do the thick and extensive sand layers resemble the thin 1964 deposits. Using the tsunami flow parameters (depth and velocity) that are indicated by the characteristics of the sand layer at the seaward side of the Crescent City marsh, then the 10 to 13-foot water depth in the marsh in 1964 could be considered similar to the depth of the tsunami at the inland-most part of the marsh during the "Y" and some of the earlier large, local events. Hence the run ups from the local paleo tsunamis were clearly much higher at the beach than 23 to 26 feet (above MLL W) attained in 1964.

As a comparison the 1964 Alaska earthquake produced near-field tsunami on Kodiak Island that are characterized by multiple fining-upward sand sequences in marshes at Women's, Middle, and Kalsen bays. The 1964 sand in the Kodiak tsunami deposits consists of both Katmai ash, which is a medium to course sand size pumice, and similar size black lithic sand derived from shale, slate, and sandstone. At most sites these two components are in roughly equal amounts, but at some sites one or the other of the two sand types predominate. Where multiple sand layers from successive wave pulses are preserved the denser lithic sand forms the lower part of a couplet with the Katmai ash sand at the top of each layer (Carver and McCalpin, 1996). The Kodiak sand layers were deposited by run up that crested about 1 0 to 18 feet above the marsh surfaces. No erosion of the marsh surface occurred at Women's Bay where water was 10 feet above the marsh, but erosion of the Myrtle Creek marsh at Kalsen Bay occurred where water depth was 12 feet above the marsh.

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The characteristics of sand sequences at Kodiak are remarkably similar to the prehistoric tsunami deposits in the Crescent City marsh at Sand Mine Road. Based on comparison with sand sheets deposited in marshes in Kodiak in 1964, we estimate they must have been at least as deep as 12 feet because they eroded the marshes. Considering that the 1964 tsunami that was 23 to 26 feet (MLL W) did not erode the marsh, the paleotsunamis that crossed the marsh must have been at least 5 feet deeper, more than 28 to 31 feet above mean lower low water for event "Y," and possibly higher for event "W," where waves crossed the beach at Crescent City (Table 9-4).

Mofjeld and others (1997) have used the precise timing of the last Cascadia subduction earthquake (event "Y") derived from the documented historic observations in Japan (Satake and others, 1996) to reconstruct the tide stage along the Pacific Northwest coast during the earthquake. Satake and coworkers back-calculate the time of the earthquake from arrival times of the tsunamis reported in Japan to be about 0500 UT on January 27, 1700 (9:00PM PST on January 26, 1700). Mofjeld and coworkers calculated the tide stages in Cascadia for that date indicate the earthquake occurred during a low, neap tide. They report the tide level reached a minimum of about 1.4 feet at 0541 UT (9:41 PM PST) at Humboldt Bay, about the time of the earthquake. The following high tide at Humboldt Bay was at about 1230 UT (4:30AM PST) and was about 5.9 feet. Since near field tsunamis from great subduction earthquakes produce tsunami wave trains that arrive at intervals over many hours, the initial tsunami pulses probably arrived during low water and contrast with the 1964 distant tsunami that arrived on the northern California coast at high tide. However, later waves in the tsunami wave train could have come ashore during the subsequent high tide stage, comparable to the 1964 tsunami. The tide stage during earlier Cascadia tsunamis is unknown.

Lagoon Creek - At Lagoon Creek, particle-size distribution, extent, structure, and the elevation of sand sheets help constrain wave run up height, inundation distance, and unit discharge for the "Y," "W," "U," and "S" events (Abramson, 1998). Flow parameters at Lagoon Creek were estimated by analysis of the particle-size distribution for the coarse-sand fraction of tsunami layers across the marsh as done elsewhere by Atwater and Moore (1992) and Moore (1994).

Abramson (1998; also in Carver and others, 1998) estimated the water depth for inundating flows that deposited sand at Lagoon Creek was 11 to 46 feet above the marsh surface using a Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-30 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

range of wave velocities from 6.5 to 16.5 feet per second (Table 9-5). The analyses resulted in estimates of unit discharge for flows carrying sand above the marsh surface at the time of inundation. This unit discharge, calculated for the "Y," "W," "U," and "S" events, places limits on flow parameters for these inundation events. Runup-height estimates (above MLL W) at the beach berm of between 18 feet (minimum, event "U," using a velocity of 16.5 ft/s, and assuming complete berm erosion) and 52 feet (maximum, event "S," using a velocity of 6.5 ft/s assuming no berm erosion) resulted from these analyses. Elevation of the marsh surface was interpreted as the base of the sand layer in Core 4, approximately 2,000 feet inland from the coast. Core 4 is the approximate location at which sand grain size begins to fine away from the beach. The calculations selecting an intermediate velocity value of 10 feet per second that is believed to be representative or "best estimate" of water depths are shown on Table 9-5; using this velocity, run up height estimates range between 26 and 33 feet (MLL W). Runup estimates for event "Y" range from 24 to 44 feet (MLL W), and the preferred estimate is 33 feet above mean lower low water.

Orick-At Orick and Big Lagoon, Yurok oral histories provide several accounts of inundation levels (Carver and Carver, 1996). These stories identify house sites and other landmarks as flooded by an ancient tsunami (event "Y"). Run up heights indicated by these stories at Orick are in the range of 66 to 69 feet above mean lower low water (Figure 9-15). These unusually high run up heights, if accurate, may reflect the effects of a nearby submarine landslide, or tsunami focusing and constructive wave interference that caused an unusual high runup at this location.

Humboldt Bay-Runup estimates at Humboldt Bay are based on the evidence of overtopping of the South Spit during each of the Cascadia subduction zone events. The lack ofpre-event-"Y" morphology or soil development on the South Spit, and sand sheets having basal unconformities in the bay margin stratigraphy at the south end of the spit at the South Bay site reflect substantial landward flows across the spit.. However, the absence of sand on the same paleoseismic horizons in the wildlife refuge on the east side of South Bay, some 2.5 to 3 miles southeast of the spit, suggests that these waves may not have transported sand all the way across the bay. The tsunami sands found at Hookton Slough, which is south of the South Bay National Wildlife Refuge site, are believed to be sand that was incorporated from the readily available sand in the tidal channels and sand flats in South Bay northwest of Hookton Slough (Patton and others, 2002).

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Overtopping of the South Spit by the most recent three or more Cascadia subduction zone tsunamis shows the run up height of each of these tsunamis had to be higher than 18 to 23 feet above mean lower low water, the average and maximum height of the spit dunes (MLLW) for about the past millennium (Leroy, 1999). The characteristics of the tsunami sand layers, including extensive tsunami erosion of the salt marshes adjacent to the spit, indicate flows over the marsh were greater than the historical inundation levels (21 to 25 feet) at Crescent City from distant-source tsunamis (discussed above). The tsunami run up at the South Spit had to be significantly higher than the top of the narrow dune field because the tsunami eroded the spit and had enough velocity to rip up and erode the highly vegetated marsh surface on the bay margin.

The sand sheet deposited in the South Bay by event "Y" thins more rapidly than the correlative sand sheet at Lagoon Creek, suggesting that the depth of the tsunami above the marsh decreased rapidly as it spread into the bay and was less than 13 to 33 feet (Table 9-5), the depth of estimated run up flows at Lagoon Creek for event "Y", as discussed above (Abramson, 1998).

We judge that the depths that correspond to run up heights are less than 20 to 40 feet for event "Y" at the south spit.

The absence of tsunami sand sheets on the same paleo seismic horizons in the stratigraphy in the northern part of Humboldt Bay and the Mad River Slough, and the presence of an uneroded soil capping old dunes on the North Spit indicate the dunes on the northern part of the North Spit have not been overtopped, thus, runups have been less than 53 to 72 feet, the average and maximum elevation of the dunes (MLL W) at the North Spit.

Radiocarbon-dated strata are mantled by concentrations of scattered-pebbles and cobbles within the dune stratigraphy on the seaward side of the North Spit, but landward of the high tide mark (Leroy, 1999) (Photo 9-14). These pebbles extend to an elevation of 38 feet above mean lower low water (Carver, 2002b ). These pebbles and cobbles are interpreted by Leroy (1999) to be from erosion within the dunes during extreme tides. However, because they are 30 feet higher than mean higher high water, Carver (Carver, 2002b) interprets them to be from transport of clasts onto the dunes during inundation by a large tsunami, probably the "Y" event, or possibly the "W" event. The inundation event that deposited them must have been higher than 3 8 feet, the present elevation of the clasts. Comparison to tsunami transported pebbles and small Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-32 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

cobbles, similar to those on the North Spit, from the 1964 Alaska earthquake at the Myrtle Creek marsh at Kalsen Bay, Kodiak Island, provides an insight to potential runups. The Myrtle Creek marsh was littered with similar but larger rocks where water depth was less than 12 feet above the marsh. This suggests that the tsunami surge at the North Spit was several feet higher than 38 feet, but less than 50 feet above mean lower low water. Carver (2002b) estimates that the runup was between 3 5 and 40 feet.

Given the observations at the South Spit, and the constraint that the water did not top the northern part of the North Spit, the estimate of run ups at Lagoon Creek, and the analysis of the historical tsunamis at Crescent City and on Kodiak Island (Table 9-4) we estimate past Cascadia-generated tsunami had run up heights of about 30 to 40 feet above mean lower low water at the sand spits facing the open coast at Humboldt Bay.

9.4.7 Potential for Local Landslide-Generated Tsunamis Landslide-generated tsunamis can contribute to the wave train of a seismic generated tsunami and can cause locally higher run ups on the affected coast. The compilation by Lander and others (1993) notes that of fifteen high-quality tsunami reports associated with earthquakes along the west coast of the U.S. since 1812, eight to thirteen were caused by or included submarine landslides. Five of the landslide-related tsunamis affected the coast of southern California; four affected the central California coast or San Francisco Bay. Only one affected the northern California coast, at Crescent City; it was due to an 1873 earthquake north of the Oregon border.

Submarine landsliding is a common and ongoing process off the California coast. Clarke and others (1985) note its prevalence in the broad, southern California continental borderland, possibly the reason for the large fraction of landslide-related tsunamis in southern California reported by Lander and others (1993). Offshore northern California, high sedimentation rates and steep sea-floor topography are conditions that produce instability and promote submarine landslides. Particularly steep slopes are present along the Mendocino escarpment, along part of the outer continental slope, and in the Eel and Trinity submarine canyons. Mapping of the continental margin offshore of northern California has identified many ancient landslides on the sea floor (Field and others, 1980). Most of these probably were seismically generated. Given Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-33 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

the frequent occurrence of strong earthquakes in historical time in the offshore area north of Cape Mendocino, it is surprising there is only one report (1873, Crescent City) of a landslide-related tsunami in this region. This apparent dichotomy may be because the infrequent large long-duration megathrust earthquakes in this area may have already caused failure of any marginally stable slopes leaving few slopes susceptible to additional large landslides and there have been none historically.

Another potential offshore landslide area is the Eel River basin where seismic reflection profiles show an area of ridge and swale topography along the continental slope and gully-type topography (the Humboldt slide) (Gardner and others, 1999; Lee and others, 2002). The Humboldt slide was originally been interpreted as a shallow sediment failure along rotated blocks (e.g., submarine landslide) by Field and others (1980) but Gardner and others (1999) has been recently reinterpreted as a series of sediment waves caused by turbidity currents (Lee an*d others, 2002) or internal tidal waves (Cacchione and others, 2002). It is also possible that several of the 50 turbiditite deposits reported from in the Eel River basin by Nelson and others (2000) were triggered by earthquakes on the Cascadia subduction zone. The landslides that caused the turbidities were probably far enough away and not large enough to cause a large landslide generated tsunami at the coast.

Analysis of the paleotsunamis in northern California suggests possible locally high tsunami runup during event "Y" (January 26, 1700) at Orick, where the estimated runup height from native oral histories of 66 to 69 feet (above MLL W) (Figure 9-15) exceeds the estimates at Crescent City (higher than 28 to 31 feet MLL W) and Lagoon Creek (26 to 33) to the north and Humboldt Bay (30 to 40 feet) to the south. The Clarke and Field (1989) geologic map shows several regions of unstable sediment deposits on the continental shelf between 12 and 31 miles to the northwest and west of Orick. However, the continental slope in this area is generally shallow and not conducive to landslides.

If a landslide originated at one of these locations, it is likely that its tsunami would have affected other areas, as well as Orick. Unfortunately, there are too few locations along the coast where reliable run up-height estimates for the 1700 tsunami have been measured to allow testing of the landslide hypothesis for the anomalous wave height at Orick. Another explanation for this Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-34 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

apparently high run up may be a combination of the azimuth of wave arrival, wave amplification, and focusing of the waves caused by local effects from seafloor topography.

Recent high-resolution sea-bottom imaging by Goldfinger and Watts (200 1) indicates the presence of very large landslide masses along the Cascadia continental margin, but their subdued geomorphic appearance indicates that they are old, estimated at~ 110,00 yrs, 450,000 yrs and 1.2 million yrs (Appendix 9A). Although huge slide masses, such as these, could generate very large local tsunamis, no such events, other than possibly Orick, have been preserved in the geologic record for at least the past approximately 3,000 years at the sites studied. Such catastrophic events appear to be infrequent compared with the occurrence of tectonically generated tsunamis from ruptures of the Cascadia subduction zone, even though the long duration of such large events should be effective in triggering large landslides.

9.4.8 Summary of Results of the Paleotsunami Study The key results of our geologic study of past tsunamis in northern California can be summarized as follows:

The stratigraphic evidence from Crescent City, Lagoon Creek, and South Bay (Figure 9-7) shows the northern California coast has experienced at least eight large-runup tsunamis during the past 3,600 years. Six of these correlate with those of the Cascadia subduction zone events recognized in Oregon and Washington. The record includes events ("Y", "W",

"U", "S", "N", "L") at about 300, 1,150, 1,350, 1,650, 2,550, and 2,950 years ago. Regional distribution of these events suggests that most of the northern California tsunamis have been generated by large earthquakes (magnitude ~9) on the Cascadia subduction zone from long fault ruptures along the northern California, Oregon, and Washington coast. The stratigraphic evidence also indicates at least two local tsunami events. One is recorded at Crescent City, Lagoon Creek, and in the lower Eel River Valley at about 850 years BP. The other is at Hookton Slough at about 3600 years BP. Only at the South Bay site is there evidence of storm waves over topping of the beach berm, and these have distinctive characteristics that differentiate them from tsunami deposits.

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Potential tsunamis from the Cascadia subduction zone could generate I wave run ups along the open coast at Humboldt Bay. The height would probably be greater if the earthquake also triggered one or more large submarine landslides off the adjacent coast; however, no evidence of such larger, landslide-generated tsunamis in the past 2,000 and probably the past 3,600 years has been found in Humboldt Bay.

At Crescent City, evidence for at least five smaller, distant-source tsunamis was found.

These events were similar in general to the 1960 Chilean and 1964 Alaskan tsunamis.

Although runups as high as 21 to 25 feet above mean lower low water based on geological evidence (tidal records were 20 to 21 feet) were observed in Crescent City during the 1964 event, the run ups in Humboldt Bay were only about 10 feet above mean lower low water ( 4.4 feet above the tide level at the time). The run ups for the 1964 tsunami are the largest in recorded history for distant tsunamis striking the Humboldt Bay area. No geologic evidence (sand deposits) of distant-source tsunamis was found inside Humboldt Bay.

We found no indication that a significant (sand-carrying) tsunami runup has ever reached the area around the Humboldt Bay Power Plant. The northern North Spit directly blocked tsunamis from reaching the northern part of Humboldt Bay; however, tsunami runups more than 18 to 23 feet above mean lower low water would cross the South Spit and the southern end of the North Spit and we estimate paleotsunamis had heights of30 to 40 feet as they reached the spits. Tsunamis with lower runups could have crossed the sandbars that partially blocked the entrance channel to Humboldt Bay (Figures 9-21 and 9-22) prior to dredging the channel in 1860, but these would have dissipated rapidly as they spread out into Humboldt Bay. In any case the marsh deposits at and near the Humboldt Bay ISFSI site contain no evidence of sand being deposited from tsunami inundation.

9.5 ADDITIONAL ASSESSMENTS OF TSUNAMI HAZARD To augment and support the interpretation of the tsunami evidence discussed above, we have considered two additional types of data: well-documented historical tsunami records worldwide to estimate the possible tsunami run ups appropriate for the Cascadia subduction zone, and tsunami inundation analyses performed by others for the Humboldt Bay area.

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9.5.1 Empirical Comparisons of Worldwide Tsunami Runup Heights George Plafker, retired expert from the US Geological Survey, reviewed the empirical worldwide data regarding tsunamis for PG&E (Plafker, 2002b) to help constrain the wave run up height for a plausible tsunami source along the Cascadia subduction zone by comparison with other large historical tsunamigenic earthquakes worldwide (Figure 9-19). Included in this group are those events for which source mechanism, moment magnitude, and fault slip have been calculated and for which there are relatively complete observational data, such as local tsunami runup height, arrival times, and coseismic shoreline displacement. The data are presented in Appendix 9A.

For most tsunamigenic earthquakes, the associated tsunamis are generated primarily by regional coseismic vertical tectonic displacement of the sea floor. In some subduction zone environments, such as Cascadia, the associated tsunamis primarily are generated by seaward thrusting along the subduction zone megathrust and subsidiary faults that splay off the subduction zone and break to the sea floor through the upper thrust plate (see report by Plafker, 2002a, included as Appendix 2A and discussed in Section 2). Within this category are the three largest tsunamigenic earthquakes that have occurred in convergent continental margin environments during the 20th century: the 1960 Chile, the 1964 Alaska, and the 1979 Colombia earthquakes. As illustrated in Figure 9-19, the average maximum runups of tectonically-generated tsunamis increase approximately linearly with moment magnitude.

Some tsunamigenic earthquakes have waves generated by both tectonic displacements and earthquake-triggered submarine landslides in coastal areas. These include the 1964 Alaska, the 1992 Flores Island, the 1998 Aitape, the 1946 Aleutian, and possibly the 1993 Hokkaido events (Appendix 2A). As shown on Figure 9-19, slide-augmented waves may be as much as four times higher than the waves generated tectonically by the same earthquake. In other earthquakes, such as the 1994 Mindoro strike-slip earthquake, waves having maximum run ups of 23 feet appear to be generated entirely by near-shore submarine landslides (Figure 9-19).

For some tsunami genic earthquakes, maximum run ups are locally as much as 2 Yz times larger than would be expected for their magnitudes (Figure 9-19). These include the 1992 Nicaragua Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-37 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

and the 1994 Java events. For some of these earthquakes, some of the relatively high runups may be attributed to peculiarities of wave build-up due to interaction of the tsunami with the sea floor and shoreline topography. However, it is more probable that these generally higher runups resulted from a contribution to the wave train by unrecognized offshore landslides, rather than from a fundamental difference in earthquake mechanism, wave interactions, or bottom and shoreline configurations. The effect of near shore and off shore landslides is well documented for the 1964 Alaska earthquake and the 1998 Aitape earthquake, respectively (Appendix 9A).

The 1946 Aleutian earthquake is unique in that it generated both a very high near-source runup of 42.7 m (137 feet) (Appendix 9A), and a high far-field runup of more than 16m (52 feet) in Hawaii (Figure 9-19). Distribution and arrival times of the near-field tsunami strongly suggest a near-field source. Comparison of this tsunami to others shows that the near-source run up is more than six times higher than that expected for tectonically generated tsunamis from comparable-magnitude earthquakes (Figure 9-19). Origin of the far-field tsunami is unknown.

With regard to the Cascadia subduction zone, earthquake-triggered submarine landsliding on a very large scale might account for the waves generated by those events for which the tsunami is too large for the earthquake magnitude. As previously discussed (Section 9.4. 7) large submarine landslides have been mapped on the sea floor offshore of northern California. The anomalous high run ups reported at Orick possibly resulted from a tsunami that was enhanced by an offshore landslide. Recent detailed bathymetric mapping of the Cascadia continental margin (Goldfinger and Watts, 2001) has revealed several enormous landslide masses off shore of Oregon that have features interpreted as indicative of large and sudden movements of thousands of square miles of the lower continental slope. These appear to have occurred at infrequent intervals and inferred to be hundreds of thousands of years old by the thickness of the overlying sediment and the inferred sedimentation rates. The presence of these large offshore submarine landslides suggests a mechanism for generating anomalously large tsunamis at infrequent intervals. However, no geologic evidence for such tsunamis has been found in the late Holocene coastal stratigraphy in northwestern California or other places along the Cascadia coast.

For a magnitude 8.8 tsunamigenic earthquake on the Cascadia subduction zone (Section 5),

empirical worldwide tsunami data indicate such an earthquake would generate average Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-38 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

maximum runup heights along the northern California coast of 31 feet (mean sea level [35 feet MLL W]). The run up range for magnitude 8.5 to 9.2 is 28 to 3 7 feet [32 to 41 feet MLL W])

(Figure 9-19; Table 9-4). This generally agrees with the findings and estimate of30 to 40 feet for the wave height offshore of Humboldt Bay for paleotsunami studies in northern California.

9.5.2 Analytical Models of Potential Tsunami Inundation Six analytical studies of potential tsunami inundation have addressed the potential tsunami hazard to Humboldt Bay. These studies span the period since 1965, and use a variety of approaches to assess potential tsunami effects on the Humboldt Bay coast, inside the bay, and in the vicinity of the Humboldt Bay Power Plant. In each study, the run up height at the coast at Humboldt Bay has been estimated, and in several cases, the runup height at the power plant has been assessed. We summarize each study's approach below and use the estimates as they pertain to the evaluation of the ISFSI site As discussed in Section 9.2, the reference level of mean lower low water is used to facilitate comparison of the results of each study to the topographic setting of the ISFSI site (Figure 9-20).

The elevation of mean lower low water is set at 0, which is 3.7 feet below mean sea level, and the tidal range between mean lower low water and mean higher high water is 6.9 feet. The highest reported tide in the ISFSI site vicinity since 1920 was 12.5 feet (MLLW) (PG&E, 1985b ). The yard elevation of the Humboldt Bay Power Plant is 12 feet (MLL W), the reference level for all surveys at the Humboldt Bay Power Plant (elevation 8.3 feet above mean sea level).

As sketched on Figure 9-20, the proposed ISFSI site is at elevation 40 feet, and the Buhne Point hill varies in height along the bluff facing Humboldt Bay from 75 feet on the northwest to 24 feet on the southeast (MLL W).

Wiegel (in PG&E, 1966) - Shortly after the occurrence of tsunami inundation at Crescent City and elsewhere along the northern California Coast due to the 1964 Alaska earthquake, PG&E was asked by the U.S. Atomic Energy Commission to assess the protection of the Humboldt Bay nuclear power plant against tsunamis. PG&E retained civil engineering Professor R. L. Wiegel of the University of California at Berkeley, a widely recognized expert on tsunamis and their engineering impact, to evaluate the likelihood of tsunami flooding at the power plant.

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Wiegel (in PG&E, 1966) reviewed the data on historic tsunami waves in Humboldt Bay, noting that the largest were associated with the March 1964 Alaskan earthquake, which had a maximum run up height of 4.4 feet above the tide level (9.6 feet above MLL W) at the power plant intake (Table 9-1). To augment the observations at Humboldt Bay, Wiegel used frequency distribution functions of observations at localities that had more data: Crescent City harbor and The Presidio in San Francisco, California; Hilo, Hawaii; and a compilation of Japanese runup data. He assumed a Poisson distribution of run up height occurrences, anchored by the maximum run up of 4.4 feet above the tide stage reported at the Humboldt Bay Power Plant intake in 1964, and extrapolated using the general shape of the tsunami run up heights versus frequency for the other localiti~s. Table 9-6 illustrates Wiegel's calculated probability levels and associated run up heights above mean lower low water level.

In addition to distant tsunamis, Wiegel also considered the probability of locally generated tsunamis. He extrapolated the frequency of occurrence of offshore earthquakes north of the Mendocino escarpment based on historic seismicity. For a magnitude 8 earthquake having an approximate recurrence of 800 years, he estimated a tsunami having a run up of about 25 feet on the open coast, and about one-half this value at Buhne Point. He concluded, "Based upon present evidence, there appears to be little likelihood of the generation of a large tsunami in a region near Humboldt Bay." At the time of his analysis, in late 1964, the existence of the Cascadia subduction zone as a potential local tsunami source was yet to be recognized.

PG&E (1985b)- In June 1985, PG&E prepared a Memorandum Report to respond to a Nuclear Regulatory Commission question on flood hydrology pertaining to the decommissioning of Unit 3 at the Humboldt Bay Power Plant. Potential tsunamis were calculated in two ways.

The evaluation of tsunami flooding levels performed by Wiegel (in PG&E, 1966) was augmented using a report prepared by Brandsma and others (1979) for the NRC, in which offshore wave heights and time histories are presented for coastal segments of the United States due to distantly generated tsunamis. Using a Corps of Engineers procedure (Camfield, 1980) and Brandsma and others' maximum tsunami wave of +/-5.2 feet at a point offshore in water of moderate depth (600 feet), PG&E (1985b) computed the wave runup at the mouth of Humboldt Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-40 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Bay to be 16.1 feet above mean lower low water. This runup height would decrease as the wave propagated through the bay to the power plant site, although no quantitative analysis of the attenuation was done.

In the second approach, PG&E (1985b) used information from a study by Houston and Garcia (1980) that predicted tsunamis for the west coast of the U.S. for flood insurance purposes.

Houston and Garcia's (1980) 100-year tsunami runup at the entrance to Humboldt Bay was estimated to be 10.6 feet above mean lower low water, and the 500-year tsunami runup was estimated to be 20.7 feet above mean lower low water. Similar to the above procedure, no specific analysis was performed to predict water levels at the power plant site itself.

Whitmore (1993)- In the numerical analysis by Whitmore (1993), Cascadia subduction zone source parameters were used to compute inundation wave amplitudes along the coast of Washington, Oregon, northern California, and adjacent areas to the north and south. The largest event analyzed was magnitude 8.8 that ruptured from central Washington to between Eureka and Crescent City. The fault rupture was 400 miles long, dipped 13 degrees, and the maximum sea-floor uplift was 12 feet. At points along the coast opposite the modeled earthquake, the maximum computed tsunami amplitude was 19 feet, with an average maximum amplitude of about 15 feet. Maximum amplitudes were computed at three locations within Humboldt Bay (Eureka: 1.7 feet, Fields Landing: 0.66 feet, and Bucksport, between Eureka and Fields Landing: 2.8 feet). The maximum amplitude of 8.7 feet was calculated on the ocean side of the North Spit, just to the south of the end of the modeled fault rupture. The Bucksport location is considered to be the most similar to the Humboldt Bay ISFSI site. Although technically the wave half-amplitudes are the predicted height of the potential run up, we consider the full amplitude to be closer to actual run up elevation (MLL W) because of the wide grid spacing used in the model and to account for asymmetry of the tsunami waves in this report.

National Oceanic and Atmospheric Administration (Bernard and others, 1994)- Following the occurrence of the April25, 1992, Cape Mendocino shallow thrust earthquake, the National Oceanic and Atmospheric Administration evaluated potential tsunami inundation along the northern California coast associated with possible Cascadia subduction zone events. The results Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-41 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

of the study were intended to be used for emergency planning purposes and, as such, are generalized.

The planned approach for the study (Bernard and others, 1994), included application of seismic source models for the Cascadia subduction zone to predict the generation of significant tsunami waves impinging on Humboldt Bay and Crescent City, followed by numerical modeling of inundation in these two areas of interest. The initial results of the seismic source modeling indicated the Cascadia subduction zone produced tsunami wave amplitudes that were judged to be unreasonably small. Therefore, Bernard and others (1994) evaluated the complexities of recent tsunamis generated by earthquakes in Nicaragua (1992), Indonesia (1992), and Japan (1993), and used an empirical approach to estimate the incident wave amplitudes at Humboldt Bay. Using tsunami observations associated with the 1964 Alaska and 1993 Hokkaido earthquakes, they judgmentally derived a 1 0-meter (33-foot) incident wave at a 50-meter (164-foot) water depth to be used in inundation models.

The inundation modeling for Humboldt Bay is described in Appendix H of Bernard and others (1994) in terms of the computer modeling input, procedures, and output, and is accompanied by a small-scale map of the inundation area and the 1 00-meter grid used for the modeling. A 1 :24,000-scale map of the Humboldt Bay region that shows the inundation boundary also is provided. Bernard and others state (1994, Appendix H, page 67), "The inundation levels inside the harbor reached 3 meters at some locations... " (10 feet referenced to mean sea level).

Because the small-scale map and 1 :24:000-scale map are somewhat disparate, we conservatively consider 10 feet as the Bernard and others (1994) run up estimate for the ISFSI site vicinity.

Bernard and others, 1994 also state (page 67), "All the Humboldt Spit was flooded." The South Spit has a maximum elevation of about 23 feet above mean lower low water. The southern end of the North Spit is similar to the South Spit in elevation, but the central part and northern end of the North Spit ranges from 56 to a maximum of about 73 feet in elevation above mean lower low water. For an input wave of 33 feet in the near offshore, their statement in Appendix H seems problematical regarding the higher portions of the North Spit.

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Lamberson and others (1998)- Roland Lamberson, Professor at Humboldt State University, has developed, along with his students, a numerical tidal model calibrated for Humboldt Bay.

During 1997, they performed a pilot study (Lamberson and others, 1998) to assess the feasibility of using their current finite-difference tidal model to simulate tsunami wave amplitudes and water velocities inside Humboldt Bay. They tested their model at low tide (0 set at mean lower low water), using an arbitrary input set of three large (4 to 6 meter amplitude) waves at the mouth of Humboldt Bay, having a period of 15 minutes. At the entrance to Humboldt Bay the third wave had the maximum wave height of 8 meters (26 feet MLL W). A wave overtopping the spits was not included in their model, although the input wave clearly would have washed over the South Spit and the southern portion of the North Spit.

In their model, the maximum flooding at the ISFSI site occurred during the second wave, and had an elevation of 5 meters (16.4 feet) above mean lower low water. Current velocities at the ISFSI site were a maximum of2 meters (6.6 feet) per second. Lamberson and others (1998) concluded their model performed well.

Myers and others (1999)- Edward Myers, a Ph.D. student, and a team of researchers from the Oregon Graduate Institute developed a finite element model for propagation of Cascadia subduction zone tsunami waves from their source near the plate interface off the coast of the Pacific northwest, to the coast. To generate the tsunamis, they used various rupture models for the Cascadia subduction zone as presented in Priest and others (2000). These models assume a geometry of the plate interface and vary the rupture dimensions by adjusting the locations and amounts of slip on the seaward and landward transition zones around a central locked zone.

They estimated regions and amounts of seafloor uplift corresponding with each of these rupture scenarios, assumed the sea floor uplift was directly transferred to the sea surface as the initial conditions for their model. They then propagated the tsunami wave trains through their finite element grid toward the coast, and reported the estimated wave heights and run-up velocities associated with each of the scenarios.

In their study, the authors reported their results for a number of locations along the coast from Cape Mendocino to the northern Olympic Peninsula. These results depend on a relatively coarse finite element grid, and are most useful to estimate tsunami-focusing mechanisms offshore, but are considered approximate for estimation of run up at the coast (A. Baptista, personal Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-43 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

communication, 2002). The authors chose two sites _for detailed estimation of runup characteristics: Seaside and Newport, Oregon. The finite element grid was much denser than the regional grid at these two sites to permit detailed estimation of run up routes, flow velocities, and runup heights. The authors report that predicted wave heights and runup velocities are very sensitive to grid density, reinforcing the notion that estimates of run-up outside of Seaside and Newport should be considered approximate. Furthermore, Dr. Baptista (Personal communication, 2002) reports that runup velocities predicted by these models are much less accurate than wave heights.

This model predicts wave heights at the coast at Humboldt Bay between 17 and 30 feet (MLL W) and flow velocities between 3 and 13 ft/s, but they did not model runups within Humboldt Bay.

At Klamath, near Lagoon Creek, they predict wave heights between 17 and 46.5 feet (MLL W) and flow velocities between 6.5 and 15 ft/s, but preferably around 10 ft/s.

Discussion-Table 9-7 summarizes the results of the various studies. For each study, the runup height of the wave at the mouth of Humboldt Bay is listed; in cases where an offshore wave in shallow water was specified, the runup was taken to be equivalent to the offshore wave height (Figure 9-2). We show the estimated runup heights at the Humboldt Bay ISFSI site at mean lower low water and at mean higher high water (Table 9-7; Figure 9-20). The latter value was obtained by adding the tide differential (6.9 feet) to the tsunami runup height.

The Wiegel (in PG&E, 1966) and PG&E (1985b) studies were based on distant tsunamis only, and were performed prior to the knowledge that the Cascadia subduction zone could produce a very large earthquake, and that such earthquakes could produce significant tsunamis. Even so, their maximum tsunami inundation estimates greatly exceed those of the 1964 earthquake (Table 9-1), which is considered by Lander and others (1993) to be the largest potential distant tsunami on the northern California coast. The geologic record at Crescent City (Carver and others, 1998) shows no evidence of significantly larger distant tsunamis during the past several thousand years, unless the circa 850 years BP event (Crescent City "W") is an unusually large distant event. Our preferred interpretation is that it is local, possibly related to the event that caused subsidence of the Eel River valley. The conservatism used in the 1966 and 1985 studies resulted in maximum values of 21 feet (MLL W) and 27 feet (MHHW) (Houston and Garcia, Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-44 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

1980) that are somewhat lower than to those derived from consideration of local tsunamis caused by Cascadia subduction zone earthquakes, but considering the knowledge at the time are considered remarkable.

The computer model studies have progressed markedly in the past 20 years, but the results should be taken as a guide to how tsunamis may impact a coast and spread inland. Most use sinusoidal waves for the analysis, but clearly this is not what occurs (discussed in 9.2).

Importantly the models need to have a fine grid that adequately characterizes the subocean and shore topography (Myers and others, 1999; A. Baptista, Personal communication, 2002).

Nonetheless, the modeling results to date provide useful information and insights that help limit the estimates of run up at the coast off Humboldt Bay and in the ISFSI site area.

In analyzing a large Cascadia earthquake, the input tsunami wave height of 10 meters (MSL) at an offshore water depth of 50 meters (167 feet), as selected judgmentally in the Bernard and others (1994) study, is comparable to the 30- to 40-foot (above MLLW) paleotsunami runup wave height we have estimated at the Humboldt Bay North and South spits. These values are significantly greater than the values computed by Whitmore (1993) for various locations along the coast. Even his maximum amplitude value of 6 meters (19 feet) appears to be unacceptably low, considering the evidence for tsunamis crossing the South Spit during the past several thousand years. Lamberson and others (1998) selected an arbitrary value of about 26 feet (8 meters) as model input, but they easily could have chosen a larger value. As mentioned above, the model from Myers and others (1999) produced maximum coastal wave height estimates (which were also labeled "Maximum Runup") at Humboldt Bay between 17 and 30 feet above MLLW, depending on the model for rupture of the Cascadia subduction zone. The authors emphasize that Humboldt Bay is at the periphery of their grid, and that these values are much less reliable than those from central Oregon, particularly Seaside and Newport.

Nonetheless, they describe these estimates as "reasonable" (A. Baptista, Personal communication, 2002).

Thus the lack of paleotsunami sand deposits in the vicinity of the plant and at other places around the bay may reflect the absence of a significant wave or a lack of a source of sand near the site. If we assume a source of sand is available, then the lack of tsunami sands near the Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-45 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

ISFSI site provides a possible height constraint for paleotsunamis at the site. The analysis to constrain water depth and velocity of the inundating flows that deposited paleo tsunami sands at Lagoon Creek (Table 9-5) (Abramson, 1998) provides insights to this issue. If paleotsunamis entered Humboldt Bay and induced inundating flows of similar depths on marshes and mud flats, we could reasonably expect them to deposit sand layers as well, provided there was a source of sand near the marsh. The elevation of the tidal marshes near the ISFSI site is 5 to 7 feet above mean lower low water; these marshes extended for about one-half mile north of the ISFSI site (Figure 4-4, Section 4) prior to the construction of jetties in about 1900 to stabilize the entrance to Humboldt Bay, and were 3 to 4 kilometers from the sand spits at the coast. Any tsunami entering the bay had to cross 500 meters of spit, traverse 1 kilometer of bay and 1 Yz to 2 kilometers of marsh to reach the ISFSI area. It is unlikely that sand from the spits would reach the ISFSI area because the tsunami sands from Lagoon Creek had settled out beyond about a kilometer inland from the beach berm. Any tsunami sand near the ISFSI would have to be from a local source, such as a beach along Buhne Point or tidal channels in the bay. Nonetheless we assume that a source of sand existed for the following calculation: when the marsh elevation ( 5 to 7 feet) is added to the minimum depth of flow expected to transport and deposit a sand layer, the value is less than 24 to more likely 34 feet above mean lower low water. Any runups higher than this are presumed to have been large enough to have deposited sand layers in the marshes near the ISFSI site. Thus, the evidence for no sand deposits near the ISFSI site suggests that the maximum estimated run up at the ISFSI site was less than 24 to 34 feet (MLL W), which is slightly higher than the estimated run up elevations at the coast based on the several models (Table 9-7).

9.6 ADDITIONAL FACTORS INFLUENCING THE TSUNAMI HAZARD AT THE ISFSI SITE Several additional factors need to be considered in order to understand the uncertainties in the estimates of the tsunami hazard at the Humboldt Bay ISFSI site.

9.6.1 Estimated Runup at the Open Coast at Humboldt Bay Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-46 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

The range of maximum estimates of run up height for the Cascadia paleotsunamis along the open coast of northern California at the five sites where paleotsunami information is available varies between 18 to 52 feet, or to 69 feet if the data for Orick is included, but is more likely between 26 to 33 feet above mean lower low water from the analysis of the events at Lagoon Creek marsh (Table 9-4).

For the "Y" event, which has the most data for a single event, run up estimates vary at different sites along the coast. At Lagoon Creek, run up estimates for this event range from 24 to 44 feet (MLLW). The maximum runup of 69 feet at Orick (based on Yurok oral histories) is presumed to describe the "Y" event, as it likely represents the most recent very large tsunami at that site.

At the North Spit, deposition of gravel and cobbles within the sand dunes constrains run up to be somewhat higher than 38 feet above MLLW (Section 9.4.6). The maximum open-ocean tsunami run up height at the mouth of Humboldt Bay from a local subduction-generated tsunami is constrained by the 53-to 72-foot elevation (i.e., less than 53 to 72 feet above MLLW) of the uneroded dunes on the North Spit, which have not been overtopped. As mentioned in Section 9.4.6, preferred estimates for tsunami wave height at the mouth of Humboldt Bay based on evidence from the North and South Spits are between 30 and 40 feet above mean lower low water.

This variability in run up heights is well within the observed range of variability in run up heights observed at different locations along coasts adjacent to historical great subduction earthquakes, including Chile in 1960, and Alaska in 1964 (Figure 9-19, Appendix 9A)

Berm Erosion - The runup height estimates at the Lagoon Creek beach berm and the South Spit are based on the assumption that the sand berms used as the elevation baselines for the calculations were not eroded by the initial rise in water level, and persisted as high barriers during the successive tsunami pulses. However, observations of similar sites in Chile show barriers composed of sand erode rapidly, and do not persist during the tsunami. Because tsunamis are dispersive wave trains and do not completely drain before the next wave arrives, and the highest inland runups usually occur after the initial wave in the wave train, the erosion of the barrier by initial wave pulses leads to more rapid inundation and more extensive run ups inland by later waves. However, the presence of a relatively continuous stratigraphic sequence Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-47 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

spanning at least the past 3000 years and the lack of significant unconformities in the marsh sediments behind the berm at Lagoon Creek indicate that if the berm was eroded by past tsunamis its elevation was not reduced below that of the marsh impounded behind it.

Velocity-The finite-element modeling of a Cascadia tsunami by Meyer and others (1999; Section 9.5.2) indicates the run up height above tide level ranges from about 3 to 9 meters (1 0 to 30 feet) at Crescent City, and about 3 to 12 meters (1 0 to 39 feet) at Klamath. If maximum high tide coincides with maximum run up at these sites predicted run up heights would be 17 to 3 7 feet at Crescent City and 17 to 46 feet at Lagoon Creek. Modeled wave velocities for the tsunamis at these locations are about 3 meters per second (1 0 ft/sec ).

The sediment transport model that relates particle deposition to water velocity used by Abramson (1998) and Carver and others (1998) at Lagoon Creek to estimate the maximum water depth of past tsunamis (Section 9.4.6) was calculated using a range of 2 to 5 meters per second

( 6.5 to 16.5 feet per second). The minimum inundation velocity was estimated from the probable time and distance of tsunami run up. In this case, assuming a tsunami wave period of one hour, the time during which flows are actually flooding the marsh would be somewhat less than 15 minutes (one quarter of the period, minus the time of sea level rise required to overtop the beach berm). The minimum distances the waves traveled inland for events Y, W, U, and S ranged between 1260 and 1330 meters (approximately 4,130 and 4,360 feet) inland based on the presence of sand layers and marine diatoms in cores (Carver and others, 1998). Dividing this minimum distance by the estimated time of flooding yields approximate rates of 1.4 -1.5 meters per second ( 4.5-5 feet per second). Considering that a 1-hour wave period is conservatively long (Section 9.2), the time of inundation is somewhat less than one-quarter of the period, and the runup distances documented in cores are minimum distances, we estimate a reasonable minimum run up flow velocity of 2 meters per second ( 6.5 feet per second) at Lagoon Creek (Abramson 1998; Carver and others, 1998). The maximum runup velocity of 5 meters per second (16.5 feet per second) is averaged over the distance of particle transport, and is based on comparison to velocities measured in large rivers and tidal bores worldwide (Abramson, 1998; Carver and others, 1998). For example the tidal bore at the Amazon River is 16 feet (5 meters) high, and attains speed of20 feet/second (6.2 m/s) (Easterbrook, 1993).

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-48 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Because the velocity value of 6.5 feet per second is believed to be low and because runup height calculated from the particle size model is inversely proportional to the velocity, the maximum runup heights are somewhat overestimated at this low velocity. Table 9-5 shows the water depths (consistent with the particle-size distributions observed in the Lagoon Creek marsh) and associated runup heights at Lagoon Creek derived using three velocities: 6.5 feet per second (minimum), 16.5 feet per second (maximum), and 10 feet per second (the velocities believed to be reasonable as predicted by Myers and others, 1999). The preferred values for runup heights at Lagoon Creek are 26 to 33 feet above mean lower low water, based on a flow velocity of 10 feet per second (3 meters per second) as the most reasonable estimate of run up heights.

Flow depths reported in Table 9-5 refer to the depth of flow over the marsh surface. The marsh elevation for each event is taken to be the elevation of the sand layer on the marsh surface in the stratigraphy at core location 4 (Figure 9-11 ), which is about 2000 feet inland, the distance inland where the course fraction sand-size begins to decrease. All runup estimates reported in Table 9-5 are relative to modem MLL W.

We assume that the elevation of the Lagoon Creek site remains tectonically stable and does not change in the analysis of tsunami events, i.e. no net uplift or subsidence.

Tide Stage - The tsunami wave height required to produce the water depths and runup heights observed for paleo tsunamis depends on the stage of the tide at the time of the tsunamis. Because the tide stage is unknown for all but the most recent (event "Y") of the paleo tsunamis along the northern California coast, only a range and limiting bounds can be estimated. (However, there is some evidence that large earthquakes preferentially occur at low tides (Platker, written communication November 17, 2002). At low tide, the wave amplitude must increase to generate the run up heights estimated at the site, and conversely, a higher tide stage requires a lower wave height.

Because a high or low tide lasts only a few hours each day, the probability of any of the largest wave pulses of paleo tsunamis arriving during high tide levels is small (about 1 in 6 for either a high or low water stage), and the probability that the maximum height wave pulses from all of the six Cascadia tsunamis in the stratigraphic record of northern California occurred during a high tide is very small (about 1 in 1,296). The normal semi-diurnal tide range along this part of Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-49 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

the coast is about 6.9 feet. The most likely tide stage would be near mean sea level, about 3 feet.

An example of the influence of tides on tsunami run up is on Kodiak Island in 1964 (Kachadoorian and Plafker, 1967 and Plafker and Kachadoorian, 1966). There the highest wave heights were generally associated with one of the first three waves in the tsunami wave train, but, because these waves arrived at relatively low tide stages, they did not result in the highest runup heights on the coast. Later, smaller waves arriving during high tide reached higher on the coast at some locations. At Crescent City the 1964 tsunami coincided with a high spring tide (Wilson and Torum, 1968) and hence the runup was near maximum there. These examples illustrate that the tidal factor is important, both in interpreting the paleotsunami wave heights, and estimating future runups.

Open-Coast Runup Estimate-We estimate future runup on the open coast at the mouth of Humboldt Bay due to a local Cascadia tsunami will be about 30 to 40 feet above mean lower low water. This estimate is based on six separate analyses of potential runup height values summarized in Table 9-4:

1. Geologic and stratigraphic evidence from marshes along the northern California coast,

2. Topographic constraints and geologic evidence of paleotsunamis at the North and South

spits,

3. Calculations of the water depth at Lagoon Creek, using the intermediate flow velocity of 1 0 feet per second.

4. Empirically predicted runup height from worldwide data of tsunami runup heights from subduction-generated tsunamis, particularly continental margin areas, such as those in Alaska, Chile, Peru and Colombia (Figure 9-19)

5. Oral history accounts of the 1700 AD earthquake, and

6. The results of the tsunami modeling of the northwest coast and Humboldt Bay 9.6.2 Run up Heights at the ISFSI Site Although the estimated tsunami runups at the North and South spits and the mouth of Humboldt Bay are 30 to 40 feet (above MLLW), the runup after entering the bay will be significantly lower on the eastern shore of the bay than on the open coast. Two factors need to be considered to understand the uncertainties in the estimates of the tsunami hazard at the Humboldt Bay ISFSI site: the change in bathymetry and shoreline since 1850, and tectonic uplift and coseismic Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-50 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

downwarping that accompanies an earthquake on the Cascadia Subduction zone and the Little Salmon fault.

The shoreline and channel at the entrance to Humboldt Bay have changed dramatically since the early 1800's. The earliest maps show that the North and South spits partially blocked Humboldt Bay prior to about 1860 (Figures 9-21 and 9-22), so the effects of the excavated Humboldt Bay shipping channel and post-1860 erosion in the Buhne Point area on future tsunami run up in the bay and at the ISFSI site is uncertain. The bay entrance and shoreline at Buhne Point has changed in several ways the since the last subduction event:

1. Modification of the entrance of the bay,

2. Changes in the depth of the bay between the mouth and the plant site, and

3. Regression of the shoreline at Buhne Point.

When the first map was made in the early 1800's by the Russian explorers (Figure 9-22) and later in the 1850's (Figure 9-21) the Bay entrance was very narrow and shallow with the channel confined between two overlapping spits, all factors that would dampen the effect of a tsunami reaching the ISFSI site area. With the artificially wide and deep open entrance to today's bay, waves are much more likely to enter the bay through the mouth and retain much more energy than before the mouth was modified. The deep dredged ship channels in the bay between the mouth and Buhne Point have replaced the much shallower, muddy-bottomed reach between the mouth and the site, allowing a wave to move from the mouth to the site with less attenuation.

Buhne Point hill, however, would continue to protect the ISFSI site from the direct impact of the tsunami runup front (Figure 9-20), and tend to reduce runup heights at the ISFSI site because it is on the lee side of Buhne Point.

Tectonic uplift and subsidence near the ISFSI site is probable during major Cascadia subduction zone earthquakes that are accompanied by large displacements on the Little Salmon fault system (Section 2.0). The paleoseismic record in the Humboldt Bay area shows subduction zone events are associated with coseismic subsidence of synclines at Arcata Bay on the footwall of the Mad River fault zone and South Bay on the footwall of the Little Salmon and Table Bluff faults.

Between these synclines is an area, including Humboldt Hill, Buhne Point and the part ofNorth Spit opposite Eureka, that is being uplifted on the hanging wall of the Little Salmon fault. Bay Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-51 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

margin sediments at locations above the hanging wall of the Little Salmon fault, including the Eureka anticline, which is considered a hanging wall structure, have no evidence (salt marsh peat abruptly overlain by intertidal mud) of coseismic subsidence. In contrast, coseismic subsidence stratigraphy is found in the cores of the large synclines at Arcata Bay-Mad River Slough and the South Bay syncline (South Bay and Hookton Slough sites) (Valentine and others, 1992;Valentine, 1992; Carver and others, 1998; Patton and others, 2002).

Because the ISFSI site is located on the upthrown block of the Little Salmon fault system, it appears reasonable to assume that future large local tsunamis impinging on Humboldt Bay would be influenced by displacement on the Little Salmon fault. The investigations in Humboldt Bay at the Swiss Hall (Witter and others, 2002) and Hookton Slough (Patton and others, 2002) sites, which are in the marshes bordering South Bay, and at other locations in the south Bay (Valentine, 1992) indicates a small amount of coseismic emergence on the hanging wall of the fault and a larger amount of subsidence of the footwall associated with past slip events. The amount of coseismic subsidence is reflected by the abrupt changes in sedimentary and paleontology indicators of intertidal zonation (Peterson and others, 1997, Valentine, 1992) in the bay margin sediments. The net tectonic elevation change over seismic cycles is approximated by the thickness of intertidal mud overlying marsh peat and soils (Valentine, 1992, Patton and others, 2002) and ranges up to 6 feet of subsidence (this ignores eustatic rise in sea level, which may approach 2 mm/yr according to Douglas, 1991). The long-term record of cumulative displacement on the Little Salmon fault shows the same relationship: predominate subsidence of the footwall and lesser emergence of the hanging wall. Thus, the down side of the fault is underlain by thousands of feet of relatively young (Quaternary) sediments and a thick Neogene Wildcat Group section is preserved (if the Table Bluff fault is included, about 12,000 feet of Neogene section in the Eel River Valley lies below sea level), while on the upthrown side (hanging wall) only a small part of the Wildcat section is preserved, suggesting many thousands of feet of net uplift. During a tsunami genic subduction earthquake involving the Little Salmon fault the vertical land level changes in the vicinity of the ISFSI site would have two effects.

Uplift of the hanging wall would raise the ISFSI site slightly, possibly a foot or two, and tend to reduce the level of tsunami flooding in the aftermath of large tsunami genic earthquakes near the coast. This may help explain the lack of observed paleotsunami sand deposits in the area of the Humboldt Bay Power Plant. However, subsidence of up to 6 feet would tend to increase the Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-52 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

runup in Humboldt Bay because South Spit would be coseismically lowered and increase the tsunami flooding in South Bay.

Attenuation of Run up Heights in Humboldt Bay - The values for the attenuation factor for tsunami wave height in Humboldt Bay can be approached in several ways. Plafker and Kachadoorian (1966) report that on Kodiak Island, "The highest recorded run ups were along exposed beaches and bluffs, whereas the runup heights in adjacent sheltered embayments and segments of the coast protected by offshore reefs were substantially lower." Comparisons of run up heights on open coasts and nearby bays along the northeast shore of Kodiak Island show that the 1964 maximum run up heights at Cape Chiniak and Narrow Cape were about 1 0 meters (33.3 feet) above tide level. At Cape Chiniak, trees along the shoreline were destroyed up to an elevation of 31.5 feet above the highest tide level on the night of the tsunami (Plafker and Kachadoorian, 1966). The highest runup heights for these same waves were 18.4 feet in Kalsen Bay, and 12.8 feet at the naval station at Woman's Bay. Kalsen Bay is large, deep, and open to the ocean, whereas the naval station at Woman's Bay is more protected, and the bay has a narrow and relatively shallow entrance. Both bays are significantly less sheltered than Humboldt Bay. The attenuation of the tsunami run up heights at these two bays relative to the open coast in 1964 was 0.6 and 0.4, respectively.

Although runup is expected to be lower in the ISFSI site area than on the open coast, the amount is uncertain. There is no measurement of the runup at the open coast at the mouth of Humboldt Bay during the 1964 Alaska tsunami, the tsunami produced a maximum wave height of 21 to 25 feet at Crescent City and of 12.6 +/-0.5 feet on the coast at Trinidad, about 30 miles north of the bay entrance (Table 9-1 ). At the ISFSI site, the wave height was 3.8 feet. If the open-ocean value at Trinidad is representative of the run up at the mouth of the bay, the attenuation factor was about 0.3, but the sand spits were not overtopped, so this attenuation is certainly low.

Previous tsunami inundation studies for modeled tsunami wave heights in Humboldt Bay also have estimated attenuation amounts. Those range from 30% to 63 %(Table 9-7). The analysis of Lagoon Creek data suggests that if inundating flows reached 24 to 34 feet (above MLL W) at the marsh around the ISFSI, they would deposit sand, at the site, assuming that a nearby source of sand from within the bay was available to supply sediment. Because there is no sand or other Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-53 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

evidence that tsunami runup has ever reached the site, past runups in the late Holocene are assumed to be less than that elevation.

Based on the above range of the attenuation factors, and because of the proximity of the ISFSI site area to the mouth of the Humboldt Bay channel, we estimate that for the analysis of the.

ISFSI site, an attenuation value between 0.7 to 0.9 is appropriate to characterize the expected reduction in run up height from the seaward side of the bay entrance to the site. Applying this to the estimated maximum of 30 to 40 feet (MLL W) at the coast, the estimated run up at the ISFSI site is between 21 and 36 feet (MLLW) and 23 and 38 (MHHW). As a check, back calculating the run up height using the estimated attenuation of 0. 7 to 0.9 applied to the maximum estimate of24 to 34 feet above the marsh near the ISFSI (from the Lagoon Creek analysis), the runup at the open coast at Humboldt Bay would likely have to be 27 to 38 and 34 to 49 feet (MLLW),

respectively, to carry sand into the ISFSI site area (Table 9-7).

9.7

SUMMARY

OF TSUNAMI HAZARD AT THE HUMBOLDT BAY ISFSI SITE The tsunami hazard at the Humboldt Bay ISFSI site is dominated by a local tsunami generated by a magnitude ~9 earthquake on the Cascadia subduction zone, resulting from rupture of the zone from the Humboldt Bay area north to Canada. This tsunami is expected to flow strongly through the mouth of Humboldt Bay, as well as wash over the South Spit and the southern part of the North Spit.

The runup height from a local Cascadia-generated tsunami on the open coast at the mouth of Humboldt Bay is estimated to be as much as 30 to 40 feet above mean lower low water at the bay entrance. This estimate considers evidence ofpaleotsunamis at the North Spit, and assumes overtopping and erosion of the sand barriers and marsh at the South Spit. It compares well with the predicted runup height estimates from historical tsunamis in continental margin settings in Alaska, Chile, Peru, and Colombia, as well as runup estimates for paleotsunamis at Lagoon Creek and Crescent City.

The vault for the placement of the dry casks at the ISFSI facility is near the top ofBuhne Point hill (Figures 1-1 and 9-20). The top of the vault is at an elevation of 44 feet above mean lower low water. This elevation is higher than the tsunami height estimates at mean Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-54 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

lower low water considered in this study (Table 9-7), which include bounding estimates for distant tsunamis, modeling of locally generated tsunamis associated with the Cascadia subduction zone, and tsunami heights from geologic evidence of no sand-transporting tsunamis inundating the region around the ISFSI site.

Using our estimate of 30 to 40 feet above mean lower low water for the runup height of the tsunami at the bay entrance, and an attenuation factor of0.7 to 0.9, the inundation height would be 21 to 36 feet above mean lower low water if the tsunami occurred at low tide, or 28 to 43 feet above mean lower low water if the tsunami occurred at high tide at the ISFSI site area. The offshore bathymetry at Humboldt Bay is smooth and wide, and topographic enhancement of tsunamis is not expected at the ISFSI site.

Even if a tsunami runup flowed above the ISFSI elevation, the tsunami hazard at the proposed ISFSI site is negligible, because the casks can be temporarily wetted without harm and they will be contained in underground vaults which protect them from damage by flowing water and damage from water-born debris.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-55 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Table 9-1 OBSERVATIONS OF RUNUP ELEVATIONS AT HUMBOLDT BAY AND OTHER LOCATIONS IN NORTHERN CALIFORNIA, FROM THE 27-28 MARCH 1964 ALASKA EARTHQUAKE (PG&E, 1966)

Pacific Gas and Electric Company Humboldt Bay ISFSI Maximum Run up Elevation above MLLW Location (feet)

U.S. Coast Guard Station, Humboldt Bay, North Spit 9

Municipal Marina, Eureka, Humboldt Bay 10.8 +/-2 King Salmon (entrance to King Salmon Slough),

10.4 +/-2 Humboldt Bay Humboldt Bay Power 9.65 Plant Intake (0.6 mile upstream on King Salmon Slough), Humboldt Bay 9.6 Pier at Trinidad 17.5 +/-2 Crescent City 20 to 21 Pebble Beach, about 2 miles north of Crescent about 15 City Harbor Ship Ashore Restaurant, just inside entrance of 12+/-2 Smith River Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 Tide Level at Time of Maximum Runup, Elevation above MLLW (feet) probably 5.9 probably between 5.7 and 6.1 probably 5.9 5.9 5.2 probably between 4.2 and 5.3 5.1 about 5 probably about 5 9-56 Elevation of Maximum Runup above Tide Level (feet)

Remarks probably 3.1 U.S. Army Engineer District, San Francisco probably U.S. Army between Engineer District, 4.7 and 5.1 +/-2 San Francisco probably U.S. Army Engineer District, 4.5 +/-2 San Francisco Note from G. E.

Altman, PG&E:

3.8 Time of maximum runup, 5:00AM, 28 March 1964 Note from G. E.

4.4 Altman, PG&E:

Time, 1 :30 PM, 28 March 1964 probably U.S. Army between 12.2 Engineer District, and 13.3 +/-2 14.9 to 15.9 about 10 about 7 +/-2 San Francisco U.S. Army Engineer District, San Francisco U.S. Army Engineer District, San Francisco U.S. Army Engineer District, San Francisco Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

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Sand layer overlying freshwater deposits 5

Thins inland 6..

Fines inland 7

Rip-up clasts 8..

Sharp basal contacts 9..

Erosive basal contacts 10..

Sharp upper contacts 11..

Well sorted 12..

Similar grain-size distribution as beach 13 Similar lithology as beach..

14 Landward transport of landslide debris 15 Woody debris mixed in 16 Trash layer on top: wood,..

peat, mud, sand 17 Normally graded 18..

Multiple normally graded..

pulses 19 Pulses separated by silt partings 20 Flopovers between pulses.

21 Allochthonous marine diatoms 22 Broken, but well preserved diatoms 23 "Beach" diatoms 24..

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01

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Table 9-3 CASCADIA SUBDUCTION ZONE EVENTS Pacific Gas and Electric Company Humboldt Bay ISFSI Event Years Before Present (1950)

(calibrated 2-sigma values)

"Y" 250 (January 26, 1700, 9:00PM)

"W"

~1,100 "U"

~1,300 "S"

~1,600 "N"

~2,500 "L"

~2,900 (from Atwater and Hemphill-Haley, 1997)

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-59 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Table 9-4 OPEN COAST RUNUP ESTIMATES FROM P ALEOSEISMIC SITES ALONG THE NORTHERN CALIFORNIA COAST AND WORLD WIDE DATA Pacific Gas and Electric Company Humboldt Bay ISFSI Site Estimated Runup Height at Coast (feet above MLLW)

Crescent City Higher than 28 to 31 feet I

Lagoon Creek 18 to 52 feet Most likely 26 to 33 feet Orick 66 to 69 feet North Spit A) Somewhat higher than 38 feet; less than 50 feet estimate 3 5 to 40 feet B) Less than 53 to 72 feet South Spit A) Higher than 18 to 23 feet B) Less than 20-40 feet World Wide Data (Appendix 9A)

A) 35 feet B) 30 to 40 feet Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-60 Comments Comparison of paleotsunami deposits with deposits in Kodiak Island from the 1964 Alaska earthquake.

Table 9-6 -values calculated using particle size and settlement velocities for "Y", "W",

"U", and "S" events. Most likely runup is judged to be 26 to 33 feet.

From Native American oral history; high elevation may be anomalous local runup.

A) Pebble layer at elevation of 38 ft. in dunes provides minimum estimate. Adding 12 feet depth to transport pebbles gives possible 50 feet B) Height of dunes provides maximum estimate.

A) Height of dunes B) Height of marsh plus estimated depth of 13 to 33ft. based on comparison with event "Y" at Lagoon Creek.

Figure 9-19 Empirical relationship for a tectonic runup vs. magnitude. Runup for MLL W adds 3. 7 feet to MSL.

A) For a 8.8 Cascadia event runup is 31 feet MSL B) For magnitude range 8.5 to 9.2 runup is 26 to 3 6 feet MSL Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Table 9-5 ESTIMATED RUNUP HEIGHTS AT LAGOON CREEK FROM THE SEDIMENT TRANSPORT MODEL (modified from Abramson, 1998)

Maximum Event particle size (mm)

"Y" 2

"W" 3

"U" 2

"S" 10 Notes:

Pacific Gas and Electric Company Humboldt Bay ISFSI Most likely Most likely depth of water velocity above marsh surface at time (ft/sec) of deposit

[m/sec]

(ft)

[m]

6.5-16.3 13-32.5

[2-5]

[4-10]

6.5-16.3 14.5-36

[2-5]

[4.5-11]

6.5-16.3 11.5-29.5

[2-5]

[3.5 -9]

6.5-16.3 16.5-45.5

[2-5]

[5-14]

Most likely depth of water referenced to MLLW (ft) 20-39.5 21.5-42.5 18.5-36.5 23.5-52.5 Measured from the stratigraphy at Core 4 that is 2000 feet inland from the beach berm.

Reasonable velocities based on empirical data on natural flows in rivers and tidal bores Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-61 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Table 9-6 WIEGEL'S (in PG&E, 1966) ESTIMATES OF TSUNAMI RUNUPS AND THEIR PROBABILITY AT HUMBOLDT BAY POWER PLANT Pacific Gas and Electric Company Humboldt Bay ISFSI Maximum Tsunami Runup (feet above MLL W) 19 13 11 6.5 Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-62 Probability Level 10% in 1 000 years 1 0% in 100 years 10% in 50 years 10% in 10 years Section 9.0 Tsunami Potential Rev. 0, December 27,2002

Table 9-7 ESTIMATES OF MAXIMUM RUNUP ELEVATIONS AT THE ISFSI SITE Pacific Gas and Electric Company Humboldt Bay ISFSI Estimated Maximum Estimated Maximum Runup Runup at at Humboldt Bay ISFSI Site (feet)

Researcher Basis Bay Entrance and Attenuation (%)

(feet above MLLW)

MLLW Calculated Runups - Distant Tsunamis Wiegel (in PG&E, Calculated -

19662) 1 0% in 1000 years 19 PG&E (1985b2) using Brandsma & others Calculated 16.1

<16.1** 3

( 1979) procedure PG&E (1985b"), using Calculated -

Houston & Garcia 1 00-year tsunami 10.6

<10.63 (1980) procedure 500-y_ear tsunami 20.7

<20.73 For comparison:

Observations for~ 150 maximum historical

~15 9.6 distant tsunami years Calculated Runups -Local Tsunamis Wiegel (in PG&E, Judgment 25 12.5 19662) 50%

Whitmore (1993)

Cascadia M 8.8; modeled 8.7 2.8 wave amplitude that we consider equal to runup 32%

Judgment for input wave 33 10 Bernard and others amplitude (10m) offshore; model and judgment used (equal to 30%

(1994) for inundation input wave)

Wave of arbitrary amplitude 26 16.4 Lamberson and (6 m) offshore of as input others (1998) to model 63%

Results of finite-element 30

<23 Meyer and others, model for tsunami wave (1999) propagation 1 Mean higher high tide (MHHW) is 6.9 feet higher than mean lower low tide (MLLW) 2 NRC docketed report 3 Assumes runup at Humboldt Bay ISFSI site is less than runup at coast.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 9-63 MHHWI 26

<233

<17.53

<27.63 16.5 19.4 9.7 17 23.3

<30 Section 9.0 Tsunami Potential Rev. 0, December 27, 2002

Table 9-7 (Continued)

ESTIMATES OF MAXIMUM RUNUP ELEVATIONS AT THE ISFSI SITE Pacific Gas and Electric Company Humboldt Bay ISFSI Estimated Maximum Estimated Maximum Run up Runup at at Humboldt Bay ISFSI Site (feet)

Researcher Basis Bay Entrance and Attenuation (%)

(feet above MLL W)

Runup Estimates from Paleo tsunami Studies Stratigraphy and judgment 18-40

<18-40

<18-40 Estimate from study of for run up at South Spit past tsunamis (Carver Minimum sand-carrying Likely depth

"<35 to 85" and others, 1998; this water depth (13 to 17ft) to deposit report) added to elevation ( 5 to sand (19 to 27 (from back

<24 to 34 7 ft) of marsh ft) added to calculation elevation ( 5 to using 0.3 and 7 ft) of marsh 0.7 from ISFSI site Notes: Numbers in Italics have been calculated by adding tidal range of 6.9 feet Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 9-64 Section 9.0 Tsunami Potential Rev. 0, December 27,2002

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Wilson, D., 1989, Deformation of the so-called Gorda Plate: Journal of Geophysical Research,

v. 94, no. B3, p. 3065-3075.

Witter, R. C., and Kelsey, H. M., 1996, Repeated abrupt changes in the depositional environment of a freshwater marsh-a record of Late Holocene paleoseismicity at Euchre Creek, south coastal Oregon (abs.): Geological Society of America Abstracts with Programs, v. 28, no. 5, p. 125.

Witter, R.C., Patton, J., Carver, G.A., Kelsey, H.M., Garrison-Laney, C., Koehler, R.D., and Hemphill Haley, E., 2002, Upper plate earthquakes on the western Little Salmon fault and contemporaneous subsidence of southern Humboldt Bay over the past 3,600 years, northwestern California: U.S. Geological survey, National Earthquake Hazards Reduction Program Final Technical Report, Award number 01HQGR0125, 44 p.

Witter, R.C., Kelsey, H.M., and Hemphill-Haley, E., in review, Great Cascadia earthquakes and tsunamis of the past 6,700 years, Coquille River estuary, southern coastal Oregon:

Geological Society of America Bulletin, submitted May 2002.

Wong, I. G., 2002. The 1873 Brookings, Oregon Earthquake and its implications for intraplate seismicity and hazard within the Central Cascadia Subduction Zone, Seismological Research Letters, v. 73, no. 2, p. 214-215.

Woodward-Clyde Consultants, 1980, Evaluation of the potential for resolving the geologic and seismic issues at the HBPP Unit No.3: Summary to Pacific Gas and Electric Company, San Francisco, 74 p. plus appendix volume.

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 10-29 Section 10.0 References Rev. 0, December 27,2002

Wright, S. G., 1991, UTEXAS3, A computer program for slope stability calculations: Shinoak Software, Austin, Texas.

Yamaguchi, D. K., Atwater, B. F., Bunker, D. E., Benson, B. E., and Reid, M.S., 1997, Tree-ring dating the 1700 Cascadia earthquake: Nature (London), v. 389 (6654), p. 922-923.

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Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 10-30 Section 10.0 References Rev. 0, December 27,2002

COMPARISON OF THE SOUTHERN CASCADIA SUBDUCTION ZONE WITH THE TECTONIC SETTING OF THE 1964 ALASKA EARTHQUAKE by George Plafker, Ph.D.

Plafker Geohazard Consultants Prepared for Pacific Gas and Electric Company George Plafker August, 23, 2002 Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 2A-1 Appendix2A Rev. 0, August 23, 2002

CONTENTS Arc Setting...................................................................................................................................... 3 Relative Plate and Block Motion.................................................................................................... 4 Historic Seismicity in the Arc-Transform Transition Region........................................................ 5 Forearc Fold and Thrust Belt.......................................................................................................... 5 Coseismic Tectonic Displacements................................................................................................ 6 Earthquake Recurrence Intervals.................................................................................................... 7 Coseismic Deformation and Tsunami Generation.......................................................................... 7 References Cited............................................................................................................................. 9 FIGURES Figure 2A-l. Land-level changes associated with the 1964 Alaska earthquake.

Figure 2A-2. Simplified tectonic model for present crustal deformation along Pacific-North American plate boundary in southern and southeastern Alaska.

Figure 2A-3. Major structural features along the Gulf of Alaska margin.

Figure 2A-4. Inferred rupture zones of major plate-boundary earthquakes and the "Yakataga seismic gap" along the Gulf of Alaska margin.

Figure 2A-5. Tectonic uplift and subsidence, and surface faults at Montague Island associated with the 1964 Alaska earthquake.

Figure 2A-6. Submarine extension of the zone of maximum uplift and faulting on Montague Island.

Figure 2A-7. Schematic cross sections showing the suggested mechanisms for the 1964 Alaskan earthquake and the 1960 Chilean earthquake.

Figure 2A-8. Profiles and sections of coseismic deformation associated with the 1964 Alaska earthquake and a hypothetical large megathrust earthquake across the southern Cascadia margin.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 2A-2 Appendix 2A Rev. 0, August 23,2002

COMPARISON OF THE SOUTHERN CASCADIA SUBDUCTION ZONE WITH THE TECTONIC SETTING OF THE 1964 ALASKA EARTHQUAKE The purpose of this report, is to consider the ways in which the southern end of the Cascadia subduction compare with, and differ from, the eastern end of the Aleutian subduction zone, which was ruptured most recently in the great 9.2 (M) 1964 Alaska earthquake. This comparison is made because of the striking overall similarities between the southern and eastern ends, respectively, of these two arcs. References for data sources on the 1964 earthquake are given in the text and figure captions. Discussion and description of the Cascadia subduction zone, seismotectonics, paleoseismology, and geology of Cascadia are presented in Section 2 of PG&E (2002) and are not repeated here.

Both the eastern Aleutian and Cascadia arcs are continental margin arcs with shallow-dipping megathrusts. Both have wide forearc accretionary sequences of Mesozoic and Cenozoic rocks that strike obliquely into the continental margin at one end so that a wide cross-section of the deformation zone is exposed on land. They both have complex transition zones where they intersect the continental margin in which the structural styles change within a broad, complex zone from dominantly near-orthogonal compression to dextral strike-slip. Until the great 1964 Alaska earthquake, historic seismicity in both regions was low and it continues to be low in the megathrust region of the Cascadia arc since the last great earthquake there about 300 years ago. They both have long late Holocene paleoseismic records of major earthquakes that document sudden regional vertical displacement of shorelines and accompanying tsunamis. Because of these striking similarities, I infer that regional warping, faulting, and tsunami generation associated with the 1964 Alaska earthquake is the best analog available for forecasting tectonic displacements and associated tsunamis that are likely to accompany future large Cascadia subduction zone earthquakes.

Arc Setting The 1964 Alaska earthquake occurred at the eastern end of the Aleutian arc, which is defined by the Aleutian trench, the arc of active volcanoes of the Aleutian Islands, Alaska Peninsula, and Wrangell Mountains, and an associated zone of high seismicity that is mainly related to the Aleutian megathrust (Figure 2A-1 ). The Aleutian subduction zone is within oceanic crust in its western part. To the east, it extends along the continental margin of North America from the western end of the Alaska Peninsula to the rupture region of the 1964 earthquake at the eastern end of the Aleutian arc in the Gulf of Alaska where it trends obliquely onto the continental margin. The earthquake occurred as a complex rupture along ~800 km of the Aleutian megathrust between the Pacific and North American plates and by large-scale subsidiary thrust faulting within the upper plate.

Interpretation of the 1964 earthquake as a result of convergence and thrusting along this plate boundary provided strong support for the theories of plate tectonics and seafloor spreading that were emerging as the dominant theme in earth science during the late 1960's.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 2A-3 Appendix2A Rev. 0, August 23, 2002

Similarly, the Cascadia arc is defined by subduction of the oceanic Juan de Fuca and Gorda plates beneath the continental North American plate and by the Cascades volcanic arc. From northern Vancouver Island to southern Oregon the arc trends roughly parallel to the continental margin. In northern California the seaward part of the arc intersects the continental margin and connects with the San Andreas dextral-slip fault system via a complex structural zone of fault-bounded tectonic blocks (Carver, 1987; Clarke, 1992).

Relative Plate and Block Motion In Alaska, relative motion between the Pacific and North American plates, when averaged over the last 3 million years (Ma) increases from 49 mm/yr in southeastern Alaska to 77 mm/yr at the west end of the Aleutian arc. In the 1964 earthquake area it is 58 mm/yr (Figure 2A-2).

The primary boundary between the Pacific and North American plates is the Queen Charlotte-Fairweather right-slip transform zone in southeastern Alaska and the Aleutian megathrust system of thrust, right-oblique, and right-slip faults that extend from the western Gulf of Alaska to the western end of the Aleutian Islands. Significant northwest-southeast relative motion is concentrated mainly along the boundaries of the Yakutat, Saint Elias, and Wrangell structural blocks in the complex region between the northern Gulf of Alaska, the Denali fault system, and the western Alaska Range (Figure 2A-2). Most of the Pacific-North American relative motion in the northern Gulf of Alaska region is taken up by dominantly strike-slip faulting of about 52 mm/yr on the northwest-trending Fairweather transform fault between the Yakutat and Saint Elias blocks, by shortening and deformation along the northeast-trending fold and thrust zone that represents an extension of the Aleutian megathrust zone between the Yakutat and Wrangell blocks, and by shortening and deformation (58 mm/yr) between the Pacific and North American plates (Figs. 2A-2 and 2A-3).

Rates of right oblique thrusting ( <1 0 mm/yr) are inferred to be relatively low along nearly east-west trending structures such as the Transition and Chugach-Saint Elias fault zones that bound the southern and northern margins of the Yakutat block, respectively. Slip rates of 10 to 20 mm/yr occur on the eastern Denali and Totschunda right-slip faults between the Wrangell block and North American plate. North of the Denali fault deformation within the North American plate is widespread but relatively minor. Indicators of principal horizontal stress directions in Alaska are broadly compatible with the relative Pacific-North American plate motions; however, the style of faulting within plates and blocks is generally more variable than along boundaries (Figure 2A-3).

The general transition from convergence along the Aleutian megathrust to right-slip in southeastern Alaska is comparable to the change in structural style in the transition from the southern Cascadia subduction zone to the region south of the Mendocino triple junction.

There are two noteworthy differences, however. In Alaska the change is due to the concave-southward oroclinal bend around the Gulf of Alaska that formed in the Paleogene whereas in Cascadia it is due to differences in motion of the oceanic plates north and south of the Mendocino triple junction.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 2A-4 Appendix2A Rev. 0, August 23, 2002

In addition, deformation in the transition zone between strike-slip and thrust faulting in Alaska is complicated by the presence of the Yakutat block (Figure 2A-2), a large allochthonous block that has moved with the Pacific plate from a source area at least 600 km to the southeast along the continental margin (Plafker and others, 1994). Continued northwestward movement of the block results in complex folding and thrust faulting along its margins, and subduction beneath the continental margin to the north and northwest with consequent rapid uplift, great topographic relief, and exceptionally active seismicity.

Historic Seismicity in the Arc-Transform Transition Region During the 20th century, most of the plate boundary in the complex transition region between the east end of the 1964 rupture of the Aleutian megathrust and the transform fault system of southeastern Alaska has ruptured in a series of plate-boundary and intraplate earthquakes (Figure 2A-4). The entire transform boundary of the composite Pacific plate and northern Yakutat block boundary experienced major earthquakes in 1958 (7.3 M) and 1972 (7.4 M) and the segment to the southwest ruptured in the 1949 (8.1 M) Queen Charlotte earthquake.

Rupture of a down-dip segment of the Aleutian megathrust and associated intraplate splay faults occurred during the Mount St. Elias earthquake in 1979 (7.5 M). A segment of the plate boundary about 1 00 km long between the 1979 and 1964 earthquakes, termed the "Yakataga seismic gap" is not known to have been filled by an historic earthquake (Tobin and Sykes, 1968). However, at least part of this area may have been within the focal region of a great series of earthquakes (8.1-8.5 Min 1899 that resulted in about 1 m of coastal uplift near the center of the gap at Cape Yakataga and 14 m of emergence near the head of Yakutat Bay (Thatcher and Plafker, 1977). The southern boundary of the Yakutat block is strongly coupled to the Pacific plate as indicated by a general absence of seismicity and late Cenozoic deformation within the block. Minor relative Pacific-Yakutat movement is suggested by the occurrence of an earthquake ( 6.3 Ms) with a thrust mechanism along the Transition fault.

In the Cascadia forearc, virtually all the historic seismicity is in the structurally complex Mendocino triple junction region where relative plate motion changes from about 30 mm/yr convergence north of the Mendocino fracture zone to dominantly right-slip south of the zone.

The southern Cascadia transition region is similar to the eastern Aleutian arc-transform transition in that they are both characterized by complex faulting and seismicity along both plate and block boundaries that reflect the change from dominant compressional shortening in the arc to dominant dextral-slip in the transform margin.

Forearc Fold and Thrust Belt In general, the overwhelming majority of the active faults and folds are related to the Aleutian megathrust and its northeastward extension onto the continental margin to the junction with the Fairweather transform (Plafker and others, 1994). Pacific oceanic crust is being subducted along the Aleutian megathrust and the composite Pacific plate and oceanic lower crust of the Yakutat block is being subducted in the region east of the Kayak Island zone (Bracher and others, 1994). Mesozoic and Cenozoic subduction has resulted in accretion of a complex of flysch and ocean crust rocks along the continental margin that ranges in width from ~300 km at the east end of the 1964 rupture zone to ~200 km at the southwest end. In the northern part of the rupture zone, dip of the megathrust is northwest at Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 2A-5 Appendix2A Rev. 0, August 23, 2002

9° beneath the zone of active faulting and it is 16-17 km deep at the inner margin of the zone near Montague Island.

The zone of active faulting associated with the megathrust near the northeastern end of the 1964 earthquake rupture zone is at least 150 km wide as evidenced by major thrust displacement on the Patton Bay fault zone in 1994 (Figure 2A-2). Within the Yakutat block to the northeast the deformed belt widens an additional 125 km between the Kayak Island zone and the Pamplona zone where it crosses the continental slope and extends onshore between the Bering and Malaspina Glaciers (Figure 2A-3). The Pamplona zone is presently the eastern leading edge of the deformation front as evidenced by growing folds, young faults, and active seismicity.

In summary, the data from the 1964 Alaska earthquake clearly show that intraplate deformation can take up much of the plate convergence, and that this deformation can extend as far as 150 km landward from the plate boundary at the trench to areas where the megathrust is ~ 17 km deep. In southern Cascadia, comparable intraplate deformation is manifested by the active Little Salmon and Mad River fold and thrust zones which occur in a segment of the forearc region where the megathrust is 12-18 km deep (Figure 2A-8a).

Coseismic Tectonic Displacements The 1964 Alaskan earthquake resulted from rupture of a segment of the eastern Aleutian megathrust 650-800 km long and 150-250 km wide (Figure 2A-5). This major tectonic event was characterized by: (1) shallow seismicity ( <30 km), with most of the earthquakes located between the Aleutian trench and the zero isobase between the zones of major uplift and subsidence; (2) regional vertical displacements in a broad asymmetric downwarp of up to 2 m centered over the Kodiak, Kenai, and Chugach Mountains with flanking zones of marked uplift of up to 11.3 m on the seaward side and minor uplift to about 0.3 m on the landward side that extends north of the Alaska Range; and (3) horizontal displacements that involved measured systematic shifts of the land in a generally seaward direction of up to 18 m in the region between the Anchorage and Montague Island areas. Data on coseismic displacements in the 160-km-wide segment of the rupture zone seaward of Montague island are available only at Middleton Island near the edge of the continental shelf where there was 3.5 m of uplift.

Subordinate northwest-dipping intraplate reverse faults, the Patton Bay and Hanning Bay faults, displaced the surface on Montague Island. The Patton Bay fault, with at least 7.9 m dip-slip displacement, is part of a zone of imbricate thrust faults that extends to the southwest on the continental shelf ~500 km. Evidence of young submarine faults, and folds, and possible coseismic uplift of the sea floor was found along the zone off Kodiak Island by marine geophysical surveys (von Huene and others, 1972). Two of the largest aftershocks lie within this uplift zone (Figure 2A-6). In addition, slip on a northwest dipping thrust fault that is seaward of Middleton Island is suggested by the 3.5 m uplift and northeastward tilting of the island during the 1964 earthquake.

Dislocation modeling of the horizontal and vertical displacement data for the 1964 earthquake (Figure 2A-7 (A)) require that coseismic slip be partitioned between the Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 2A-6 Appendix2A Rev. 0, August 23,2002

megathrust and known and inferred intraplate faults. Similarly, the vertical displacement data (Figure 2A-7 (B)) for the great (9.5 M) 1960 Chile earthquake (Figure 2A-7 (B)) can not be modeled using slip solely on the megathrust; the best-fit dislocation model requires an intraplate fault with dip of about 35° that intersects the surface offshore on the upper continental slope (Plafker and Savage, 1970; Barrientos and Ward, 1989)).

As in the 1964 Alaska, and probably 1960 Chile, earthquakes, faulting and neotectonic deformation in the southern Cascadia margin is likely to be partitioned between the megathrust and the active intraplate faults that splay off the megathrust in the forearc region.

Coseismic slip on intraplate faults within the active Little Salmon and Mad River fold and thrust zones in southern Cascadia could result in nearshore or onshore surface ruptures, vertical displacements, and tilts comparable to those that accompanied the Alaska earthquake.

Earthquake Recurrence Intervals Paleoseismic data at the Copper River delta in the eastern Aleutian arc indicate that 8 large pre-1964 megathrust earthquakes occurred in the same region in the last 5,600 years (Plafker and others, 2000). Because each of these paleoearthquakes involved sudden regional uplift (0.5-2.5 m) comparable to the 2m coseismic uplift at the same localities in 1964, they are interpreted as probable subduction zone events. Recurrence intervals range from,...,400-900 years and average,...,700 years. For the orthogonal plate convergence rate of~ 50 mm/yr, maximum dip slip/event ranges from 20-45 m and averages ~35m. The penultimate event was ~750 years ago and plate convergence during that interval was 37.5 m. Thus, only about 50% of this slip budget appears to have been recovered by coseismic slip in 1964.

In southern Cascadia, paleoseismic data indicate somewhat shorter recurrence intervals for 6 or ?large paleoearthquakes in the past 2,900 years (summarized in Sections 2 and 9 of the report, Seismic Hazard Assessment for the Humboldt Bay ISFSI Project). Recurrence intervals range from a minimum of,..., 200-300 yrs to a maximum of,...,900 yrs and average 430-520 years. For the orthogonal convergence rate of,...,30 mm/yr, maximum dip-slip per event ranges from 6-27 m and averages,..., 14 m. The most recent event was the earthquake in 1700 AD some 300 years ago and the southern part of the megathrust has been loaded ~9 m during this 300-year interval.

Coseismic Deformation and Tsunami Generation The 1964 Alaska earthquake tsunami was generated offshore by sudden coseismic uplift of the continental shelf and slope. The tsunami crest, as determined from initial arrival times at the adjacent coast, corresponds with the maximum uplift on the southwestward offshsore extension of the Montague Island zone of intraplate faults (Figure 2A-6). Maximum tsunami runup height of 12.7 m (described in Section 9 ofPG&E, 2000 and its Annex 9A by Platker, 2002), closely matches the maximum coseismic onshore uplift (11.3 m) along the Montague island zone, despite marked differences in the local bathymetry and configuration of the coast in the earthquake source region Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 2A-7 Appendix 2A Rev. 0, August 23, 2002

The intraplate splay faults on Montague Island and off Middleton Island accommodated

~80% of the 18-20 m maximum available slip on the Aleutian mega thrust and most of the vertical tectonic displacements (Figure 2A-8-(A)). No more than 3 m of the regional uplift is attributable to slip solely on the seismogenic segment of the mega thrust, which has an average dip of9° (Brocher and others, 1994).

The Alaska data demonstrate that a major fraction of the total fault slip can be partitioned between the gently dipping megathrust and intraplate splay faults that break relatively steeply to the surface. As a consequence, the vertical component of seafloor uplift can be considerably larger than for an equivalent displacement entirely on the megathrust. For tsunami generation, this means that the initial wave at the source is higher and closer to shore than it would be for slip entirely on the megathrust. Similarly, in southern Cascadia an earthquake on the subduction zone alone would result in less than 2 m tectonic uplift for an average dip of 8° on the megathrust and 12m slip (for 30 mm/yr orthogonal convergence and the average 470-year recurrence). The fact that most large megathrust earthquakes (those that rupture the full extent of the megathrust) in this part of the arc have been accompanied by tsunamis with large runups strongly indicates that they involve slip on one or more offshore intraplate faults such as those in the Little Salmon and Mad River zones (Figure 2A-8 (B)). Thus, partitioning of a significant fraction of the total slip onto steeply dipping intraplate thrusts is entirely consistent with the data on neotectonic deformation and paleotsunamis along the southern Cascadia margin.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-0 1 2A-8 Appendix 2A Rev. 0, August 23, 2002

References Cited Barrientos, S.E., and Ward, S.N., 1990, The 1960 Chile earthquake; inversion for slip distribution from surface deformation: Geophysical Journal International, vol.103, no.3, pp.589-598 Bracher, T.M., Fuis, G.S., Fisher, M.A., Plafker, George, Moses, M.J., Taber, J.J., and Christensen, N.I., 1994, Mapping the megathrust beneath the northern Gulf of Alaska using wide-angle seismic data: Journal of Geophysical Research, v. 99, no. B6, p.

11,663-11,685.

Carver, Gary, 1987, Late Cenozoic tectonics of the Eel River basin region, coastal northern California: San Joaquin Geological Society Miscellaneous Publication 37, p. 61-72.

Carver, G. A., Abramson, H., Garrison-Laney, C. and Leroy, T., 1999, Paleotsunami Evidence from Northern California for Repeated Long Rupture (M 9) of the Cascadia Subduction Zone [abs.]: Seismological Research Letters, v. 70, no. 2, p. 245.

Clarke, S. H. Jr., 1992, Geology of the Eel River Basin and adjacent region: Implications for late Cenozoic tectonics of the southern Cascadia subduction zone and Mendocino triple junction, American Association of Petroleum Geologists Bulletin v. 76, p. 199-224.

Plafker, George, 1965, Tectonic deformation associated with the 1964 Alaska earthquake:

Science, v. 148, p. 1675-1687.

Plafker, George, 1969, Tectonics of the March 27, 1964, Alaska Earthquake: U.S.

Geological Survey Professional Paper 543-I, 74 p.

Plafker, George, Kachadoorian, Reuben, Eckel, E.B., and Mayo, L.R., 1969, Effects of the earthquake of March 27, 1964, on various communities: U.S. Geological Survey Professional Paper 542-G, 50 p.

Plafker, George, and Savage, J.C., 1970, Mechanism of the Chilean earthquakes ofMay 21-22, 1960: Geological Society of America Bulletin, v. 81, p. 1001-1030.

Plafker, George, 1972, The Alaskan earthquake of 1964 and Chilean earthquake of 1960; Implications for arc tectonics: Journal of Geophysical Research, v. 77, no. 5, p. 901-925.

Lahr, J.C., and Plafker, George, 1980, Holocene Pacific-North American plate interaction in southern Alaska: Implications for the Yakataga seismic gap: Geology, v. 8, p. 483-486.

PG&E, 2002 in preparation, Seismic hazard assessment for the Humboldt Bay ISFSI Project, Report to the USNRC.

Plafker, George, 1987, Application of marine-terrace data to paleoseismic studies: in Crone, A.J. and Omdahl, E.M., eds., Proceedings of conference XXXIX-Directions in Paleoseismology: U.S. Geological Survey Open-File Report 87-673, p. 146-156.

Plafker, George, Gilpin, L.M., and Lahr, J.C., 1994, Neotectonic map of Alaska, in Plafker, G., and Berg, H. C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology ofNorth America, v. G1, Plate 12, 1 sheet with text, scale 1:2,500,000.

Plafker, George, Carver, G.A., and Clarke, S.H., Jr., 2000, Seismotectonics ofthe 1964 Alaska earthquake as an analog for future tsunamigenic southern Cascadia subduction earthquakes [abs.]: Proc. of Penrose Conference 2000, Great Cascadia Earthquake Tricentennial, Program Summary and Abstracts". Oregon Department of Geology and Mineral Industries, Special Paper 33, p.

Plafker, George, 2002, A review of empirical data on tsunami runup versus earthquake source parameters: Report to Pacific Gas and Electric Company, 18p. plus figures.

Humboldt Bay ISFSI Project Technical Report TR-HBIP-2002-01 2A-9 Appendix2A Rev. 0, August 23, 2002

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