DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY … · U.S. GEOLOGICAL SURVEY Santa Cruz Seismic...

DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Santa Cruz Seismic Investigations

Following the October 17, 1989 Loma Prieta Earthquake

by

K. King1 , D. Carver1 , R. Williams 1 ,

D. Worley1 , E. Cranswick1 , and M. Meremonte1

Open-File Report 90-307

This report is preliminary and has not been reviewed for conformity with U.S, Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Golden, Colorado

1990

SANTA CRUZ SEISMIC INVESTIGATIONS

FOLLOWING THE OCTOBER 17, 1989 LOMA PRIETA EARTHQUAKE

BY

King, K., Carver, D., Williams, R., Worley, D., Cranswick, E., Meremonte, M.

INTRODUCTION

Ground shaking induced by the aftershocks from the October 17,1989 Loma Prieta, California, earthquake provided a unique opportunity to study the effects of the ground motions induced by earthquakes and to contribute to the on-going process of developing methods to mitigate damage from earthquakes. This report presents results of a preliminary inspection of the seismic data acquired in the Santa Cruz, Calif., area during the Loma Prieta earthquake aftershock sequence. The report shows site information, seismic time histories (seismograms), and response spectra derived from data recorded at selected sites during the aftershock sequence. The data in this report will be useful to other researchers working with the complete data set, which will be available on a read-only compact disk in the near future.

Thirty-six sites in the Santa Cruz, Calif., urban area were occupied with portable seismic systems during the aftershock sequence. The selection of sites in the urban area took into consideration the site geology and spatial locations. The seismic systems were meticulously calibrated and impressed with accurate world time to produce high-resolution data for additional future studies. The objectives of the instrument deployment were:

Comparison of Aftershock Seismograms With Main Shock Seismograms

The comparison of the amplitude, phase, and spectral content of the aftershock ground motions with amplitudes, phase, and spectral content of the ground motions from the main shock will allow several scientific investigations, including source investigations. To achieve this objective, portable seismic systems were co-located with seismic instruments that recorded the main shock at the University of California, Santa Cruz, Saratoga, Hall's Valley, and Moss Landing Thermal sites.

Urban Area Site Response

To achieve this objective, a total of 31 sites were located on various geologies, topographic relief, and areas which had different degrees of damage from the main shock within the Santa Cruz, California urban area.

Basin Seismic Model

Eleven sites were located on a general east-west line across the San Lorenzo River alluvial basin in the Santa Cruz urban area. Several aftershocks were recorded at these sites. The data show the variance in the ground response and may, through ray-tracing, show a subsurface model of the basin.

Topographic EffectsSix sites were instrumented with portable seismic systems located across an area of

high relief in the northwestern part of Santa Cruz. The data may show a ground motion variance due to topographic relief.

Refraction Study

Shallow subsurface seismic refraction profiles were made at ten urban sites in Santa Cruz. Shear wave profiles were made at seven of the ten sites. The data will supplement the site data for the site response study and will be useful for developing near-surface velocity values for other studies.

Chimney Damage Study

A large portion of the urban residential structures were visually inspected for degree of chimney damage to supplement the damage-intensity investigations.

Field Data Consolidation SystemThe project field tested and experimented with a new field-data-consolidation system

using off-the-shelf PC systems. A PC-oriented processing system which consolidates field data from portable seismic digital systems was used to facilitate the data-storage management and data inspection during the field operation.

ACKNOWLEDGMENTSAny field operation such as this is deeply indebted to the residents, and city, county,

and State personnel.Even though they were in a sense "in harms way", we received complete cooperation

and support from all. A special thanks is due to Armand Levesque, schools, Ruby Quintanar, parks, and Larry Erwin, public works, of the city of Santa Cruz.

All efforts were in cooperation and coordination with various U.S. Geological Survey researchers under specific coordination of Paul Spudich, Tom Holzer and Kaye Shedlock. A Tarr (USGS) supplied base maps and integrated data into a GIS system. Authors are USGS employees Denver, Co.

DATA ACQUISITION

Aftershock Data

Ground shaking induced by the aftershocks of the Loma Prieta earthquake were recorded by portable digital seismic systems at 36 sites in the Santa Cruz urban area (figs. 1 and 2). The seismic systems used triaxial velocity-sensing transducers that have a natural period of 0.62 second and damped at 60 percent of critical. The data are digitally recorded on magnetic tape at 200 samples per second per channel.

Digital recordings of the induced ground motion from thirty aftershocks were recorded at four or more locations in the city of Santa Cruz and vicinity (fig. 3). Between five and eleven seismic systems per study were installed at temporary locations in the Santa Cruz city area. The seismometers were leveled, oriented to magnetic north, and calibrated for each event using standardized procedures (Carver and others, 1986). All seismometers were either buried and tamped approximately 0.2 - 1.0 m deep or attached to a firm, well coupled surface with epoxy cement. The procedure for triggering (turning on) the recording system to record the induced ground shaking from the aftershocks was to use a long term average-short term average (LTA/STA) algorithm within the field system. The system's internal control is set to activate the recording system for a 30 s duration when the STA = 0.2 s data signal exceeds the LTA = 6.6 s by a ratio of approximately 3-6 dB. The system is continuously active and buffers 6.6 s of data which are recorded with at least 30 seconds of data following triggering. The field systems' clocks were set with a master-clock which was synchronized with UTC. Time corrections have been entered into the data base and are included with the data that will be distributed. Most times are considered to be ± 0.003 s in accuracy. Some of the peak motions produced clipped traces due to overloading the amplifiers. These are shown on the various figures by an asterisk. The clipped data traces are included since the signals contain such useful information as duration, phase and arrival times, and relative amplitudes.

Sites were selected on the basis of four criteria:1 The spatial distribution of sites was according to a line distribution across the

study area.2 The site was at or near the location of an intensity observation or a recording

of the large ground motions from the main shock of the Loma Prieta earthquake.

3 The surficial geology of the site was known.4 The site could accommodate future subsurface investigation by drilling or by

short seismic-reflection and -refraction lines.The surficial geology of all sites is known from Clark (1981). Twenty locations were

located on various thickness of marine terrace deposits (Qm), which is defined as unconsolidated moderate-yellowish brown fine sand and granular gravel. Fourteen locations were on alluvium (Qal), unconsolidated gravel, sand and silt. One site was on metasedimentary rocks (Sch), mainly pelitic schist and quartzite; one site was on the Purisima Formation (Tp) a thick-bedded yellowish-gray tuffaceous siltstone with interbeds

of bluish-gray fine-grained andesitic sandstone; one site was located on marble which contains interbedded schists, and one site was located on the third floor control room of the thermal electrical generating plant at Moss Landing (fig. 2, table 1).

Refraction Data

A 12-channel digital-seismograph with 8-Hz (resonant frequency) geophones was used in this study to acquire compressional- (P-wave) and shear-wave (5-wave) seismic-refraction profiles at 10 sites in Santa Cruz (fig. 2). Compressional P-wave ground motion was induced by either an 8-lb sledgehammer impact on a steel plate placed on the ground surface, or a buried explosive charge equal to about 1/12- to 1/3- Ib dynamite. The explosive shot holes were 1.0-1.5 m deep. The explosive charges were used at two sites (B and F of table 3) where long geophone arrays could be used and where high seismic-velocity subsurface (bedrock) was not detected using the sledgehammer. 5-waves were produced using sledgehammer impacts against a timber oriented perpendicular to the geophone array and placed under the wheels of a car or truck. To facilitate identification of the S-wave, two 5- waves were recorded, one each from hitting opposite ends of the timber to reverse the polarity.

Three reversed seismic refraction profiles were obtained at each site except for sites B and F. The seismic source for the profiles was offset from the nearest geophone by 3.05, 15.2, and 30.5 m, combined with a 3.05-m geophone interval. The available space at sites B and F allowed additional profiles with 220 m between the explosive charge and the farthest geophone and a 9.1-m geophone interval.

The slope-intercept analysis method was used for the preliminary analysis with the first-arrival time visually determined from the seismograms. Plotting the first-arrival times against the offset distance produced a travel-time graph from which the seismic-velocity structure versus depth was determined.

Chimney Damage

The approximate degree of damage to 1,950 chimneys was documented to supplement the intensity studies. Figure 4 indicates the spatial distribution of the damaged chimneys in the city of Santa Cruz. The inventory does not include the business-mall sections of the city since most of the damage of those areas was extreme and the structures were not similar to the urban areas. Data were obtained by an actual count of residential homes.

DATA PROCESSING

Data consolidation and preliminary reduction was accomplished in the field during operations. The field computer system used to perform processing of the data from the portable digital seismographs deployed in Santa Cruz is illustrated in figure 5. The hardware consisted largely of IBM PC-compatible microcomputer components which were

installed in a local motel. The waveform analysis, and data consolidation software had been developed for the Armenia Project of the Eurasian Seismic Studies Program (Cranswick and others, (1989). The digital cassette playback system was connected by an RS-232 port to a PC/AT-compatible microcomputer equipped with a 40-Mb hard disk. The data for each event were stored in an ASCII file which was data-compressed and then transferred to a larger and faster microcomputer (IBM PS/2,20MHz, Model P70) for data consolidation and storage. The latter computer was equipped with a write-once-read-many (WORM) optical disk drive for archiving the raw data, a 120-Mb hard disk for interim data consolidation, and a laser printer/plotter. A portable laptop computer was used to calculate spectra and spectral ratios of the seismograms. Earthquake-summary information, which was downloaded via modem from computers at USGS, Menlo Park, was correlated with the field records.

DISCUSSION

Aftershock Data

Thirty-two sets of seismograms (particle-velocity, time histories) from the documented data are shown (fig. 6-42). Table 1 gives the locations of each site and the events that were recorded at that site. Table 2 gives the location of the aftershocks. Table 3 shows the compressional and shear wave velocities recorded at locations A-J shown on figure 2.

The earthquake seismic time-histories shown are derived from the recorded digital data. The time length (15 seconds), filtering (20 Hz., 4-pole, low-pass), and component orientation are identical on all of the figures. The amplitude scale bars allow visual comparison. Figures 8, 11, and 14 show comparisons of the sites located at or near California State strong-motion instrument locations. Figures 12-23 show seismograms from sites located near areas of low damage compared to those located near areas of high damage. Figures 27 - 37 show the seismograms from an east-west line which transected the alluvial basin in Santa Cruz, Figures 40 - 42 show the seismograms from the east-west topographic line. All of the events were located 10 - 30 km from central Santa Cruz except for the event represented in figure 42 which was located on the Hayward fault approximately 50 km from the city of Santa Cruz.

Figures 43 - 49 show examples of spectra derived from the digital data. The vertical and horizontal scales are identical on all of the sepctra to allow visual comparisons. For each event at each site, a 10-s time duration of digitally recorded ground motion was selected for analysis, centered on the largest 5-wave amplitude. The window was tapered with a whole-cosine bell (Hanning window) before being transformed by a standard Fast Fourier Transform (FFT) program. It was not necessary to normalize spectral amplitudes for window length because all spectra in this study were derived from time series of identical duration. The spectra were smoothed by a moving 5-point averager in each of 90 equal- frequency band-widths from 0.5- to 30-Hz.

Figures 43 and 44 show the spectra at sites located near sites which recorded the main shock, (LOE and FRE), and areas of low-to-moderate damage. Figures 54, 46, and

47 show the spectra of sites located on the east-west basin line across Santa Cruz. Sites BAR and KLA are near or on rock sites; TRE is on the marine terrace; and WAL through BAS are on varying depths of alluvium. Sites WAL> BLA, and CE2 in figure 4 were in areas of heavy damage from the Loma Prieta earthquake. The spectral amplitudes are greater in the areas of heavy damage and sites located on the alluvium basin and less in those areas underlain by the older and consolidated materials.

Refraction

Refraction profiles were made at 10 sites in Santa Cruz. A summary of the preliminary interpretation of the seismic refraction data is presented in table 3. Three distinct seismic-velocity layers in the 0- to 20-m-depth range below the surface were detected at all sites except where noted in table 3.

The P-wave seismic velocity (Vpl) of the near-surface layer ranges from 155 m/s to 338 m/s; the layer is only about 1- to 3-m thick. This layer probably represents the dry, aerated loose soil horizon at each site and 80% of the sites had a Vp less than the Vp of sound in air at sea level.

Layer 2 is immediately below layer 1 and had a Vp range of 623 m/s to 2459 m/s with a wide range of thickness. Sites with a Vp2 of between 1500 m/s and 1675 m/s indicate the presence of an unconsolidated saturated material (sand, clay, and silt). At sites A, B, E, and F the base of layer 2 was not detected.

The Vp3 at all sites is equal or larger than Vp2 while the Vp3 at sites C, D, E, G, and J was 2,500 m/s or greater suggesting the detection of bedrock at these sites. At site G, the refraction profile was located such that the line was perpendicular to the cliff face and started about 10 m from its base. In the cliff face, an indurated sandstone rock was exposed; we believe the 4,157 m/s Vp3 at this site suggests detection of this sandstone in our refraction profile at about 14.5 m depth 50 m south from the base of the cliff. In table 3 a question mark following the thickness estimate means that the value is a minimum thickness for that layer and that the base of that layer was not detected. At sites A, B, H, and I, a material with a bedrock velocity was not detected.

The 5-wave velocity (Vs) of layer 1 ranged from an extremely slow 76 m/s and 91 m/s at sites F and G respectively to 198 m/s at site A. At site G, this extremely low Vs was maintained for a depth interval of almost 4 m. These two sites, plus site B, which are located in the San Lorenzo River flood plain (B, F, and G), show that the V5 does not increase much in the 15 m depth range targeted in these profiles. At site F layer 2 is about 73 m thick and has a velocity of 111 m/s with an increase to 168 m/s for layer 3. The S- wave data recorded at sites located outside of the San Lorenzo flood plain show an increase in Vs at much shallower depths than the flood-plain sites, and eventually reach velocities (above 600 m/s) which were not detected at the flood-plain sites. Site G show a Vs of 497 m/s, but because this site was located near the cliff close to the high-velocity sandstone, the higher Vs is not surprising.

Chimney Study

The general distribution of the chimney damage may indicate a microzonation of damage/or ground motion. Approximately 3,890 homes were visually inspected. Of the homes inspected, 1,356 had chimneys with no apparent visual outside damage and 594 had chimneys that were severely damaged. The homes on the marine-terrace deposits in the northwest part of the town seem to have more chimney damage than the homes located on similar geology in the southern and eastern part of the city fig. 4).

REFERENCES

Carver, D.L., Cunningham, D.R and King, K.W., 1986, Calibration and acceptance testing of the DR-200 digital seismograph: U.S.Geological Survey Open-File Report 86-430, 28 p.

Cranswick, E., King, K.W. and Banfill, R., 1989, A program to perform time and frequency domain analysis of vector timeseries' recorded by portable autonomous digital seismographs: U»S. Geological Survey Open-File Report 89-172, 62 p.

8

Table 1 Station Locations

CODE

ALMBARBASBESBLACBLCE2CEDCENDIGEFFESCESTGILHIGHILHLUHOLKALKEYKINLAULAVLOEMAIMAYMONNOBNPAPACROSSBRSHETREWALWAS

ADDRESS

407 Almar AveGlenn Coolidge Dr313 Barson St126 Plymouth St223 Blackburn St738 Cable St121 Cedar St205 Cedar St809 Center St224 Dickens Wy126 Effey St110 Escalona Dr621 Highland Av116 Gilbert Ct548 Highland Av139 Hillcrest Ter1504 Laurent St125 Hollywood Av112KalkarDr215 Keystone1722 King StSe comer Laurel & Front St105-111 Barson StU.C. Santa Cruz315 Main St225 May Av,137 Monterey St340 Nobel Dr2117 N Pacific AvPacific & Soquel721 Highland Av239 Branciforte Av122 Sherman St154 Trescony St116 Walti St333 Washington St

LATITUDE(N)

36.954936.981136.971036.986436.967936.970236.968236.968736.974436.979836.976036.977036.97%36.990236.980036.97%36.980036.974736.978836.985536.%3936.%9336.970536.000536.966036.977836.957536.970536.979736.973436.979336.971936.972236.965936.968036.9683

LONGITUDE(W)

-1210421-1210530-1210180-1210209-1210303-121.9944-1210250-1210250-1210281-122.0482-1210086-122.0323-122.0377-1210079-1210364-122.0323-122.0418-122.0367-122.0435-12100%-122.0461-1210220-122.0205-122.0561-122.0213-122.0179-122.0263-122.0515-122.0262-1210247-122.0399-122.0126-1210388-1210361-122.0322-1210273

ELEft

4137015402140101018365631002601722582452709526092858880045224224012102656090652518

GE(

QmmQalQalQalQmQalQalQalQmQmQmQmQmQmTpQmQmQmQmQmQalQalschQmQalQmQmQalQalQmQmQmQmQalQal

Table 1. Santa Cruz seismic station locations and surface geology. Qal - Alluvium. Holocene, unconsolidated gravel, sand and silt. Qm = Marine Terrace Deposit, Pleistocene, unconsolidated moderate fine sand and granular grave. Tp = Purisima Formation. Upper Miocene to Pliocene, thick bedded tuffacious siltstone with interbeds of fine-grained andesite sandstone, sch » Metasedimentary rocks. Mesozoic or Paleozoic, mainly pelitic schists and quartzite. m = Marble. Mesozoic or Paleozoic contains interbedded schist Elevation in feet, 1 foot - 0305 m. Coordinates reference to 1927 RA. datum.

Table 2 Aftershocks documented

Fig.

6789101112131415161718192021222324252627

282930313233343536373839404142

UTCday-hr-min

294-00-11294-00-49294-00-49294-01-02294-08-32294-12-54294-22-14295-00-31295-14-24295-14-24296-21-27297-04-48297-07-02297-08-56297-14-45297-22-26298-01-27299-09-01299-12-18299-12-18299-13-26302-13-10

302-13-10302-13-10302-13-10302-20-44302-21-55303-04-52303-06-31303-11-17304-06-50304-08-34306-05-12306-10-11307-10-48308-00-20308-07-16

LOCATION deg-min deg-min

DEPTH SIZE km coda

37-06.1137-02.0137-02.0137-02.7037-06.4937-08.0437-02.9637-033536-58.7336-58.7337-06.7537-02.4737-10.0337-103937-043737-103837-03.7737-02.1237-03.1437-03.1437-02.7437-02.62

121-58.53121-5238121-5238121-51.59122-0037121-5722121-53.65121-54.43121-49.57121-49.57121-55.98121-48.94121-57.92121-58.07121-54.97121-58.17121-50.18121-53.71121-52.58121-5158121-54.84121-5525

13.811.911.910.713.44313.012315.615.6824.85.75.712.64310.011.411.111.1152143

2.54.14.12.62.73.14.42^3.73.73.0322&252325423.6(22 as(22 as(2.4 as2.9

(as No. 23, vertical component only)(as No. 23, horizontal-EW component only)(as No. 23, horizontal-EW component only)37-03.5137-04.4137-032736-55.0337-03.1137-07.5837-03.7037-03.7037-03.7736-55.2737-063337-4635

121-54.09121-5431121-5422121-41.90121-49.40121-59.85121-48.62121-48.09121-55.53121-4425122-04.15122-2222

13.810214.511.79.91228.47.813.410.111.87.1

222.92^2.93.6223.42^233.12.022

* day is in number of days in year; October 21 » 294

STATIONS RECORDED (code rentable 1)

LOE,DIC,ESC,BESLOE,DIC,ESC,BESLOE,HAL,SAR,FRE,CO1LOE£>IQESC,BESLOEJMQESCJBESLOE,DIC,ESC,BES,FRE,CO1KAL,HOUPAC,CED,HAL,SAR,CO1,FREKAL,HOUPAC,CEDKAL,HOL,PAC,CEDKAL,SAR,HAL,FRE,CO1LOE,LO2,KIN,SHE,PACLOE,LO2,SHE^ON,PAC,CEDLOE,L02,SHE,KIN,PAC,CEDLOE,LO2,SHE,KIN,PAC,CEDLOE,LO2,SHE,KIN,PAC,CEDLOE,SHE,KIN,CED,BASBAR,KAL,SHE^ON,ALMIX>E,BAR,KALALM,MON,MAI,BAS

No. 20) No. 22) No. 21+EFF)

BAR,KAL,TRE,WAL3LA,WAS,CE2,IAU,LAV,BAS,SBR

BAR,KAL,TRE,WAL,BLA,CE2,LAV,BAS(as No. 27+LAU)(as No. 28+SBR)(as No. 29)(as No. 29)(as No. 29)(as No. 29)BAR,GIL>MAY,CBL,KEY,HIL,NOB)NAP,CENCEN,GIL,HIL^1AY,NAP,CBUKEYBAR,KAL,HLU,ROS,HIL,NOB,CEN,MAY(as No. 36)KAL,EST^nG,HIL,CEN,MAY,NOB

TABLE 3. Shallow seismic refraction results in Santa Cruz.

10

Site Code

ABcV-*

DE F G H I T

Site Code

AB C D E F G H I J

P-v/ave dataXT TT XT TT

pl ^J>1 P^ ^T*2

m/s m m/s m

338 1.8 1593 23? 229 3.8 1622 10? 213 1.0 1524 5.8120 20 Q51 S2J£i\J A».V/ sJJ J*£*

204 1.8 2459 9? 168 1.5 1631 70? 155 1.1 1290 13.4 169 0.6 623 1.2 381 1.8 726 0.9O1^ 1 1 1 &TI1 *J CZ1J l.j 10 /J 3.J

Location

Bay View Elem School County Admin. Bldg Garfield Park Lighthouse Park Mission Hill School Neary Lagoon Santa Cruz High School Star of the Sea Park Trescony Park Westlake Park

VP3

m/s

17682477*»~ / / 104Q«?V^T7

3049

4157 1642 1524 2QQ4A»77^

5-wave dataTT \7 TT \/ TT \T TT*lp3 si '^^1 s2 "^^2 s3 '^s3m m/s m m/s m m/s m

198 2.7 378 3.7 628 >14? >60? 137 3.0? ? ?

>10? 127 0.6 152 2.1 889 >12? 76 2.4 111 7.3 168 >11?

> 7? 91 3.7 137 8? 497 > 4? >15? 122 0.9 595 >15? >14? 110 1.0 300 5.5 701 >10?

X^/ ^A\y O WvlVV^ V«OLO

Definition of terms use in above table

Vp! = P-wave velocity of layer at the surface. Hpt = Thickness of layer with Vpl. Vp2 = ^-wave velocity of layer below H^. Hp2 = Thickness of layer with Vp2. Vp3 = P-wave velocity of layer below Hp2. Hp3 = Thickness of layer with Vp3.

Vsl = 5-wave velocity of layer at the surface. H^ = Thickness of layer with Vgl. V^ = 5-wave velocity of layer below H^.

Thickness of layer with V^. S-wave velocity of layer below Thickness of layer with V^.

H

38

SANTA CRLg (study area)

122

Figure l.~ Location map showing Santa Cruz study area, main shock, and general outline of aftershock zone.

PACIFIC OCEAN

UPPER yiOCENE TO PLIOCENE SEDIMENTARY SEQUENCE

:':: ALLUVIUM

!.... RIVER TERRACE DEPOSITS ^

KB! MARINE TERRACE DEPOSIT

PURISIMA FORMATION

SANTA CRUZ MUDSTONE

SANTA MARGARITA SANDSTONE

CRYSTALLINE PLUTONIC AND METAMORPHIC ROCKS

'-:-'. QUARTZ DIORITE

WE GRANITE AND ADAMELLITE .

,-' METASEDIMENTARY ROCKS

MARBLE .,t naaras

ZIOLC

Figure 2.-- Surficial geologic map of the city of Santa Cruz showing primary geologic units, DR-200 recording stations (3 letter code), and seismic reflection/refraction recording sites (letters A-J).

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2148

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1641

2032

22 373835 " "

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05' 122* 55 Longitude45'

Figure 3. Epicentral locations (black squares) of earthquakes included in this report. The number above the square indicates the figure number which shows the seismograms for that aftershock (see table 2). Approximate epicentral distances described in figures 6-42 were measured from location SC in Santa Cruz. Earthquake locations for figure numbers 8,15,19 and 25 are not shown in this figure as they are essentially co-located with earthquakes 7,14,18 and 24, respectively, Earthquake 42 is located outside the boundary of this map on the Hayward fault.

14-

2 K1LOUTDQ

Figure 4. Santa Cruz map showing surficial geology and spatial distribution of damaged chimneys. Numbers refer to number of damaged chimneys per city block. Geologic units as show on Fig.2.

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Figure 12. Earthquake time-history. l=vertical component; 2s=horizontal (NS) component; 3=horizontal (EW) component; *=clipped data. Epicenter is approximately 14 km Northeast of central Santa Cruz. Sites in areas of low to high damage from the Loma Prieta earthquake.

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Figure 16. Earthquake time-history. l=vertical component; 2=horizontal (NS) component; 3=horizontal (EW) component; *=clipped data. Epicenter is approximately 17 km North of central Santa Cruz. Sites in areas of low to moderate damage from the Loma Prieta earthquake.

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DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY … · U.S. GEOLOGICAL SURVEY Santa Cruz Seismic...

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Transcript of DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY … · U.S. GEOLOGICAL SURVEY Santa Cruz Seismic...