U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

IMAGING AND GROUND MOTION IN URBAN SEDIMENTARY BASINS: 

THE SANTA CLARA AND SAN BERNARDINO VALLEYS

by

Stephen Hartzell, Robert Jachens, Thomas Brocher, and Arthur Frankel

U.S. Geological Survey Open-File Report 98-244

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any 

use of trade, product, or firm names is for discriptive purposes only and does not imply 

endorsement by the U.S. Government.
Introduction

Following the 1994 Northridge earthquake in Los Angeles, California, and the-1995 

Hyogo-ken Nanbu earthquake in Kobe, Japan, ground motion records became available 

that demonstrated the importance of deep basin structure for strong-ground motion and 

its correlation with observed damage patterns (Iwata et al, 1996; Pitarka et a/., 1996; 

Kawase, 1996; Hartzell et al, 1997; Wald and Graves, 1998; Graves et al, 1998). The U.S. 

Geological Survey Earthquake Hazards Program Five Year Plan 1998-2002 (Page et a/., 

1998) recognized the significant hazard and risk that earthquake ground motion in urban 

basins presents, and called for a workshop to choose an earthquake-prone urban basin as 

a focus for future investigations. On April 22, 1998 a workshop was held at the Menlo 

Park, California, office of the U.S. Geological Survey for the purpose of selecting an urban 

sedimentary basin to conduct 3D wave propagation and ground motion studies. That 

workshop chose the Santa Clara Valley as the primary study area and the San Bernardino 

Valley the secondary study area. These selections were later endorsed by the NEHRP 

Earthquake Program Council.

The purpose of this report is to give a brief review of our current knowledge of the 

structure of these two basins and the work that is needed to better understand the ground 

motion hazards that exist. Figures 1 and 2 show the probabilities of exceeding the current 

design criterion in a 30 year period for rock sites in the Santa Clara and San Bernardino 

Valleys, respectively (Frankel et a/., 1996). Effects of the sediment-filled basins are not 

considered. Both areas show a significant hazard, with San Bernardino being higher than
Santa Clara. The Santa Clara area, however, has a higher risk due to a greater population 

density and industrial development.

Basin Structure Information

Santa Clara

Brocher et al (1997) have developed a detailed 3D velocity model for the entire San 

Francisco Bay area. The Santa Clara Valley is a section of this model. The data used 

to construct the model come from a variety of sources: geologic considerations, seismic 

reflection and refraction, gravity inversion, water wells, and drill-hole logs. The major 

units are: 1) mantle, 2) mafic lower crust 3) basement middle crust (Franciscan complex), 

4) basement middle crust (Salinian plutonic and metamorphic), 5) Cretaceous and Tertiary 

sedimentary rocks, 6) Pliocene and Quaternary sediments. Within the basement and 

overlying units the model velocities are gradients. For the region of the Santa Clara 

Valley, the model includes a detailed surficial geologic classification as well. There is no 

topography in the model. Figures 3, 4, and 5 show the locations of water wells, USGS 

drill-holes, and refraction lines, respectively, that are available for the Santa Clara Valley, 

and have been used to construct the velocity model. The USGS drill-holes (Gibbs et a/., 

1975; 1976; 1977; 1992) are logged for P- and S-wave velocities and have depths from 30 to 

100m. Figure 6 shows the estimated depth to pre-Cenozoic basement (either Franciscan, 

coast range ophiolite, or Salinian granitic/metamorphic). Primary features are two deeper 

parts of the basin, the Cupertino Basin along the west edge of the valley and the Evergreen 

Basin along the east edge. The valley is bounded on the west by the San Andreas and
Monte Vista Faults and on the east by the Calaveras/Hayward fault system. How seismic 

waves from sources on these faults are affected by the sedimentary basin is a primary 

research objective.

A second velocity model of the area has been developed by Antolick et al (1997) and 

Larsen et al (1997). This model has been used to model the Loma Prieta earthquake 

(Antolick et al., 1998) and scenario earthquakes on the Hayward Fault (Larsen et al., 

1998).

San Bernardino

Less structural information is available for the San Bernardino basin. What is known 

is based primarily on gravity data and a few wells that reach basement. Figure 7 shows the 

depth to pre-Cenozoic basement (Pelona Schist or San Bernardino Mtns. granitic/metamophic) 

based on gravity inversion. The San Bernardino Valley is bounded by the San Andreas 

Fault on the north side and the San Jacinto and Rialto-Colton Faults on the south- side. 

Wells that reach basement are given in Table 1 (Butcher and Garrett, 1963).
Table 1

San Bernardino Well Information

Latitude

34.0735

34.0705

34.0816

34.1363

34.0740

34.0770

34.0114

34.0018

34.0576

34.0646

34.0583

34.0953

Longitude

-117.1375

-117.1287

-117.3469

-117.2729

-117.1118

-117.0796

-117.2674

-117.0994

-117.1658

-117.1400

-117.1253

-117.3959

Depth to Basement

(ft)

390.

210.

1185.

330.

200.

100.

213.

5358.

213.

470.

250.

371.

Table 2

Rialto-Colton 1000 ft Sonic Logs

Latitude Longitude

34.0914 -117.2349

34.0800 -117.2359

34.0606 -117.2238

34.0804 -117.2216

34.0559 -117.1945

One refraction line was done in the San Bernardino Valley by Hadley and Combs 

(1974) along the Santa Ana River channel. However, their interpretation of this refraction 

line is inconsistent with the gravity inversion in Figure 7. Both show the deepest part of 

the basin to be at its southwestern edge near the merging of the San Jacinto and RialtoColton Faults. Gravity, however, shows a rapidly shallowing basin to the northeast and 

the refraction line suggests the basin remains deep.
In addition, six 1000 ft holes have been drilled in the Rialto-Colton basin and sonic 

logs obtained (Table 2) (Linda Woolfenden, personal commmimication, 1998). The RialtoColton basin lies just to the west of the San Bernardino Valley (Figure 7). Water wells have 

also been used to infer geologic cross sections through the San Bernardino Valley basin 

by Izbicki et al. (1997) and Danskin et al. (1998). Initial shallow P- and S-wave velocity 

measurements have been taken in the San Bernardino Valley using the seismic refraction 

method of Williams et al. (1997). These velocity profiles are judged to be accurate to a 

depth of 30m. Figure 8 shows the sites where these measurements have been made.

Ground Motion Availability

Earthquake ground motion recordings are needed to test modeling codes, validate 

predictions of ground motion, and update our knowledge of the 3D velocity structures.

Santa Clara

Figure 9 shows the strong-ground motion stations that recorded data for three recent 

earthquakes: 1989, MW 6.9 Loma Prieta; 1984, Mz,6.2 Morgan Hill; and 1979, ML 5.8 

Coyote Lake. In addition to these mainshock records, the Loma Prieta earthquake provided 

a significant number of aftershock records in the Santa Clara Valley. Figure 10 shows the 

aftershock station locations (Mueller and Glassmoyer, 1990) in and near the valley. The 

data from many of these stations have been used to estimate local site response (Boatwright 

et al., 1991; Fletcher and Boatwright, 1991; Hartzell, 1992). A dense array in Sunnyvale 

also recorded several aftershocks and was used to study basin surface waves (Frankel et al., 

1991).

6
Three-dimensional wave propagation modeling in the Santa Clara Valley is limited. 

Frankel and Vidale (1992), used a simple two-component velocity model (sediment and 

bedrock) based on water wells to propagate elastic waves through the valley for a magnitude 

4.4 aftershock of Loma Prieta. Recently, Antolik et al (1998) have done a 3D numerical 

simulation of the Loma Prieta mainshock using a more elaborate velocity model.

San Bernardino

Figure 11 shows the location of strong motion instruments in the San Bernardino 

Valley. These stations recorded the 1992, Mw 7.2 Landers earthquake and the 1991, Mw 

5.6 Sierra Madre earthquake among other smaller events. In addition, there is a recently 

completed 100m deep drill-hole at the San Bernardino Central Fire Station (34.1052, - 

117.2798) (Rogers et al., 1998). This hole is instrumented with both surface and down-hole 

broadband seismometers. Also, many aftershocks of the Landers and Big Bear earthquakes 

were recorded on three dense arrays deployed in the San Bernardino Valley (Frankel, 1994).

Three-dimensional waveform modeling in the San Bernardino Valley is more limited 

than in the Santa Clara Valley. Studies by Frankel (1993; 1994) used alluvium and basement units and a depth to basement based on limited well information.

Future Work

Imaging

The relevant properties of alluvial basins must be well specified before their seismic responses can be accurately calculated. The properties include: 1) the geometry of the basin

7
(the shape of the sediment/basement interface as well as structures within the sedimentary 

column), 2) the velocity structure, 3) attenuation, and 4) the location and orientation of 

significant faults. In other words, spatial seismic velocity changes are of particular interest, whether at the basement interface or within the basin itself, that produce a significant 

modification of the wavefield. The most efficient and direct means of acquiring this information is by active-source seismic reflection/refraction profiles. These experiments should 

be designed to maximize resolution in the upper 5 km of the crust or to the depth of 

the pre-Cenozoic basement. Both the Santa Clara and San Bernardino Valleys have very 

little data of this type. Consideration needs to be given to what field procedure is best to 

accomplish this goal, that is, reflection versus refraction, Vibroseis source versus explosion 

source, etc.

Inversion of gravity and magnetic data have also proved useful in defining basin geometry as well as the dip on faults. Langenheim et al (1997) modeled gravity and aeromagnetic 

data to help constrain the dip of the Monte Vista Fault on the western margin of the Santa 

Clara Valley. Efforts should be made to see that all available well information is included 

in these inversions to help constrain the solution.

In addition, shallow imaging of the top 30 to 100m is needed to accurately determine 

P- and S-wave velocities. These profiles are needed to calculate the local site response 

caused by shallow velocity variations and to distinguish shallow from deeper effects.

A typical finite-difference calculation to an upper frequency of I Hz may require a 

grid spacing of approximately 100m with physical properties specified at this interval. 

Resolution of large urban basins in 3D at this scale is not a practical objective. The
key to productive research will be the discovery of those structures that contribute most 

significantly to the observed ground motion, such as structures causing focusing or basin 

edge effects (Gao et a/., 1996; Hartzell et a/., 1997; Graves et a/., 1998), and to image them 

as well as possible.

Ground Motion

Ground motion measurements are vital for observing the response of an alluvial basin 

and for checking model predictions. At present, ground motion instrumentation in both 

the Santa Clara and San Bernardino Valleys is limited to a few widely spaced strong motion 

instruments. For the Santa Clara Valley there are a few additional aftershock recordings 

of the Loma Prieta earthquake. Data from many more free-field recording sites are needed 

with a spacing of approximately 1 km. The close spacing is required to resolve spatial 

differences caused by subsurface structures that have been observed in other sedimentary 

basins (Hartzell et a/., 1997). These instruments should be designed to record weak motions 

as well as strong motions to enable the recording of background seismicity (Mi,3.0). This 

flexibility will allow the instruments to be moved to new locations fairly frequently.

Modeling

The accuracy of 3D wave propagation codes needs to be checked by comparing the 

results of different algorithms. Existing 3D velocity models should be used to simulate 

both aftershock and mainshock data records to determine the weaknesses of these models. 

The velocity models can then be modified to produce better comparisons with the data.
These new models need to incorporate and be checked against new basin imaging data as 

it becomes available.

Improvements in 3D wave propagation codes are needed to make large problems more 

efficient and practical. Variable grid sizing would reduce the size of the problems by using 

larger grid spacing for regions with higher seismic velocities. Other improvements would 

include models with topography and codes designed to take advantage of multi-processor 

CPUs.

Acknowledgements

This report was significantly improved by the contribution of figures and information by several individuals: David Boore, Wes Danskin, Jim Gibbs, Vicki Langenheim, 

Doug Morton, Chuck Mueller, Paul Spudich, David Wald, Carl Wentworth, and Linda 

Woolfenden. The clarity of the report was improved by reviewers Rob Wesson and Rus 

Wheeler.

References

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2-D and 3-D models of broadband wave propagation in the San Francisco Bay region 

and north coast ranges: Seism. Res. Lett., v. 68, p. 328.

Antolik, M., Dreger, D., Stidham, C., Romanowicz, B., and Larsen, S. (1998), ThreeDimensional numerical simulation of the seismic wavefield of the 1989 Loma Prieta 

earthquake: Science, submitted.

10
Boatwright, J., Fletcher, J., and Fumal, T. (1991), A general inversion scheme for source, 

site, and propagation characteristics using multiply recorded sets of moderate-sized 

earthquakes: Bull. Seism. Soc. Am., v. 81, p. 1754-1782.

Brocher, T., Brabb, E., Catchings, R., Fuis, G., Fumal, T., Jachens, R., Jayko, A., Kayen, 

R., McLaughlin, R., Parsons, T., Rymer, M., Stanley, R., Wentworth, C. (1997), A 

crustal-scale 3-D seismic velocity model for the San Francisco Bay area, California: 

EOS, Trans. Amer. Geophys. Un., Supplement, v. 78, no. 46, p. F435-F436.

Danskin, W., Woolfenden, L., and McPherson, K. (1998), Hydrology, description of computer models, and evaluation of selected water-management alternatives in the San 

Bernardino area, California: U.S. Geological Survey Open-File Report 98-XXX.

Butcher, L., Garret, A. (1963), Geologic and hydrologic features of the San Bernardino 

area, California: U.S. Geological Survey water-supply paper 1419, 114 p.

Fletcher, J., and Boatwright, J. (1991), Source parameters of Loma Prieta aftershocks and 

wave propagation characteristics along the San Francisco Peninsula from a joint 

inversion of digital seismograms: Bull. Seism. Soc. Am., v. 81, p. 1783-1812.

Frankel, A. (1993), Three-dimensional simulations of ground motions in the San Bernardino 

Valley, California, for hypothetical earthquakes on the San Andreas Fault: Bull. 

Seism. Soc. Am., v. 83, p. 1020-1041.

11
Frankel, A. (1994), Dense array recordings in the San Bernardino Valley of Landers-Big 

Bear aftershocks: Basin surface waves, Moho reflections, and three-dimensional simulations: Bull. Seism. Soc. Am., v. 84, p. 613-624.

Frankel, A., Hough, S., Friberg, P., and Busby, R. (1991), Observations of Loma Prieta 

aftershocks from a dense array in Sunnyvale, California: Bull. Seism. Soc. Am., v. 

81, p. 1900-1922.

Frankel, A., and Vidale, J. (1992), A three-dimensional simulation of seismic waves in the 

Santa Clara Valley, California, from a Loma Prieta aftershock: Bull. Seism. Soc. 

Am., v. 82, p. 2045-2074.

Frankel, A., Mueller, C., Barnhard, T., Perkins, D., Leyendecker, E.V., Dickman, N., 

Hanson, S., and Hopper, M. (1996), National seismic-hazard maps: Documentation 

June 1996: U.S. Geological Survey Open-File Report 96-532, 110 p.

Gao, S., Liu, H., Davis, P. M., and Knopoff, L. (1996), Localized amplification of seismic 

waves and correlation with damage due to the Northridge earthquake: evidence for 

focusing in Santa Monica: Bull. Seism. Soc. Am., v. 86, p. S209-S230.

Gibbs, J., Fumal, T., and Borcherdt, R. (1975), In-situ measurements of seismic velocities 

at twelve locations in the San Francisco Bay region: U.S. Geological Survey OpenFile Report 75-564, 87 p.

12
Gibbs, J., Fumal, T., and Borcherdt, R. (1976), In-situ measurements of seismic velocities 

in the San Francisco Bay region-Part II: U.S. Geological Survey Open-File Report 

76-731, 145 p.

Gibbs, J., Fumal, T., Borcherdt, R., and Roth, E. (1977), In-situ measurements of seismic 

velocities in the San Francisco Bay region-Part III: U.S. Geological Survey Open-File 

Report 77-850, 143 p.

Gibbs, J., Fumal, T., Boore, D., and Joyner, W. (1992), Seismic velocities and geologic 

logs from borehole measurements at seven strong-motion stations that recorded the 

Loma Prieta earthquake: U.S. Geological Survey Open-File Report 92-287, 139 p.

Graves, R., Pitarka, A., and Somerville, P. (1998), Ground motion amplification in the 

Santa Monica area: Effects of shallow basin edge structure: Bull. Seism. Soc. Am., 

submitted.

Hadley, D., and Combs, J. (1974), Microearthquake distribution and mechanisms of faulting in the Fontana-San Bernardino area of southern California: Bull. Seism. Soc. 

Am., v. 64, p. 1477-1500.

Hartzell, S. (1992), Site response estimation from earthquake data: Bull. Seism. Soc. 

Am., v. 82, p. 2308-2327.

Hartzell, S., Cranswick, E., Frankel, A., Carver, D., and Meremonte, M. (1997), Variability 

of site response in the Los Angeles urban area: Bull. Seism. Soc. Am., v. 87, p. 

1377-1400.

13
Iwata, T., Hatayama, K., Kawase, EL, and Irikura, K. (1996), Site amplification of ground 

motions during aftershocks of the 1995 Hyogo-ken Nanbu earthquake in severely 

damaged zone-Array observation of ground motions in Higashinada Ward, Kobe 

City, Japan: J. Phys. Earth, v. 44, p. 553-561.

Izbicki, J., Danskin, W., and Mendez, G. (1997), Chemistry and isotopic composition 

of ground water along a section near the Newmark area, San Bernardino County, 

California: U.S. Geological Survey Water-Resources Investigations Report 97-4179.

Kawase, H. (1996), The cause of the damage belt in Kobe: "The basin-edge effect", constructive interference of the direct S-wave with the basin-induced diffracted/Raleigh 

waves: Seism. Res. Lett., v. 67, p. 25-34.

Langenheim, V., Schmidt, K., and Jachens, R. (1997), Coseismic deformation during the 

1989 Loma Prieta earthquake and range-front thrusting along the southwestern 

margin of the Santa Clara Valley, California: Geology, v. 25, p. 1091-1094.

Larsen, S., Antolik, M., Dreger, D., Stidham, C., Schultz, C., Lomax, A., and Romanowicz, 

B. (1997), 3-D models of seismic wave propagation: Simulating scenario earthquakes 

along the Hayward Fault: Seism. Res. Lett., v. 68, p. 328.

Larsen, S., Antolik, M., Dreger, D., Stidham, C., Schultz, C., Lomax, A., and Romanowicz, 

B. (1998), 3-D simulations of scenario earthquakes along the Hayward Fault: in 

preparation.

14
Mueller, C., and Glassmoyer, G. (1990), Digital recordings of aftershocks of the 17 October 

1989 Loma Prieta, California, earthquake: U.S. Geological Survey Open-File Report 

90-503, 147 p.

Page, R.A., Mori, J., Roeloffs, E.A., and Schweig, E.S. (1998), Earthquake hazards program five year plan 1998-2002: U.S. Geological Survey Open-File Report 98-143, 37

Pitarka, A., Irikura, K., Iwata, T., and Kagawa, T. (1996), Basin structure effects in the 

Kobe area inferred from the modeling of ground motions from two aftershocks of the 

January 17, 1995, Hyogo-ken Nanbu earthquake: J. Phys. Earth, v. 44, p. 563-576.

Rogers, A., Mueller, C., Tinsley, J., Gibbs, J., and Koesterer, C. (1998), Instrumentation 

of two vertical strong-motion arrays at Cerritos College, Norwalk and downtown 

San Bernardino, California: U.S. Geological Survey Open-File Report 98-XXX.

Wald, D., Graves, R. (1998), The seismic response of the Los Angeles basin, California: 

Bull. Seism. Soc. Am., v. 88, p. 337-356.

Williams, R., Stephenson, W., Odum, J., and Worley, D. (1997), High-resolution surfaceseismic imaging techniques for NEHRP soil profile classifications and earthquake 

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501, 42 p.

15
Figure Captions

Figure 1. Contours of probability of exceeding the design criterion (0.4g) in a 30 year 

period for the Santa Clara Valley. The estimates are for rock sites.

Figure 2. Contours of probability of exceeding the design criterion (0.4g) in a 30 year 

period for the San Bernardino Valley. The estimates are for rock sites.

Figure 3. Distribution of water wells in and around the Santa Clara Valley used in the 

construction of the three-dimensional velocity model of the area. (Brocher et a/., 

1997).

Figure 4. Distribution of drill-holes, logged for P- and S-wave velocity, with depths of 30 to 

100m used to construct the three-dimensional velocity model of the area. (Brocher 

et a/., 1997).

Figure 5. Shot points (large solid circles) and receiver locations (small open circles) for 

seismic surveys in and around the Santa Clara Valley used to constrain the threedimensional velocity model of the area. (Brocher et a/., 1997)

Figure 6. Depth to pre-Cenozoic basement for the Santa Clara Valley taken from the 

three-dimensional velocity model of the area (Brocher et al., 1997). Small open 

squares are gravity stations on basement outcrop. Small open triangles are wells 

and seismic control on depth to basement.

16
Figure 7. Depth to pre-Cenozoic basement for the San Bernardino Valley from inversion 

of gravity measurements (Jachens, unpublished).

Figure 8. Sites with shallow P and S-wave velocity measurements to a depth of 30m base on 

seismic refraction (solid triangles). Pilot 650m long refraction line to map shape of 

alluvium/bedrock contact (solid square) (Robert Williams, personal communication, 

1998).

Figure 9. Distribution of triggered strong-ground motion instruments in and around the 

Santa Clara Valley for recent earthquakes.

Figure 10. Distribution of aftershock recorders for the Loma Prieta earthquake in and 

around the Santa Clara Valley. (Mueller and Glassmoyer, 1990).

Figure 11. Distribution of strong-ground motion instruments in the San Bernardino Valley. 

Solid triangles (USGS), open triangles (CDMG), gray triangle on San Andreas Fault 

(TerraScope).

17
Santa Clara Valley 

Probability of Exceeding 0.4g in 30 Years for Rock Sites

-122° 12' -122° 06' -122° 00' -121° 54' -121° 48' -121° 42'

37° 24'

37° 18'

37° 12

37 06'

37° 24'

37° 18'

37° 12'

37° 06'

-122° 12' -122° 06' -122 00' -121° 54' -121° 48' -121° 42'

Bay mud

Quaternary alluvium

Tertiary and Quaternary sed. rock

Mesozoic sed. and metased. rock, Franciscan complex

Mesozoic igneous rock and schist

Figure 1
34° 12'

34° 06

San Bernardino Valley 

Probability of Exceeding 0.4g in 30 Years for Rock Sites

10km

-117° 24' -117° 18' -117° 12' -117° 06'

34° 00

-117° 00'

Rock

Pleistocene Alluvium

Holocene Alluvium

-117 24' -117 18 -117 12' -117 06' -117 00'
Water Wells

IN5 O

1 .5 01-2 345 6 7 MILESi

' 1.50 1 2 3 4 5 B 7 KILOMETERS !

am
uiara vaiiey r-iegion 

Vejocity data

SCALE 1:300000 

1.50 1-2 3 4 5 6 7 MILES

1.50 1 234567 KILOMETERS i

mm 

02 4 6 8101214 THOUSAND FEET i

 m
Shot Points and Recievers

1.50 1-2 3 4 5 6 7 MILES

02 4 8 8101214 THOUSAND FEET

 I
DEPTH TO PRE-CENOZOIC BASEMENT

10

c: 

-s 

rt>

122'15* 122' 121*45'

rv>CO

37*30'

37*15"

- 37'30'

37*15'

km

-0.1

-1.1

-2.1

-3.1-4.1

-5.1

-6.1

-7.1

122*15' 122* 121*45'

10 15 20 25 30 35 40 45 50 KM
DEPTH TO BASEMENT

117*30' 117*25' 117*20' 117*15' 11 no- il 7*05"

CO

C -5

34'10'

34*05'

34*

34*10'

34*05'

Depth Km

te*

-0.05

-0.55

-0.80-1.05

 1.30

-1.55

-1.80-2.05-2.30

-2.55

117*30' 117*25' 117*20' 117*15' 117*10' 117*05'

10 15 25 KM
T

CO

c: 

-s 

ro

oo

San Bernardino Vp & Vs Measurements - 4/98

~, _ t NTchblsofrPark " ''"M

1996 DeLorme Street AflasfDS/UL/>

Mag 12.00

Thu Apr 30 16:12 1998 

Scale 1:87,500 (at center) 

, 2 Miles

2 KM

Local Road 

Major Connector 

State Route

 * Primary State Route

- Trail

Interstate/Limited Access

Rest Area with facilities

Utility/Pipe

Railroad

Small TownLarge City

Public AirportPopulation Center

Water

River/Canal

* Intermittent River
Triggered strong motion stations

+ 1979 Coyote Lake

x 1984 Morgan Hill

< 1989 Loma Prieta

Approx boundary Qal

rte

 37

122

List of triggered stations may be incomplete

Not all triggered stations were digitized

Stations triggered in 1980 Livermore events not shown

Figure9 26
,02 - - ,03

C\J

01

ZZL 01 ,03
M.90 0 ZLL

N.OO

C\J

co

N.2L

N.90