ML20247C428
| ML20247C428 | |
| Person / Time | |
|---|---|
| Site: | Satsop |
| Issue date: | 05/17/1989 |
| From: | Alexander Adams Office of Nuclear Reactor Regulation |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8905240466 | |
| Download: ML20247C428 (30) | |
Text
{{#Wiki_filter:_ _ _ _ _ _ _ A 4 UNITED STATES
,f i NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555
$ j
,/ May 17, 1989 Docket No. 50-508 APPLICANT: Washington Public Power Supply System (WPPSS)
FACILITY: WNP-3 SUBJECT.
SUMMARY
OF MEETING WITH REPRESENTATIVES OF WPPSS AND CONSULTANTS TO SUMMARIZE THE GEOSCIENCE ISSUES FOR WNP-3, JANUARY 11, 1989 INTRODUCTION On January 11, 1989 representatives of the NRC, WPPSS and WPPSS consultants met in the NRC offices in Rockville, Maryland, to review and discuss the overall geoscience program for WNP-3 as has been presented tc the staff. This safetywas to be the evaluation final(SER report wrap-)up meeting tothe
. In particular, enable theofstaff purpose to prepare a the meeting was for WPPSS to sunnarize the results of deterministic studies conducted with respect to the seismic hazard to the site represented by the Cascadia Subduction )
Zone. The attendance list is provided in Enclosure 1. A copy of the slide ' presentations is provided in Enclosure 2. DISCUSSION WPPSS representatives presented the reasons for the studies and its goal regarding the WNP-3 project. A reassessment of the near-future power needs of the Pacific Northwest is underway and WPPSS desires to present a case to the Northwest Power Counsel, who is responsible for the power needs of the area, that WNP-3 is still a viable source for contributing to those power needs in spite of significant seismologic and geologic issues that have arisen over the past few years. WPPSS would like to have these issues reviewed by the NRC in a SER, even if it identifies some open issues, within the next few months. The staff agreed to attempt completion of an SER by June, 1989 but that the evaluation would not be a complete evaluation in view of the unknowns regarding f the Cascadia Subduction Zone, and the considerable amount of research currently underway in the region by the USGS and others. The subjects of this meeting were: the capability of the Cascadia Subduction Zone; the maximum magnitude earthquake from the Cascadia Subduction Zone; attenuation of ground motions from such an earthquake; and ground motions at the WNP-3 site. These topics are addressed in detail in the June 30, 1988 l WPPSS report entitled, "Cascedia Subduction Zone, An Evaluation of the Earthquake Potential and Implications to WNP-3." l i Dr. G. Davis, University of Southern California, presented information I concerning the capability of the Castedia Subduction Zone, a brief summary of which follows: ) e905240466 890717 yes noccxosoggg6 'Foi I a i 3 ( ,
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~2- May 17, 1989 The Juan de Fuca oceanic crustal plate was subducting beneath the continental Horth American Plate in a North 50 degree East direction at a rate estimated at the latitude of the Columbia River to be about 4 cm/ year up to about 500,000 years ago. As there have been no magnetic reversals in the last 700,0')0 years, it.is not possible to calculate the precise current rate of convergence, so it is assumed to be the same.
The Juan de Fuca Plate is bounded to the north across the Nookta Transform Fault by the very small Explorer Plate, and to the south across the Blanco Fracture Zone, by the small Gorda Plate. These two mini-plates are being severely deformed internally due to their involvement in the transition from the subduction tectonics of the Juan de Fuca Plate and the strike slip tectonics
, of the Queen Charlotte Fault along coastal Canada to the r. orth and the San Andreas Fault-Medocino Fracture Zone to the south.
Because of the absence in the historic past of large subduction zone thrust earthquakes associated with the Cascadia Subduction Zone, there are two end member hypotheses concerning the seismic potential of this zone. The plates are either locked along their interface, and stress will eventually be released as a great earthquake; or the two plates are slipping past each other seismically and stress is not accumulating that could result in a great earthqua ke. Analysis of the Cascadia Subduction Zone and comparison with apparently similar subduction zones around the Pacific Ocean lead two seismologists, Heaton and Kanomori (1984), to conclude that the Cascadia Subduction Zone is locked and a great earthquake is likely in the future. This conclusion was based on the youth of the ocean plate ar.d rate of convergence. Support for that hypothesis was provided during the last several years by geomorphological and geological studies on coastal southwest Washington and northwest Oregon, such as those of Brian Atwater of the USGS. On the other hand, some geophysicists and rock mechanists found that the Cascadia Subduction Zone is characterized by unique thermal and mechanical properties that do not require the potential occurrences of great earthquakes. Dr. David presented data that characterized the ascadia Sut, duction Zone and its tectonic state. The first data set was geodetic data, which he concluded did not demonstrate anything very convincing about the subject. The next data set was seismicity, from the historic record, and instrumental record, including data from Canadian networks and the University of Washington broad band network, and geophysical information including that from the Lithoprobe studies in Canada. Based on interpretation of these and other data the Cascadia Subduction Zone is concluded to be characterized by the following:
- 1. the state of stress in the upper crust is north-south compression;
- 2. the Juan de Fuca is folded into an east-west striking arch; ,
- 3. the plate interface on top of the arch dips east an average of about 11 i degrees then bends to a more steep dip estimated to be 50 to 65 degrees i beneath Puget Sound down to a depth of 500 kms beneath the Cascade Mountains;
May 17, 1989
- 4. north and south of the arch, dips of the slab interface range from 15 '
degrees to 20 degrees;
- 5. seismicity in the Puget Sound area is related to deformation in the upper crust above the arch in the slab;
- 6. deeper seismicity, which defines an approximate Benioff zone, is ,
occurring within the subducting slab in the vicinity of the change in l dip, and is tensional because it is associated with the upper part of the { down bending of the slab. Dr. Davis reassessed the six circum-pacific subduction zones that Heaton and f Kanomori suggested as being similar to the Cascadia, using more of the known l char ccteristics than only youth of plate and rate of convergence. These other 9aracteristics, which included heat flow, size of rupture surfaces, presence : ei c5sence of asperities on the ocean plates, volumes of sediment on the down-going plates, characteristics on accretionary wedges and backstops, presence or absence of volcanic arcs, strain classification etc., appeared to demonstrate that the Cascadia Subduction Zone is unique in many ways. Many of these characteristics as they relate to the Cascadia Subduction Zone will tend to support the hypothesis that subduction is occurring seismically. Dr. Davis summarized paleoseismic studies on Coastal Washington, and presented alternative explanations for the anomalous features found in the late Holocean a geologic record presented by Atwater and others as evidence for prehistoric great earthquakes. The data that support alternative conclusions includes similar geologic features on the relatively aseismic coasts of eastern North America and northern Europe, where the features are attributed to Holocene sea level fluctuations instead of earthquakes. Also the apparent stability of coastal marine terraces and wave cut platforms during the late Quaternary was f given as evidence against the occurrence of prehistoric great subduction zone j earthquakes. Based on the available evidence, WPPSS favors an interpretation that subduction within the Cascadia Zone is occurring seismically and the potential for a damaging earthquake is very remote. However, since that conclaion cannot be absolutaly demonstrated, WPPSS assumes that such an event can occur. Dr. Davis and K. Coppersmith of Geomatrix Consultants, presented the basis for segmenting the Cascadia Subduction Zone into three separate seismogenic zones. The approximate extent of each of the three segments are: southern Vancouver Island and the Straits of Juan de Fuca encompasses the northernmost segment; I the central segment, which contains the arch in the slab, extends from the Straits of Juan de Fuca to northern Oregon, just below the Columbia River; and the southern segment extends from northern Oregon to the Blanco Fracture Zone. The length of the central segment, which is most significant to the site is 280 kms. The width of the seismogenic section of the plate was constrained updip on the plate by the estimated western boundary of the backstop and estern boundary of the assumed aseismic accretionary wedge. The depth of the plate interface beneath this location is 20 km. The downdip (eastern) limit of the seismogenic zone was estimated to be the transition zone from brittle to plastic state l within the North Ainerican Plate at a depth of about 35 km. l l
; l May 17, 1989 The resulting width of the seismogenic part of the plate is 75 km. The area of fault rupture was calculated to be 18,750 sq km.
l ' Based on the size of the seismogenic plate segment and data on the crustal earthquakes beneath Puget Found, K. Coppersmith presented the maximum earthquakes assumed fro the n different sources. The maximum intraslab event was determined to h6 ~ 2 moment magnitude (Mw) 71/2 at a hypocentral distance of 70 km, and the maximum interslab resulted in a Mw 81/4 earthquake at a depth of 33 km beneath the WNP-3 site. Ground motion attenuation analyses from these earthquakes was presented using compiled spectral plots of numerous recordings from seismographs founded on both soil and rock during the 1964 Alaskan,1985 Chilean, and the 1985 Mexican earthquakes. He showed that the plant structural design spectra enveloped the 84th percentile plots of these spectra in most critical frequencies. CONCLUSIONS The staff reminded WPPSS that WNP-3 is a low priority project as compared to the work on operating plants and the staff resources and capabilities are limited. However, we would attempt to complete an SER by June, 1989. The SER will be written to include current findings based on review of the data we now have, and is subject to significant. changes in the future. Since there will be no further meetings on this subject, the staff may find it necessary to talk directly to WPPSS and their consultants. This meeting summary was prepared based on inputs provided by R. McMullen. NWhi o , Alexander Adams, Jr., P oject Manager Standardization and 1-Power Reactor Project Directorate Division of Reactor Projects - III, IV, V and Special Projects
Enclosures:
As stated cc: See next page
May 17, 1989 The resulting width of the seismogenic part of the plate is 75 km. The area of fault rupture was calculated to be 18,750 sq km.
. Based on the size of the seismogenic plate segment and data on the crustal earthquakes beneath Puget Sound, K. Coppersmith presented the maximum earthquakes assumed from the two different sources. The maximum intraslab event was determined to have a moment magnitude (Mw) 7 1/2 at a hvnocentral distance of 70 km, and the maximum interslab resulted in a Mw 81, earthquake at a depth of 33 km beneath the WNP-3 site.
Ground motion attenuation analyses from these earthquakes was presented using compiled spectral plots of numerous recordings from seismographs founded on both soil and rock during the 1964 Alaskan, 1985 Chilean, and the 1985 Mexican earthqua kes. He showed that the plant structural design spectra enveloped the 84th percentile plots of these spectra in most critical frequencies. ; CONCLUSIONS The staff reminded WPPSS that WNP-3 is a low priority project as compared to the work on operating plants and the staff resources and capabilities are limited. However, we would attempt to complete an SER by June, 1989. The SER will be written to include current findings based on review of the data we now have, and is subject to significant changes in the future. Since there will be no further meetings on this subject, the staff may find it necessary to talk directly to WPPSS and their consultants. This meeting summary was prepared based on inputs provided by R. McMullen. Alexander Adams, Jr., Project Manager ' Standardization and Non-Power Reactor Project Directorate Division of Reactor Projects - III, IV, V and Special Projects
Enclosures:
As stated cc: See next page l DISTRIBUTION: iDocket File. NRC PDR JSniezek PDSNP R/F AAdams > OGC l l EJordan BGrimes NRC Participants ACRS(10) EHylton PM:PD D:PDSNP AAd s- ff CMiller b 05/t7/89 05//7/89 o
4 -
.d E '
w ., , Mr. D. W. MazurJ , WPPSS Nuclear' Project No. 3 L Washington.Public Power' Supply System. '(WNP-3) Docket No. 50-508 "~ cc: Mr. Charles B. Brinkman,- Manager. ' Mr. G.. C. Sorensen, Manager
- l Washington. Nuclear Operations ,
Regulatory Programs j Combustion Engineering, Inc. Washington Public Power Supply 12300 Twinbrook Parkway, Suite 330 System Rockville,- Maryland 20852- Post Office Box 968 Nicholas-S. Reynolds, Esq. Bishop, Cook, Purcell & Reynolds Regional-Administrator, Region VI 1400 L Street, N.W. U.S. . Nuclear Regulatory Commission Washington,~DC 20005-3502' 1450 Maria Lane Suite-210 G..E. Doupe, Esq. . Walnut Creek, California 94596 e Washington Public Power Supply System 3000' George Washington Way Richland, Washington. 99352 Mr. Curtis Eschels, Chairman -
. Energy Facility Site Evaluation Council
. Mail Stop PY-11 Olympia, Washington 98505-Mr. Richard Latorre Regulatory Programs Washington Public Power Supply System h Post Office Box 968
.Richland, Washington 99352 Mr. William Ang
' Region Inspector /WPPSS 3/5 U.S. Nuclear Regulatory Commission-1450 Maria Lane - Suite 210 Walnut Creek, California 94596 Mr. Eugene Rosolie, Director Coalition for Safe Power 408 Southwest Second Avenue Portland, Oregon 97204 g
____ _____ -____--_____-_-_-_____-_- ___ D
4
^
ENCLOSURE 1 ATTENDANCE LIST FOR MEETING WTTH WPPSS CONCERNING GEOSCIENCE ISSUES FOR WNP-3 NAME ORGANIZATION Guy Vissing NRC/NRR/PDSNP Raman Pichumani NRC/NRR/ESGB Leon Reiter NRC/NRR/ESGB Dan Guzy NRC/RES/SSEB Robert Rothman NRC/NRR/ESGB Richard McMullen NRC/RES/SSEB Charles Butros WPPSS Joseph Burn WPPSS David Bosi WPPSS Richard Latorre WPPSS Bill Kiel WPPSS Donald West Golder Associates Robert Crosson University of Washington Gregory Davis University of Southern California Robert R. Youngs Geomatrix Consultants Devin Coppersmith Geomatrix Consultants
I
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-I CASCADIA SUBDUCTION ZONE l
e CAPABILITY e MAXIMUM EARTHQUAKE MAONITUDE. e GROUND MOTION ATTENI'ATION RELATIONSHIPS FOR SUBDUCTION ZONE SOURCES e SITE GROUND MOT ONS FROM POSTULATED SUBDUCTION ZONE EARTHQUAKES
, ...'t~ , J n . q CONSTRAINTS ON MAXIMUM MAGNITUDE 1 INTRA-SLAB SOURCE
- o. Historical record.
e Age of slab e ' Analogy INTERFACE SOURCE l e Physical dimensions e Analogy e 4 e
' SEGMDrfATION CHARACTERISTICS' Burbach and Frolich, 1986 e Gaps in interface seismicity e Abrupt changes in strike of Benioff zone e Abrupt changes in dips of Benioff zone ,
i e Changes in maximum depth of seismicity j e Abrupt lateral end of Benioff zone i e Apparent lateral strain caused by anomalous trench geometry Habermann et al., 1986 e Differences in rate of interface seismicity e Changes in-trend of are i e Intersecting ridges
-i e Offsets in volcanic arc i o Changes in age of volcanics 1
!. 1 l-e Changes in trench sediment thickness l e Changes in dip of Benioff zone e Changes in seafloor topography I i l l l 1 1 I i l , I i
. ~. ; *
.; ; l 1
l CASCADIA SEGMENTATION
]
Seaments 1 1 e Northern (British Columbia): 49 5*N to 47 5'N { Length: 275 km
- Slab dip: 15-20*
Strain class: 4a-5 e Central (Washington): 47 5*N to 45'N l Length: 250 km ' Slab dip: 11* Strain class: 4a-5 e Southern (Oregon): 45*N to 42 7'N Length: 330 km Slab dip: 15-20* Strain class: 3 Segment Boundaries Northern: 47 5*N e Change in slabdip e Change in slab and shallow crustal seismicity e Bend in volcanic arc Southern: 45*N e Change in slab dip e Change in slab and shallow crustal seismicity e Bend or step in late Cenozoic volenaic front
. e Change in volume of Quaternary volcanics e Change in upper plate tectonic regime i
L . ._ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ . _ . _ _ _ _ _ _ _ . _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ . . _ _ . _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ . _ _ _
r- ., INTERFACE MAXIMUM MAGNITUDE Physical Constraints Lateral extent of rupture e Segmentation e 250 km Updip extent of rupture e Accretionary wedge e Backstop e Below 20 km depth Downdip extent of rupture e Crust / mantle boundary e Rhealogy of young slab e Above 35 km depth Dimensions Length: 250 km Width: Assume 11' dip. 75 km Area: 75 km x 250 km 18,750 km2 Maximum magnitude e M, = log A + 3 99
. M, = 86 e
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NOVEMBER Volume 15 1988 Number 12 AMERICAN GEOPHYSICAL UNION { l
15,20,8 ,Lawis et aL: Subductio2 of the Juan de Fuca Plate i SW NE B SEDIMENT DEFORMATION TOFINO VANCOUVER ISLAND g' COVER F T 8ASIN UNE 1 ---*4 N : 0
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50 km 60 Fig.1. Cross sectsoa summarizing the geology and seismic reflectors under Vancouver '. stand, Lithoprobe phase I, and offshore [from Clowes et al,1987]. undertesa by graaidc and nietamorphic rocks of the Coast [Ellis et al.,1983; McMechan and Spence,1983; Spence et "lueenac Complex, a belt of sasialy quarts diorite and si.,1985] and results from two offshore seismic reflection
- t. eesdsories pessons, mismaties, and roof pendames. Rapid pr files [Saavely and Wagner,19811 Clowes et al.,1987; R.
4 Eft has anowed accelerated erosson and d1===a*= of the D. Hyndman et al, manuscript in preparation,19881 and the complex into a region of rugged mountaias. Neogene Lithoprobe Phase I profiles on Vancouver Island [Clowes et volcanics related directly to subduction of the Jana de Fuca al,1987] as well as seismicity studies (Crosson, 1983; plate laclude the young Geribaldi Volcanic Belt, defined by Rogers,1985] sugagsts a subducting oceanic crust with a dip several moderately dissected Flio-Pleistocene ~uiposite steppening from 3 at the base of the continental slope to volcanoes including Garibaldi, Cay'ey, and Menger 15 , at a distance of 35 km landward. A reflector, very clear ' , mountaias. The belt is a morthern extension of the High west of the trench but continuing under the trench, Casades in the United States (Figure 2). corrumponds to the top of the subducting oceanic crust. - Above the oceanic crust are zoras of reflectors, D and E in A combasation of models based on seismic refraction b - we
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SUMMARY
OF DEVELOPMENT OF GROUND MOTION ATTENUATION RELATIONSHIPS Empirical strong motion data (rock and soil sites) used to develop attenuation relationships (a,,, and S,) for Mw 5 to 8 events e Soil motions significantly larger than rock motions e Intraslab events (high angle events within subducting. slab) produce significantly larger peak acceleration than interface events (low angle thrust at plate interface) Site-specific numerical simulations of ground motions used to extend empirical relationships to events of Mw>8 e Magnitude scaling greater than suggested by theoretical source scaling relationships e Resulting relationships compare well with those developed by Heaton and Hartzell (1986) using alternative numerical techniques and recorded data.
WNP-3 GEOSCIENCES PROGRAM PROVI_RER Ilis HRC STArr: GEOLOGIC / TECTONIC MODEL. INCLUDING: COASTAL TERRACE STUDY BROAD 8AND NETWORK
-THERMAL-MECHANICAL EVALUATION
. PLATE COMPARISONS GEOLOGIC FAULT. INVESTIGATIONS j ASSUMED THE PLATE' INTERFACE CAPABLE SEGMENTATION -
MAXIMUM MAGNITUDE
-SEISMIC HAZARD ANALYSIS NUMERICAL SIMULATION'MODEL DETERMINISTIC EARTHQUAKE EVALUATIONS a
RESULTS: CONSISTANT BETWEEN TECHNIQUES WITHIN THE RANGE OF VALUES FOR OTHER PLANTS SUPPORT THE ADEQUACY OF THE PRESENT DESIGN REQUEST: REVIEW THE INFORMATION SUBMITTED AS A PACKAGE MAKE THE MOST SOLID CONCLUSIONS POSSIBLE , (EXPECT A DISCLAIMER RESERVING FINAL JUDGEMENT) I DOCUMENT RESULTS IN AN UPDATED NOVEMBER 1985 SER CCMPLETE IN NEXT 3 TO 4 MONTHS 2 _. _ _ _ _ _ _ _ _ _ _ - - _ . - - - - - - - - - - - - - - - - - - - - - - - - - - ' - - ~ -
L 4 [.. SPECIAL CHARACTERISTICS OF THE JUAN DE FUCA PLATE f L SEISMIC FACTORS
- ENTIRE CASCADIA SUBDUCTION ZONE PLATE INTERFACE !
IS SEISMICALLY QUIESCENT; NO KNOWN THRUST FAULT MECHANISMS OF ANY MAGNITUDE
- BENIOFF ZONu PRESENT ONLY IN PUGET SOUND AREA
- DOWNDIP LENGTH OF SEISMICALLY ACTIVE SUBDUCTED J d F SLAB (280 km) AMONG 4 SHORTEST OF 35 ZONES OR ZONE SEGMENTS (JARRARD,1986)
GEOLOGIIC FACTORS
- J d F PLATE IS SLOWEST MOVING OF ALL OCEANIC PLATES IN TERMS OF ABSOLUTE VELOCITY
- EXTREME YOUTHFULNESS OF SUBDUCTED PLATE (8 Ma --
NONE OF ZONES STUDIED BY JARRARD ARE YOUNGER;
" EFFECTIVE' THERMAL AGE < 8 Ma) i
- SLOW SUBDUCTION RATE (3.4 cm/yr -- ONLY 8-10 OF ZONES STUDIED BY JARRARD HAVE SLOWER RATES)
- CASCADIA " TRENCH" IS SITE OF PACIFIC OCEAN'S MOST VOLUMINOUS QUATERNARY SEDIMENT ACCUMULATION (AS MEASURED IN VOLUME / KM LENGTH OF TRENCHj
8 i I HEATON AND KANAMORI(1984) EQUATION j FOR " MAXIMUM EARTHQUAKE MAGNITUDE" OF ANY SPECIFIC SUBDUCTION ZONE : M w ' = -0.00889 t + 0.13 y + 7.96 where t = age of subducting plate (Ma) and v = convergence rate in cm/yr. KANAMORI(1986) CONCLUDED THAT M w IS MORE APPROPRIATELY INTERPRETED AS A PARAMETER MEASURING SEISMIC MOMENT RELEASE, RATHER THAN THE MAXIMUM
'4AGNITUDE OF THE CHARACTERISTIC EARTHQUAKE FOR A SPECIFIC SUBDUCTION ZONE
_ _ _ _ _ - - _ - - _ - _ _ - - - _ - - - - - - _ - - - - _ _ - .i
CASCADIA SU33UC":ON ZON3 T = 8 Ma V = 3.4 cm/yr Accretionary prism . 0 . . -
" * 's
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* = , ,
KVERA LAT3 SUBDUCTEON ZONE T = 8-10 Ma V = 2.3 cm/yr Narrow eceretionary DSDP "473 prism
,..,~. '""' , se,ds = .25 km '
s
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G. A. DAVIS 1/89
.i ._- _ _- _ __ . _ _ - _ - _ _ _ _ _ _ _ _ _ _ - _ - _ _ _ _ - _ _ _ _ _ _ _ _ _ - _ _ _ _ a
i ROLE OF SEDIMENTS IN FAVORING STABLE FRICTIONAL t SLIDING BEHAVIOR AS OPPOSED TO STICK-SLIP BEHAVIOR
- Sediments blanket JdF plate soon after its formation and inhibit its convective cooling by sea water; plate cools only by conduction and thus remains abnormally hot. High temperatures of sediments at time of subduction (21000 C) facilitates stable sliding behavior
-- as seen in laboratory experiments on other materials.
- Clays (esp. montmorillonite & vermiculite)in subducting marine sediments dramatically lower the coefficient of friction of sediments along the plate interface; such clays make up 44-66% of clay fraction in 10 JdF samples; compaction and dehydration of clays increases fluid pressure at the plate interface.
- Tectonically dewatered sediments, especially l clay-rich types, exhibit a marked increase in internal 1
fluid pressure; high fluid pressures lower effective stresses (confining pressure minus fluid pressure) across the plate interface -- thus favoring stable sliding.
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(Figure 6) almost all indicate strike-slip The Plate System faulting with a probable northwest southeast o rien ta tion. Despite the structural evidence East of the ridge, the seafloor is underlain within the axial valley, only one solution by the Juan de Fuca plate, the descendant of a indicates normal faulting (Jones & Johnson, much older plate called the Farallon plate by 1978). Atwater (1970). This place is now regarded as i I 10 RIDDIHOUCH ET AL.. t L. _ _ _ _ _ _ _ _ - - - - - - - - - - - - - - - - - - - - - - - - - - -}}