Strong-motion program report, January-December 1981

* WESTMORLAND, CALIFORNIA, EARTHQUAKE OF APRIL 26, 1981

e ACCELEROGRAPH STATION

UNITE~~!E.§_----- ------MEXICO

JAMES G. WATT, Secretory

Library of Congress catalog-card No. 83-600616

Free on application to Branch of Distribution, U.S. Geological Survey 604 South Pickett Street, Alexandria, VA 22304 This Strong-Motion Program report gives preliminary information on the nature and availability of strong-motion data recorded by the U.S. Geological Survey {USGS). The program is operated by the USGS in cooperation with numerous Federal, State, and local agencies and private organizations. Major objectives of the program are to record both strong ground motion and the response of various types of engineered structures during earthquakes and to disseminate this information earthquake-engineering research and design community.

This report contains a summary of the accelerograms recovered from the USGS National Strong-Motion Network during the year 1981. A brief summary of USGS strong-motion data earthquake of April 26, 1981, and a revision of strong-motion data recorded during the Livermore, California, earthquakes of January 24 and 26, 1980, are included along with summaries of recent strong-motion reports, notes on the availability of digitized data, and additional general information related to the USGS and to other strong-motion programs. contains information necessarily recorded} during 1981.

PREFACE

The data summary in table 1

Ronald L. Porcella, Editor U.S. Geological Survey, Mail Stop 77 Menlo Park, California

Availability of strong-motion information and data - - - - - -

Data sources - - - - - - - - ~ - - - - - - - - - - - - - - - - - - - - - - 10

TABLES

Table 1 - Summary of 1981 accelerograms - - - - - - - - - - - - - - - - - 11 2 - Livermore earthquake of January 24, revised accelerograph data - 5

3- Livermore earthquake of January 26, revised accelerograph data- 6

Duri~ 1981 there were 125 records recovered from the USGS National Strong-Motion Network, .compared with a yearly average of 227 records for the period 197 2 to 1980 inclusive. The most significant record set obtained in 1981 was recorded during the magnitude Westmorland in the Imperial Valley, California. This earthquake triggered accelerographs at 24 USGS stations in the region and produced significant ground accelerations at the Brawley, Salton Sea, Parachute Test, and Superstition Mountain stations (see the following report).

The produced 25 records during 1981; this brings the total number of accelerograph records to more than 100 since the network was first established in February 1973. Two strong-motion stations on Mauna Loa and Mauna Kea were triggered and produced the first records from those stations since their installations in 1979 and 1980, respectively. The most significant acceleration was recorded at the Molokai Airport station during a magnitude 5.5 earthquake on March 5, 1981; the maximum horizontal acceleration was 0.19 g with a strong duration (acceleration greater than 0.10 g) of 0.3 second (see table 1, end of report).

Many of the additional records recovered in 1981 (table 1) are related to small-magnitude earthquakes near Bear Valley, in central California, and near Anchorage, in south-central Alaska.

[The following report has been abstracted from Maley, R. P., and Etheredge, E. C., 1982, Strong-motion data from the Westmorland, California, earthquake of April 26, 1981: U.S. Geological Survey Open-File Report 81-114 9, 18 p.]

A magnitude 5.6 earthquake occurred 8 km northwest of Westmorland, California, on April 26, 1981, triggeri~ U.S. Geological Survey (USGS) strong-motion accelerographs at 24 of the 30 stations located in the Imperial Valley region. The earthquake triggered all instruments less than 2 8 km from the epicenter with the nearest record from Salton Sea, 9 km north of the epicenter (fig. 1). In addition to the

1981 STRONG-MOTION RECORDS

5.6 earthquake on April

THE WESTMORlAND, CAliFORNIA, EARTHQUAKE OF APRil 26, 1981 1981

By J. C. Switzer

McJunkin and Kaliakin (1981) reported the following accelerations for the CDMG instrumentation located in Imperial Valley: Westmorland, 7 km, 0.49 g horizontal and 0.80 g (spike) vertical; Niland, 19 km, 0.19 g horizontal and 0.13 g vertical; and El Centro, 35 km, less than 0.05 g. All CDMG instruments are located on alluvium.

S-wave minus trigger times (S-t), such as those shown in table 1, have been reported for numerous earthquakes over the past several years. instruments are usually triggered during one of the first P-wave arrivals, but this is not always true, particularly when P-waves are small and lack a strong impulsive character. Westmorland earthquake, El Centro 6 had an S-t interval of from 0.8 to 1.1 seconds larger (triggered earlier) than El Centro 1, 2, and 3 at nearly equivalent distances. In other instances, no S-t exists when instruments were triggered during later arrivals. For instance, trigger times obtained from the WWVB code at El Centro 7 and 8 are, respectively, 4 and 3 seconds later than station 6 although they are at essentially equal distances. The instrument at station 6 is housed in a small fiberglass shelter located 1.2 km east of the Imperial fault while station 7 is a large corrugated for the USGS stations.

is on granite. Although

For example, during the Maximum

olle3

Figure 1.- Strong-motion stations in the Imperial Valley region during the Westmorland earthquake of April 26, 1981.

approximately 2.8 to 3.0 seconds after the PEM data was being recorded. The trigger delay after ground accelerations nominally reached 0.01 g was a little more than 1 second in each instance. If smaller earthquake recordings are of sufficient seismological and/or engineeri~ interest an additional 2.5-second PEM could be incorporated in the existing digital systems.

----vt~tJ~/'~t..r~N·NJit''',(V'I"'vo.,./{V',f"'<Mf'""""-~~"""'"""------------

Figure 2.- USGS accelerograms greater than 0.10 g from the Westmorland earthquake of April 26, 1981 (from Maley and Etheredge, 1981).

Numerous foreshocks magnitude 3 to 4 events (Caltech Seismological Laboratory) were recorded at Salton Sea on April 26 and 27 (also reported by McJunkin and Kaliakin 1981, for CDMG's Westmorland and aftershocks of

station). Maximum accelerations equalled or exceeded 0. 05 g during four of those earthquakes; the largest 0.11 g, was recorded 5 1/2 hours after the main shock. Fourteen of the aftershocks and one foreshock occurred during the instruments normal run time thus allowing the determination of S-wave minus P-wave times (S-P) and comparison with S-t times. The foreshock S-P was 2.3 seconds compared to S-t's of 1.9 to 2.2 seconds. The aftershock S-P's within the first 30 minutes of the main shock ranged from 2. 2 to 2. 8 seconds compared to S-t's of 2.3 to 2.5 seconds. This suggests the trigger delay was a few tenths of a second at Salton Sea, perhaps no larger than 0.4 second. Five hours after the main earthquake, S-P's suddenly decreased to 1.3 to 1.5 seconds for all nine measurable shocks while S-t's ranged from 1.1 to 1.3 seconds, again indicating a few tenths of second trigger delay. The later aftershocks, at least those recorded by strong-motion instruments, had migrated nearer to the recording station compared to the main event and the closely following aftershocks. Relatively local seismicity has been observed during previous earthquake sequences. For instance, seven shocks recorded at Salton Sea within two weeks after the magnitude 6.4 October 15, 1979, earthquake had small S-t times of 0.9 to 1.6 seconds, and in one instance an S-P of 1.2 seconds, (Porcella and Matthiesen, 1980}, indicating a relatively near-source area. earthquake was 66 km from the Salton Sea station.

References: McJunkin, R. D., and Kaliakin, N. A., 1981, Strong-motion records recovered from the Westmorland, California earthquake California Division of Mines and Geology Report 81-5.1, 11 p. Porcella, R. L., and Matthiesen, R. B., 1979, Preliminary summary of the U.S. Geological Survey strollt-motion records from the October 15, 1979 Imperial Valley earthquake: Survey Open File Report 79-1654, 41 p. Porcella, R. L., and Matthiesen, R. B., 1980, Strong-motion data summary, Imperial Valley earthquake of October 15, 1979 and aftershocks, in Seismic Engineering December 1979: U.S. Geological Survey Circular 818-C, p. 3-60.

REVISED STRONG-MOTION DATA, PART Ill

One of the primary objectives of the National Strong-Motion Network operated by Geological Survey is to disseminate data from ground sites and structures that have been subjected to strong motion during earthquakes. These data are published initially as preliminary peak acceleration and strong duration values for each component on the recording (see Porcella and Matthiesen, 1979, for example) and later, after digitization and processillt, as uncorrected acceleration; corrected acceleration, velocity, and displacement; and response spectra (see Brady and others, 1980, peak-acceleration value is not directly related to The epicenter of the 1979

By R. L. Porcella

for example).

April 1981:

the U.S.

Although the frequency content or duration of strong shaking, the value is readily obtained from an accelerogram and thus is widely used as a general indicator of the severity of strong shaking in studies of the attenuation characteristics of horizontal ground motion for varying earthquake magnitudes, source mechanisms, source-site distances, and recording-site conditions (Porcella, 1983).

A recent increase in real time strong-motion data at both far- and near-source distances has resulted in numerous requests from the seismological community for more detailed lists of both the recorded peak-acceleration values and source-site distances in order to utilize better these data in studies of ground motion attenuation. Tables 2 and 3 are revised lists of both U.S. Geological Survey and California Division of Mines and Geology data from the Livermore, California, earthquakes of January 24 and 26, 1980; the peak-horizontal ground acceleration recorded at each site has been rescaled to 0.001 g and is listed together with the corresponding source-site distance.

References: Brady, A. G., Perez, V., and Mork, P. N., 1980, The Imperial Valley earthquake, digitization and accelerograph records: U.S. processing of Geological Survey Open-File Report 8Q-703, 309 p. Porcella, R. L., 1983, Revised strong-motion data, in Seismic Engineering Program Report, May-August 1980: U.S. Geological Survey Circular 854-B, p. 4-8. Porcella, R. L., and Matthiesen, R. B., 1979, Preliminary summary of the U.S. Geological Survey strollt-motion records from the October 15, 1979 Imperial Valley Earthquake: U.S. Geological Survey Open-File Report 79-1654, 41 p.

AND DIFFERENTIAL GROUND MOTIONS

Differential ground motions due to horizontally propagating surface waves are of importance in determining the stresses and displacements developed in extended structures such as large mat foundations for nuclear power stations, dams, bridges and pipelines. determining the motion of a large rigid mat foundation subjected to travelling surface waves and observations made on the relative displacements of individual foundations and their importance in bridge failure.

Reference: Journal of Earthquake Engineering and Structural Dynamics, v. 8 p. 397-404.

IN-SITU MEASUREMENTS OF SEISMIC VELOCITIES AT 19 LOCATIONS IN THE LOS ANGELES, CALIFORNIA, REGION

Studies conducted in the San Francisco Bay region have shown that average shear-wave velocity can be related to quantitative estimates of ground motion such as amplification from nuclear explosions and earthquake intensity. physical properties of the geologic materials such as texture, hardness, and fracture spacing are described during geologic mapping, a method can be used to predict shear-wave velocity from descriptions of geologic units. By measuring shear-wave velocities in representative geologic units, regional maps depicting the earthquake hazard can be compiled.

These studies are presently being extended to the Los Angeles Basin and the Oxnard-Ventura areas. To date, shear and compressional waves have been measured in boreholes at 46 locations. A previous report summarized seismic and geologic data at sites 1-27. This report presents the data for sites 28-46. At each location seismic travel times are measured in drill holes, normally at 2.5 m intervals to a depth of 30 m. Geologic logs are compiled from drill cuttings, undisturbed samples, and penetrometer samples. The data provide a detailed comparison of geologic and seismic characteristics and parameters for estimating strong earthquake ground motions quantitatively at each of the sites.

Reference: U.S. Geological Survey Open-File Report, 81-399, 121 p. (see section "Data Sources").

• Inclusion of strong-motion information sources is intended as a service to our readers and does not imply endorsement of these reports by the U.S. Geological Survey.

SOIL-FOUNDATION INTERACTION

A general method is developed for

By T. E. Fumal, J. F. Gibbs, By G. N. Bycroft

and E. F. Roth

EARTHQUAKE INSTRUMENTATION

The U.S. National Workshop on Strong-Motion Earthquake Instrumentation was held April 12-14, 1981, on the campus of Westmont College in Santa Barbara, California. The workshop was organized by a steering committee appointed by the Earthquake Engineering Research Institute and the Universities Council for Earthquake Engineering Research.

Reference: Earthquake Engineering Research Institute (see section "Data Sources") and Universities Council for Earthquake Engineering Research, 55 p. and append.

FROM THE WESTMORLAND, CALIFORNIA,

A moderate earthquake occurred 8 km northwest of Westmorland, California, at 1209 (UTC) on April 26, 1981, triggering U.S. Geological Survey (USGS) strong-motion accelerographs at 24 of 30 stations located in the Imperial Valley region. The earthquake triggered all instruments less than 28 km from the epicenter with the nearest record from Salton Sea, 9 km north of the epicenter. In addition to the network operated by the Survey, the State of California Division of Mines and Geology (CDMG) obtained records from stations located at Westmorland, Niland, and in El Centro adjacent to the former site of the Imperial County Services Building. recorded by both the USGS and CDMG networks from ground-level accelerographs (ground stations, not necessarily free-field stations) located in one-story buildings or in small fiberglass instrument shelters. Most recorders were equipped with WWVB radio receivers, receiver-timer problems, real time was impressed on only half of the records.

Reference: U.S. Geological Survey Open-File Report 81-1149, 18 p. (see section "Data Sources").

U.S. STRONG-MOTION

Edited by W. D. Iwan

EARTHQUAKE OF APRIL 26,1981

By R. P. Maley and E. C. Etheredge

but due to either poor signal or

AVAILABILITY OF STRONG-MOTION

records and the circumstances in which they were recorded are available to ·anyone involved in earthquake engineering through the computer-based Strong-Motion Information Retrieval System (SMIRS). SMIRS provides ready access to information about strong-motion records and the level of processing and analysis that has Information recorded motion and about the sites at which the motion was recorded is also provided.

With an ordinary phone line and a keyboard terminal, a SMIRS user may review the information free of charge. Once accessed, SMIRS will offer a general introduction and will tell the user how to request more detailed instructions. The user also will be given an opportunity to request a copy of the printed user's manual.

SMIRS resides in one of the U.S. Geological Survey computers in Denver, Colorado. It can be accessed by telephoning the computer center directly or by telephoning a local node in the TYMNET telecommunications telephone numbers for the Denver computer center are:

(303) 232-0058 for 300 baud modems such

(303) 232-5309 for 1200 baud modems such

(303) 232-2248 for 1200 baud BELL 212a

The TYMNET telecommunications network can be used long-distance telephone charge to Denver. TYMNET Corporation maintains local telephone numbers in many cities in the United States and in several foreign countries. TYMNET phone numbers can be obtained from TYMNET Corporation's Western Customer Service at (800) 323-7389 or TYMNET Customer Support Group at (800) 366-0149.

Take the following steps to connect your terminal to SMIRS.

1) Set the switches, keys, or buttons on the terminal that allow a choice of operating modes:
2) Plug in and turn on the terminal; turn on the

STRONG-MOTION NETWORK DATA

transmission speed= 300 baud (30 characters

on line; lower case ASCII characters; and full duplex (if you are going to dial a

half duplex (if you are going to dial By April Converse

network.

per second) or 1200 baud;

Denver directly). as those included in the TI Silent 700 terminals,

as the VADIC modems,

modems.

without on them. generated

Sa) modem too if it is a separate device. Notice whether the modem uses an acoustic coupler or whether the modem is directly connected to a telephone. An acoustic coupler will have a cradle into which a telephone handset can be inserted. Look for a label or diagram on an acoustic coupler that will show you in which direction the telephone cord should go. modem will have a switch that can be set for voice or data transmission. Telephone the USGS computer center in Denver or telephone the TYMNET number nearest you; wait for a high-pitched tone. Place the telephone handset in the cradle on the acoustic coupler or set the direct-connect switch to "data." Wait for the "carrier detect" light to turn on; this indicates that the terminal is receiving a signal from the computer or from the TYMNET equipment. If you telephoned the Denver computer center directly, skip this section, but if you telephoned a TYMNET number, the TYMNET prompts (shown underlined here) and your responses (shown in italics here) should proceed as follows:

The <CR> symbol represents the carriage-return key.

The "gsbcl234" is what TYMNET refers to as a user name, the "2361" is a location identifier for the USGS computer center, and the "sandstone" is a password. Note that the "sandstone" will not be printed at your terminal while you type.

Do not be alarmed if the first prompt comes at an odd speed.

TYMNET will now connect your terminal to a computer selecting device in Denver. If the computer selector is operating, "GSD N is online" will be printed at your terminal.

Type the carriage return key. The computer selecting device will ask you to "enter class", to which you should answer "mult":

The selecting device will answer with several lines, DENVER MULTICS" and "class mult start". Type the line-feed key. The MULTICS computer will respond with several lines that will tell you which computer you have accessed, how many other users are connected, and so forth. If your terminal will transmit both upper and lowercase characters, type:

The <CR> represents the carriage-return key and <your name> is your own name typed without any embedded blanks. It is good practice for you to A direct-connect

please type your terminal identifier e

enter class rrrult <CR>

something like "CONNECTING TO choose a version of your name that will probably be unique, and to use that version of your name every time you log onto SMIRS. That way, any messages sent to you through SMIRS will not be received by the wrong user.

Note that the word "enter" is in lower case and "SMIRS" is in upper case. 8b) If your terminal has only uppercase characters, type:

9) From now on, SMIRS will prompt you whenever it expects you to type something. All the prompt lines begin and end with two dashes; answer by typing a question mark if you do not know what is expected of you.

Do not be concerned if the computer does not respond immediately after you enter SMIRS. The response time may improve in the future, but it will always be fastest during nonworking hours (Denver time).

CALIFORNIA DIVISION OF MINES AND GEOLOGY STRONG-MOTION DATA

Processed strong-motion data from selected earthquakes are available from the California Division of Mines and Geology (CDMG). The data have been prepared by the interim CDMG strong-motion data processing system. This system is composed of a series of programs that have been developed by the California Institute of Technology, the USGS, and the CDMG, with special emphasis on the handling of long-duration film records from multiple-channel central recording instruments.

The data are grouped by phase:

Phase I Phase II

Phase III Each phase contains three-channel subgroups arranged by station. At the present time, data from the following earthquakes have been processed:

Santa Barbara earthquake of August 13, 1978

UCSB Goleta UCSB North Hall Freitas Building

Imperial Valley earthquake of October 15, 1979

El Centro free-field Uncorrected accelerations, Corrected accelerations, velocities, and displacements, Response spectra.

Channels Imperial County Services Bldg.

The data are available on standard nine-track tapes, along with a microfiche copy of the tape contents. Interested parties should contact the CDMG Office of Strong-Motion Studies (see section "Data Sources").

It is the policy of the CDMG to make all strong-motion record data promptly available to the public in a manner consistent with good data management. Requests for copies of records, personal access to record or data files, and copies of data files should be made to the Chief, Office of Strong-Motion Studies (OSMS), and should specify identity and medium of materials to be provided or reviewed. Desired access or delivery dates should be specific. When a request for copies of materials or personal access to files is received, OSMS staff will provide the requested material or will set an appointment time for personal review of files; the requestor will be notified immediately of any significant delay or other problems that prevent meeting the request. copying or other processing of materials will be based on the actual cost of producing and delivering the items, and OSMS will retain control of originals and master copies of all items.

FOREIGN STRONG-MOTION DATA

Because of the long history of close cooperation between the United States and the Central and South American strong-motion programs, much of the data from those programs are available from the same sources as the United States data (see below). Information about strong-motion data from the Western Hemisphere will be included in the Strong-Motion Information Retrieval System operated by the USGS.

The USGS does not attempt to obtain first-class copies of records from those foreign organizations that prepare data reports comparable to those prepared by the USGS. Abstracts of the data reports from such organizations are presented in this Seismic Engineering Program Report series, and through informal arrangements, copies of the data and records are made available.

NOAA WORLDWIDE STRONG-MOTION DATA

A worldwide collection of seismograms for dissemination to the scientific and engineering community is available from World Data Center A for Solid Earth Geophysics, National Geophysical Data Center (N GDC). contributing to the strong-motion data base include Australia, Italy, Japan, New Zealand, Rumania, U.S.S.R., and Yugoslavia. The USGS has furnished records from its network of cooperative strong-motion stations, including those in Central and South America.

Copies of strong-motion records are available on 35-mm film, on 70-mm film chips, as paper copies, and as digitized data on punched cards or magnetic tape. A list of most records can be obtained from the World Data Center A publication "Catalog of Seismograms and Strong-motion Records," Report SE-6. This catalog can be ordered from NGDC (NOAA) for $3.00 Charges for

Countries (see section "Data Sources").

The most significant strong-motion records recorded in the United States and Latin America between 1931 and 1971 have been copied on seven reels of 35-mm film (xl2 reduction) and 70-mm film chips (approximately x8 reduction). The film chips are available for $1.50 per chip; longer records are continued on additional chips. The 35-mm film copies can be purchased for $30 per reel, the complete set of reels for $180. There is a minimum charge of $10 per order. Check with the National Geophysical Data Center for current prices before placing an order.

Japan and Australia have supplied magnetic tapes of digitized data from stations located in the western Pacific Ocean (the Japanese Islands, New Guinea, and New Britain). strong-motion records (1933-71) were digitized by the California Institute of Technology and are now available on six magnetic tapes. digitizing post-1971 records from its network; they have generated 15 tapes of strong-motion records recorded from 1967 to 1981 in the United States, Chile, Nicaragua, San Salvador, and Mexico.

Other digitized data include punched cards containing strong-motion records from the March 4, 1977, earthquake in Rumania (recorded in Bucharest); the Gazli earthquake of May 17, 1976, in Uzbek, U.S.S.R.; and three earthquakes in the New Madrid seismic zone (located in midcontinental United States) in 1975 and 1976.

Recent acquisitions include a magnetic tape of strong-motion records triggered by a swarm of earthquakes that occurred in northern Italy near the town of Friuli in 1976; these were compiled by the National Commission for Nuclear Energy and have been given to the center for distribution. Other data include records obtained from California earthquakes near Santa Barbara in August 1978, Gilroy in August 1979, El Centro in October 1979, and Livermore in January 1980.

A table listing all digitized strong-motion records available on magnetic tape may be obtained free of charge from NOAA. Digitized strong-motion records may be purchased either in punched card format (including all three instrument components) or in tape format.

Checks or money orders should be made payable to "Commerce/NOAA/NGDC"; inquiries should be addressed to NOAA (see section "Data Sources").

For strong-motion records and data, address inquiries to the appropriate agency listed below:

1. Branch of Distribution

604 s. Pickett Street Alexandria, VA 2 2304

2. Earthquake Engineering Research Institute 2620 Telegraph A venue Berkeley, CA 94704 A series of 400 United States

OATA SOURCES

reports or information The USGS is

(804) 756-6141 (FTS) 756-6141

(415) 848-0972 National Geq:>hysical 3.

Data Center NOAA, Code E/GC11 32 5 Broadway Street Boulder, CO 80303

National Technical Information Service 5285 Port Royal Road U.S. Dept. of Commerce Springfield, VA 2 2161

NISEE/Computer Applications (415) 642-5113 5.

519 Davis Hall, UC Berkeley Berkeley, CA 94720.

Office of Strong-Motion Studies California Division of Mines and Geology 2811 "0" Street Sacramento, CA 95816

Open-File Services Section Branch of Distribution

Box 25425, Federal Center Denver, CO 80225

National Strong-Motion 8.

345 Middlefield Road, MS 77 Menlo Park, CA 94025. (303) 497-6764 (FTS) 320-6764

(703) 487-4650 (FTS) 737-4650

(916) 322-3105 (FTS) 552-3105

(303) 234-5888 (FTS) 234-5888

(415) 323-8111 ext 2881 (FTS) 467-2881 Table 1 • - Summary of U.S. acceZerograph records recovered during 1981

19.20° N * 155.47° w

record** recovered.

2-peaks

Table 1 • - Summary of U.S. accelerograph records recovered during 1981 - continued

El Centro Station 8 Cruickshank Road (USGS)t

El Centro Station 9 Commercial Avenue (USGS)t (film record) El Centro Station 9 Commercial Avenue (USGS)t (paper record) Station S-t Direction coord. (s)

230° Up 140° (s) (g)

Table 1 • - Summary of U.S. acceZerograph records recovered during 1981 - continued

2-peaks

Table 1 . - Summary of U.S. accelerograph records recovered during 1981 - continued

Parachute Test Site Imler Road (USGS)

Co ache 11 a Can a1 Station 1 (USGS) Oc ot i 11 o We 11 s Burro Bend Cafe (USGS)

0.16 315° Up 225° .13 .23

Table 1 • - Summary of U.S. accelerograph records recovered during 1981 - continued

0.07 310° Up 220° Duration5 (s)

Table 1 • - Summary of U.S. acceZerograph records recovered during 1981 - continued

19.539° N 2.1 155.580° w (g)

Table 1 • - Summary of U.S.acceZerograph records recovered during 1981 - continued

36.658° N 1.9 121.249° w Ouration5

Table 1 • - Summary of U.S. acceZerograph records recovered during 1981 - continued

37.795° N * 122.401° w Duration (s)

Table l • - Summary of U.S. acceZerograph records recovered during 1981 - continued

10 June 1981- 10 December 1981 Central California Epicenter and magnitude unknown

station owner code: ACOE - U.S. Army Corps of Engineers. CHY - City of Hayward. MWD -Metropolitan Water District of Southern California. USGS - U.S. Geological Survey. VA TA t - WWVB time code not legible or instrument not equipped with a radio receiver;

S-wave arrival minus trigger time (S- t) interval. * S-t time is questionable or cannot be determined. Direction of case acceleration for upward trace deflection on accelerogram. Horizontal components are listed as azimuth in degrees clockwise from north. Vertical components are listed as "up" or "down."

Peak acceleration recorded at ground level on one vertical and two orthogonal horizontal components unless otherwise noted. ** Denotes maximum acceleration is less than 0.05 ~ at ground level or less than 0.10 ~at non-ground-level stations.

Duration between first and last peaks of acceleration greater than 0.10 ~· - Veterans Administration. - Transamerica Building.

correlation of accelerogram with event may be questionable. Station name (owner)1

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