ML20151H066
| ML20151H066 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Carver D, Katrina King, Tarr A INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808010130 | |
| Download: ML20151H066 (14) | |
Text
7 CE0 PHYSICAL STUDIES IN SUPPORT OF SEISMIC HAZARDS ASSESSMENT OF
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SEATTLE AND OLYMPIA, VASHINGTON.
by K. King, A.Tarr, D. Carver, R.Villiams and D.Vorley U. S. Geological Survey INTRODUCTION According to plate tectonics theories, the Juan de Fuca oceanic plate and the continental North American plate are converging at about 3 4 cm/yr in a subduction zone more or less parallel to the Pacific Northwest coast.
Que consequence of this convergence is the occurrence of earthquakes, active volcanism, and tectonic deformation. The potential exists in the Puget Sound area for (1) large, shallow earthquakes along the interface of the underthrust zone, (2) major earthquakes within the cold,. brittle Juan da Fuca slab, (3) moderate earthquakes associated with active volcanism in the' Cascade Range, i
and (4) major shallow earthquakes within the North American plate landward of
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the underthrust zone. The Pacific Northwest cities of Seattle, Tacoma, and Olympia, Washington and Portland, Oregon are urban centers with significant risk from the occurrence of large earthquakes.
Major earthquakes occurred in the Puget Sound area in 1946, 1949, and 1965 (Fig. 1).
The 1949 shock caused major damage to high rise structures in Olympia; highest intensities were VII. The 1965 shock caused widespread damage in both Seattle and Tacoma, and intensity VII affects in Olympia; the highest intensity effects (VIII) were observed in West Seatcle and Harbor I
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Island.
t URBAN RAZARDS INVESTICATION The USGS is engaged in a regional earthquake hazards assessment program in the States of Washington and Oregon, concentrating on the Puget Sound and Portland urban areas.
The program is a partnership among governmental (Federal, State, and local), academic, and private entities to study how the Northwest would be impacted if a lar5e, potentially dama6tng earthquake were to occur in the region.
The program is the outgrowth of two earthquake hazards workshops (USGS, 1983; USGS, 1986) and is divided into five, interrelated components:
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Information Systems (2) Synthesis of geological and geophysical data (3) Cround motion modeling (4) Loss estimation models (5)
Implementation The studies described in this report were conducted by the Urban Hazards Field Investigations project in support of the ground motion modsling component of the urban hazards program.
The objective of the ground motion modeling component is to produce deterministic and probabilistic ground motion models and to produce maps of ground shaking hazard. One element of the ground motion modeling component is predicting relative ground response to strong vibratory motion from a model earthquake.
Relative ground response is determined from observations of ground motion a..d from extrapolations of those k
measurements into areas where ground motion data are not available. The G808010130 8G061'5 ~
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geophysical studies described in this report are designed to provide seismic data from which relative ground response values are determined and to collect geotechnical data which assist in making the extrapolations.
OBJECTIVES The o ;ectives of the Urban Hazards Field Investigations project are to:
(1) Directly record, in digital form, seismogr.as of actual vibratcry ground motion at sites in urban areas where ground response data are desired.
(2) Collect; geotechnical (geological and engineering) data from sites near where the ground motion measurements were made.
(3) Correlate geotechnical data with seismic response data by clustering sites of similar geotechnical parameters.
METHODS Relative ground response is determined by comparing the ground response at a site with a standard or reference response site.
In this study, relative ground response is obtained by dividing the Fourier amplituds spectrum of a site by the amplitude spectrum of the reference location. The resulting spectral ratio may then be smoothed or averaged over any number of bandwidths; the average spectral ratio in the value of relative ground response within that band. In the relative ground response method, it is assumed that the grcund response is due to ground conditions only, that is, the seismic inputs to the crust under the response site and reference site are essentially identical for a specified event and that any changes in response are due to difference between reference and response sites.
If it is possible to correlate those difference in terms of physical parameters and geological description characterizing the sites, it may be possible to predict ground response from geotechnical data in locations where seismic observations are not available.
Desirable seismic waveforms for the study may occasionally be masked by i
seismic noise of natural and manmade origin.
Ic is important to know the characteristics of noise to minimize the effects of contaminating a desirable signal and to help identify the frequencies of interest for site response studies.
Recordings of vibratory ground motion and geotechnical data of specific sites are the principal kinds of data which are required for predictive ground response studies. Ground motions resulting from various seismic sources were
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recorded by portable digital seismic systems for this study. The seismic data l
used in the studies described in this report were recorded by calibrated portable digital seismic systems especially for the urban hazards program.
The sources of seismic energy were both natural (microcarthquakes and microseisms) and artificial (nuclear explosions and mining explosions).
Digital recordings of induced ground vibratiens in the Seattle and Olympia areas were successfully acquired for four nuclear explosions at the Nevada Test Site (approximately 1,100 km distance) and seven mining explosions at an open pit coal mine near Centralia, Washington (about 80 km from Seattle)
(Figs. 1,2,and 3). The general procedure for recording these explosions was to manually start all recorders within 15 min of the expected arrival time of the seismic waves and record up to one hour for each, Five to eight seismic stations were installed at temporary locations in the Vest Seattle Area. The recorders at the stations continuously store
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approximately 15 seconds of data in a digital memory and will permanently 2
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store the digital data for longer duration when activated by a radio signal
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from a master station or by a local vibratory ground motion which coincides with a pre-set algrorithm. Two microcarthquakes have been recorded to date.
The time history data for one of the earthquakes and for one of the nuclear events are shown in Figs. 4,end 5.
Three separate experiments were conducted to evaluate the nature of seismic noise and microseisms in Seattle. The first experiment consisted of recording 10 min of background vibrations at five sites at three times (at 1 a.m.,
2 a.m., and 3 a.m.) on two successive dsys.
The second experiment consisted of recording 15 min of background noise at five stations during a Saturday afternoon in the Brighton district of Saattle.
Two of the sites were located on bedrock while the other three were located on varying thickness of sediment. The third experiment consisted of record bg 15 min of background noise at five stations on a windy, rainy Sunday morning in Vest Seattle, extending inland along a line about a mile long.
Seismic refraction and high resolution reflection are geophysical anthods used to determine the subsurface structural details at a site, based on the acoustic contrast across interfaces. The two methods are complementary:
Seismic refraction is most accurate in determining average velocities of layers whereas seismic re.tlection is most accurate in determining layer thickness.
In this study, seismic refraction experiments were run to determine layer velocities and approximate layer thickness at several responso sites.
Seismic reflection experiments were run, using an approximate velocity model determined from refraction, to determine more accurate thicknesa and scructure of the shallow layers.
In addition, longer reflection lines were run to detect deep interfaces which were not accessible to the refraction experiments.
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In all cases, refraction lines were run in both forward and reverse directions, and the reflection methods used the "push pull" technique to give a minimum 18-fold summsry of the common depth points.
Several data reduction techniques were used to derive the velocity model and depths frors the travel-time data. Fourteen refraction / reflection lines were run in the Seattle area (Fig.2). Examples of the high resolution reflection profiles are shown in Fig.6.
The method used to determine building response parameters is to artificially force the structure into oscillation and to record the vibration using a seismograph. An impulse delivered to the structure produces a damped j
vibration whose waveform allows a damping constant to be measured.
i A spectrum of the waveform indicates the predominant resonant period of the structure.
Ten one story single family dwellings, with brick chimneys in the West Seat::le area were tested for building response. In all cases, both the dwelling and the chimney were tested (Fig.7).
SUMMARY
Only preliminary conclusions can be derived from the present data set.
The comparison of the derived spectra from the ground motions induced by the Nevada Test Site nuclear explosions and the spectra derived from the ground motions induced by small earthquakes suggest that the spectra derived from the large nuclear event ground motions are comparable and therefore useful at frequencies less than approximately 1.5 Hz.
i The ground motions at Seattle and Portland induced by the quarry and mine blasts at the Centralia, Washington coal area are too small to be used for
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site response studies. The study has shown that ground motions induced by 3
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local earthquakes are the only technically acceptable source at this time for
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ground motion studies in these areas. The ground motions induced by the quarry blasts at Centralia are less than desirable, but are adequate for site response studies in Olympia, Washington.
The derived site response values in the Olympia, Washington area from the motions indu:ed by the Centralia quarry blasts cerrespond favorably with.the MM intensities from the 1965 earthquake; that is, the higher response sites are located at areas of higher intensities, the medium response values are at sites of medium intensities and low response values are located at areas where ro intensities or damage was reported Fig.9. The few site response values derived thus far for the sites in the West Seattle area from the motions induced by the nuclear blasts and local earthquakes do not seem to agrec as well with the 1965 MM intensity values as the Olympia data do except in a very general sense; that is, the highest response value derived for the West Seattle area from the limite,' data available is at Harbor Island which experienced higher shaking damage in the 1965 earthquake than did the West Seattle Area Figs.10,and 11..
In many cases, the noise spectrum at a response site was amplified above the corresponding reference spectrum.
Prominent peaks were apparent at several sites and remained prominent at different times of day and en different days. These results suggest that microseisms are a possiMe fourth l
source of seismic energy for ground response measurements.
i Refraction and reflection lines were run at sites to help determine 1ccal i
subsurface geologic structure, to determine near surfacs variability of seismic velocities, and to establish the velocity of bedrock.
The highest velocity ebserved, about 8,500 ft/sec, was also the velocity of the exposed bedrock unit at Seward Park. The velocity of the surface soil layer was less
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than 1000 ft/see and the velocity of the intermediate till 1syers ranged from 2,400 to 5,100 ft/sec.(Fig.6).
The low rise building testing established the range of the period and damping paramet.ers of one story houses and chimineys in the West Seattle area.
The predotinant frequency of the dweilings ranged from 5.4 Hz to 14.8 Hz. and the chimneys ranged from 6.2 Hz. to 13.7 Hz. The building dampings varied from 2.54 to a high of 64 of critical, i
The microseismic data, reflection data, refraction data and past intensity data all suggest interesting correlations to the site respons,e values; however, the amount of site response data and the number of sites under study are too small to make any but preliminary conclusions or trends. A basic conclusion is that a larger dar.a set from ground motions induced by earthquakes are needed to continue and complete the study.
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