Digital Recordings of Aftershocks of the
by
Open-File Report 90-503
17 October 1989 Loma Prieta, California, Earthquake
Charles Mueller and Gary Glassmoyer
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.
Menlo Park, California 94025
July 1990
Site Selection and Instrumentation
Sensor Orientations
Clock Corrections
After the 17 October 1989 Loma Prieta, central California, earthquake (291 00:04:15.25 UTC, MS=7.1), the U. S. Geological Survey recorded aftershocks at sites of seismologic and engineering interest. This report describes a seismic-waveform dataset collected from 17 October 1989 (291 UTC) to 14 March 1990 (073 UTC) with GEOS digital seismographs deployed from USGS offices in Menlo Park, California. It is a summary of field and data-playback information that is intended to facilitate use of the waveform data in seismologic and engineering studies. It includes station locations, instrumentation histories (trigger parameters, sensor parameters, clock corrections, etc.), listings of waveform records, preliminary seismicity listings, and information about data availability.
Our principal research goal is to understand the influence of seismic-source and wave-propagation phenomena on damaging ground motions, and to use this knowledge to predict hazards from future earthquakes. Source processes encompass the spatial and temporal variations in the excitation of seismic waves at the fault. Wave-propagation processes include amplification and deamplification as seismic energy radiates away from the fault through rocks and soils of variable seismic properties. Since shallow geology at a site (upper tens to hundreds of meters) can exert a relatively great influence on damaging ground motions, it is useful to separate wave-propagation phenomena into whole-path and site parts. In studying these phenomena, aftershock waveform recordings can play an important role. Compared to main shocks, aftershocks are simple earthquakes and aftershock records tend to be relatively dominated by wave-propagation effects. Geometrically, since aftershocks are distributed over the fault zone, aftershock records should resemble part of the main-shock ground motion contributed by localized rupture. This observation forms the basis of the empirical-Green's-function method wherein main-shock ground motion is modeled by summing aftershock records.
The Loma Prieta earthquake provided a rare opportunity that motivated the intensive aftershock-recording effort described in this report. In a seismically noisy urban environment like the San Francisco Bay area, such a large set of high quality seismic data would otherwise be unobtainable.
Data tapes are played into a DEC PDP 11/70 computer using RDGEOS software. Records are stored one-component-per-file in a compact block-binary format. Each file consists of 512-byte header and data blocks: one integer header block (256 two-byte integer), followed by one real header block (128 four-byte real), followed by one-or-more integer data blocks (256 two-byte integer). In this report, records are identified by computer filename, a unique 13-character string constructed from the start-of-record time (UTC), component, and station name: characters 1-3 = day of year (001-366), characters 4-5 = hour (00-23), characters 6-7 = minute (00-59), character 8 = second code (A-T, where A = 0.000-2.999, B = 3.000-5.999, ..., T = 57.000-59.999), character 9 = component code (1 for vertical-component acceleration, 2 and 3 for horizontal-component acceleration, 4 for vertical-component velocity, 5 and 6 for horizontal-component velocity), character 10 = '.', and characters 11-13 = station name.
The Loma Prieta dataset is summarized in Figures 1 through 9 and Tables 1 through 7. Figure 1 shows all instrument locations and an estimate of the Loma Prieta main-shock rupture area superimposed on a map of central California. Figures 2 through 8 show station locations in finer detail at progressively larger scale. Figure 9 shows seismicity (events listed in Appendix A and B - see below) at the same scale as Figure 1. Table 1 lists records from identified seismic events by time and station - Table 1a lists all records and Table 1b lists records from events recorded at five-or-more sites. During initial processing, probable seismic events (aftershocks, earthquakes unrelated to Loma Prieta, quarry blasts, etc.) are identified by a computer algorithm that finds multiple triggers in a sliding time window; the window duration is selected just long enough to include all plausible triggers from an event (accounting roughly for traveltimes). Then, seismic events that were excluded (for example, single-station recordings) and false events that were included (for example, coincidental unrelated false triggers) are identified by inspection of seismogram plots. Multiple events (two or more events included in one file) are also identified in this way. Table 1 is complete - all seismic events have been included and all false events have been eliminated to the best of our ability. Tables 2, 3, and 4 list instrument locations, instrument parameters for a typical deployment, and detailed instrument histories, respectively. Tables 5 and 6 list clock-correction data. Table 7 is a list of the data tapes recovered from each station. At the time of writing this report in July 1990 the USGS had computed preliminary origin times and locations for many of the events listed in Table 1 - this information is summarized in Table 1, Figure 9, and Appendix A and B.
In a typical deployment, each GEOS recorder was independently triggered and recorded six components of ground motion at 200 samples/second/channel: three components of ground acceleration with a Kinemetrics FBA-13 triaxial force-balance accelerometer (fba) and three components of ground velocity with a Mark Products L-22D 2-Hz triaxial geophone. Typical gain settings were 6-18 dB for acceleration and 36-48 dB for velocity. This provided a good tradeoff between numbers of triggers and dynamic range: the largest ground motions clipped the geophone channels, but were well recorded on the fba channels, while small motions were well recorded on the geophone channels, but below noise on the fba channels. Instruments were deployed at ground or basement level at quiet, secure sites. Sensors were buried in dirt or fixed in place with silicone caulk to prevent sliding during strong shaking. Table 3 summarizes GEOS parameters for a typical deployment and sensor parameters. Table 4 lists detailed station histories, including exceptions to the standard deployment.
Clock-correction data in Tables 5 and 6 were transcribed from tape playback logs and field notes. The information varies in completeness and accuracy, since both data sources can break down in various ways. Most problems are caused by GEOS failure or by incomplete or inaccurate field notes. Synchs (regardless of time standard) and WWVB error measurements are usually written to tape and recovered from tape playback logs. In case of tape failure or synching without an installed tape, however, missing data must be recovered from field notes, or, in the worst case, inferred from apparent discontinuities in clock history. Ideally, the clock error just before a synch can be computed from the difference between the old and new times reported in the tape playback log. In case of GEOS failure, however, missing data must be extrapolated (by assuming a clock drift rate based on the unit's history). Masterclock error measurements are not written to tape - they (as well as the masterclock history) must be recovered from field notes. Four masterclocks were used; the masterclock serial number (5,6,10,11) is listed in Table 5 when it is known.
It was our intention in Tables 5 and 6 to present all the clock data with a minimum of interpretation. Question marks denote missing data and inconsistent, but apparently reliable, data - readers are sure to find other cases that we have missed. In a few cases of gross timing problems the data file headers themselves were adjusted manually - no further correction is needed in these cases. When two or more clock events have the same time, they are listed in order of occurrence. We have added a leap second to the errors of clocks that were operating at 1990 001 00:00 UTC; errors were estimated by linear interpolation or extrapolation from nearby data points (straightforward - except at KOI, MON, and STQ). Time standards such as WWVB and the rubidium clock accommodated the leap second immediately (this will be reflected in hypocenter listings, for example), but several days elapsed before the GEOS clocks could be resynched. Although the masterclock histories (relative to the rubidium standard) in Table 6 are very incomplete, it is worth noting that the policy was to check masterclocks daily - they were generally not allowed to drift more than 20 ms from the rubidium standard clock.
The Loma Prieta aftershock waveform dataset described in this report is available from the U. S. Geological Survey on a single ISO-9660-standard CD-ROM optical disk (compact disk - read only memory). Waveform files have been reformatted to ASCII from the compact block-binary form used at the USGS - a sample file is presented in Appendix C. For information contact:
ES&G Data Project
U. S. Geological Survey, MS-977
345 Middlefield Rd.
Menlo Park, CA 94025
The Loma Prieta aftershock investigation was a large, complex undertaking - we are grateful to our USGS colleagues who made it a success. The field work was accomplished by M. Andrews, J. Bicknell, J. Boatwright, R. Borcherdt, L. Carroll, M. Celibi, C. Dietel, J. Fletcher, G. Glassmoyer, J. Gibbs, T. Hanks, A. Lindh, A. McGarr, C. Mueller, T. Noce, L. Seekins, E. Sembera, J. Sena, P. Spudich, R. Warrick, and L. Wennerberg, all under the joint supervision of Hanks and Spudich. A. Shakal of CSMIP and R. Maley and E. Etheridge of USGS assisted in co-siting GEOS with strong-motion recorders. D. Oppenheimer supplied the hypocenter data that is summarized in Figure 9, Table 1, and Appendix A and B. M. Celebi provided the photograph of the damaged Marina district building reproduced on the cover. The maps in Figures 1 through 6, 8, and 9 were created with F. Klein's QPLOT program. D. Boore provided the digitized main-shock rupture-zone file used in Figure 1, and H. Bundock provided the digitized coastline and fault files used in Figures 1 through 6 and 9. The Marina map ( Figure 7) was conceived by T. Hanks and drafted by R. Eis and L. Hollis. We are grateful to P. Spudich and L. Wennerberg for thorough reviews of this report.
Borcherdt, R. D., J. P. Fletcher, E. G. Jensen, G. L. Maxwell, J. R. VanSchaak, R. E. Warrick, E. Cranswick, M. J. S. Johnston, and R. McClearn (1985). A General Earthquake Observation System (GEOS), Bull. Seism. Soc. Am. 75, 1783-1825.
Maley, R., A. Acosta, F. Ellis, E. Etheredge, L. Foote, D. Johnson, R. Porcella, M. Salsman, and
J. Switzer (1989). U. S. Geological Survey strong-motion records from the northern California (Loma Prieta)
earthquake of October 17, 1989, U. S. Geological Survey Open-File Report 89-568, 85 pages.
Shakal, A., M. Huang, M. Reichle, C. Ventura, T. Cao, R. Sherburne, M. Savage, R. Darragh, and
C. Petersen (1989). CSMIP strong-motion records from the Santa Cruz Mountains (Loma Prieta), California
earthquake of 17 October 1989, CSMIP Report No. OSMS 89-06, 196 pages.
Figure 1: Station map - all stations
Figure 2: Station map - south subregion
Figure 3: Station map - south-central subregion
Figure 4: Station map - north-central subregion
Figure 5: Station map - north subregion
Figure 6: Station map - San Francisco area
Figure 7: Station map - Marina area
Figure 8: Station map - small arrays at Agnews, Duveneck, Robinwood, Stanford
Table 1: Waveform record tables - description and example
Table 1a: Records listed by time and station (all events)
Table 1b: Records listed by time and station (events recorded at five-or-more sites)
Table 3: GEOS parameters for typical deployment and sensor parameters
Table 4: Station-instrument histories
Table 6: Masterclock clock errors
Appendix A: Hypocenter listing, sorted by time
Appendix B: Hypocenter listing, sorted by location
Appendix C: Sample file reformatted to ASCII
The GEOS Home Page is at /GEOS/geos.html
Figures
Tables
Appendix
The USGS Home Page is at http://www.usgs.gov
The USGS Geologic Division Home Page is at http://geology.usgs.gov
The USGS National Strong Motion Program Home Page is at http://earthquake.usgs.gov/monitoring/nsmp
The URL of this page is /GEOS/LPE/OFR_90-503/Loma_Prieta.html
e-mail: glassmoyer@usgs.gov
or address:
Gary Glassmoyer
345 Middlefield Road M/S 977
Menlo Park, California 94025