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Historical Earthquake Re-analysis Project

R. A. Uhrhammer, R. M. Nadeau, A. Newman, R. Brito

Introduction

The objective of this USGS NEHRP funded two-year project, that commenced in March, 2000, is to characterize the spatial and temporal evolution of the San Francisco Bay Region (SFBR) seismicity during the initial part of the earthquake cycle as the region emerges from the stress shadow of the great 1906 San Francisco earthquake. The problem is that the existing BSL seismicity catalog for the SFBR, which spans most of the past century (1910-present), is inherently inhomogeneous because the location and magnitude determination methodologies have changed, as seismic instrumentation and computational capabilities have improved over time. As a result, the SFBR seismicity since 1906 is poorly understood.

Creation of a SFBR catalog of seismicity that is homogeneous, that spans as many years as possible, and that includes formal estimates of the parameters and their uncertainty is a fundamental prerequisite for probabilistic studies of the SFBR seismicity. The existence of the invaluable BSL seismological archive, containing the original seismograms as well as the original reading/analysis sheets, coupled with the recently acquired BSL capability to scan and digitize historical seismograms at high resolution allows the application of modern analytical algorithms towards the problem of determining the source parameters of the historical SFBR earthquakes.

Our approach is to systematically re-analyze the data acquired from the archive to develop a homogeneous SFBR catalog of earthquake location, local magnitude (ML), moment magnitude (Mw), and seismic moment tensor (mechanism), including formal uncertainties on all parameters which extends as far back in time as the instrumental records allow and which is complete above appropriate threshold magnitudes. We anticipate being able to compile a new SFBR catalog of location and ML which spans 1927 to the present and is complete at the ML $\sim $3 threshold, and of Mw which spans 1911 to the present and which is complete at the Mw $\sim $4.5 threshold.

In addition to the above analysis, we will also search for sequences of repeating earthquakes. Identification of repeating earthquakes, which are nearly identical in all source properties, provides an internal consistency check on the location and magnitude homogeneity in the catalog over time.

Background and Motivation

Although the 1910 to present BSL catalog of earthquakes for the SFBR appears to be a simple list of events, one must remember that it really is a very complex data set. It is easy to misinterpret observed variations in seismicity if we do not understand the limitations of this catalog. The existing 1910 to present BSL catalog of earthquakes for the SFBR is inhomogeneous in that it suffers from the three types of man-made seismicity changes identified by Habermann, 1987, namely detection changes, reporting changes, and magnitude shifts. The largest change in the detection capability of the BSL seismic station network occurred circa 1927-1931 with the installation of the Wood-Anderson and Benioff seismometers at four seismic stations (BRK, MHC, PAC, and SFB as shown in Figure 22.1) in the SFBR and the resulting increase in sensitivity lowered the threshold for detection of SFBR earthquakes by about 2 ML units. The most significant reporting change occurred circa 1942 when the BSL began determining ML for some earthquakes and by 1948 ML was routinely determined and reported for all SFBR earthquakes listed in the BSL Bulletin (Romney and Meeker, 1949). A magnitude shift occurred in 1954 when the response of the Wood-Anderson seismographs changed (owing to changing the free period from 1.0 to 0.8 seconds) (Bolt and Miller, 1975).


  
Figure 22.1: Map of the San Francisco Bay Region showing the Interim 1951-1998 Seismicity and the Stations which have housed Wood-Anderson seismographs. The solid inner box defines the SFBR (as given in the WGCEP 99 Report) and the dashed box is a 30 km buffer zone in which candidate SFBR events are also analyzed.
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The lack of a homogeneous catalog of earthquake for the SFBR which spans most of the past century, the availability of the invaluable BSL seismological archive, the interest in the Working Group on California Earthquake Probabilities (WGCEP, 1999), the funding of an initial effort with support from the USGS-PG&E CRADA, and the purchase and loan of a high-resolution wide-format digitizer by the USGS, combine to provide both an incentive and an unique opportunity to systematically re-process, using modern algorithms, the BSL seismographic records and data for SFBR earthquakes and to produce a homogeneous catalog of earthquakes for the region.

Initial Effort

During the summer of 1998, the USGS funded two students, via a USGS-PG&E CRADA, to transcribe the data from the original BSL reading/analysis sheets to computer readable form. With this funding, they were able to transcribe the reading/analysis sheets for SFBR earthquakes, working back in time from 1983 through 1944 (1984 onward was already in a computer database). The newly transcribed data along with the data already in the database were used to determine systematically the location, ML, and corresponding uncertainties of earthquakes which have occurred in the SFBR. An interim catalog of SFBR earthquakes was subsequently developed which includes hypocenter, ML, and their uncertainties, and which spans from 1951 through 1998. The catalog starts in 1951 because amplitude data, used in the determination of ML, were not registered on the reading/analysis sheets prior to that time. The interim catalog events are plotted in Figure 22.1 and available at http://seismo.berkeley.edu/herp/).

The rate of SFBR seismicity (see Figure 22.2) inferred from the 1951-1998 Interim Catalog is: log N = 4.105 - 0.890 ML. Assuming that the annual rate of SFBR seismicity (ML $\geq $ 3) is stationary, we expect to observe an average of 27 earthquakes per year.


  
Figure 22.2: SFBR rate of seismicity inferred from the 1951-1998 catalog.
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Current Effort

Since the Wood-Anderson maximum trace amplitude data used in the determination of ML were not registered on the original reading/analysis sheets kept in the BSL archive, we need to read the maximum trace amplitudes recorded by the Wood-Anderson seismograms in order to calculate ML and its uncertainty. Starting in June, 2000, we began the manpower intensive task of reading the maximum trace amplitudes registered by the Wood-Anderson seismograms for Berkeley (BRK), Mt. Hamilton (Lick Observatory; MHC), Palo Alto (Banner Station; PAC), and San Francisco (USF) that are kept on store in the BSL seismogram archive in Edwards Stadium. We began with the 1950 records and are working backward in time.

We started by compiling a list of candidate SFBR events which included all events in the UCB catalog of earthquakes that were within the SFBR (including the 30 km buffer zone) and which either had a ML of 2.8 or larger or else no ML assigned. The UCB catalog for the pre-1951 era is a composite based on the analysis of instrumental recordings and felt reports (Bolt and Miller, 1975). As we progress back in time prior to 1951, the proportion of events with no ML assigned increases an by 1942 it is virtually 100 percent. To expedite the process, we are reading the microfilm copies of the MHC Wood-Anderson seismograms (Uhrhammer, 1983) to cull out events which are below the ML 2.8 threshold and thus minimize the number of records that we have to deal with in the BSL seismogram archive.

We are currently reading seismograms which were recorded during April 1943. The seismograms are representative of the types of difficulties that can be encountered when reading Wood-Anderson seismograms. The UCB catalog lists 30 SFBR candidate events that occurred during April 1943. Only 11 of the events had an assigned ML, 14 of the events were listed only to the nearest minute implying that they were based on felt reports, and 12 of the events occurred on April 15 as part of a ML 4.2 sequence of events that occurred near Livermore (38 km NNW of MHC and 52 km SE of BRK). The times marked on the seismograms are in PWT (Pacific War Time; GMT - 7 hours) and the time marks were synchronized with a local chronometer. Clock corrections relative to a radio time standard were tabulated daily and linearly interpolated to determine the clock correction for a given event. The time corrections at USF were unreliable so we will have to use relative times (the time difference between the P-wave and S-wave onsets) in place of absolute times when using USF data to locate an event. The photographic paper was occasionally placed on the recording drums emulsion side down which resulted in faint and unreadable traces. Seismograms were occasionally fogged (inadvertently exposed to light) which also rendered them generally unreadable. Some seismograms are also difficult or impossible to read owing to the trace being out of focus due to misaligned optics.

Close inspection of the BRK, MHC, PAC, and USF Wood-Anderson seismograms for April 15-16, 1943, indicates that there are a few ML 3.0+ events in the sequence that are not listed in the UCB catalog and also that the times based upon felt reports are not reliable. The procedures used to compile the UCB catalog during that era were obviously not uniformly applied and the ML threshold for catalog completeness it not clear. Thus we have to scrutinize the seismograms to identify SFBR ML 3.0+ events that may not be in the current UCB catalog (particularly so when there is a sequence of events).

Acknowledgements

We thank William Bakun of the USGS who encouraged us to pursue this project and to obtain USGS NEHRP funding.

References

Bolt, B. A. and R. D. Miller, Catalog of Earthquakes in Northern California and Adjoining Areas: 1 January 1910 - 31 December 1972, Seismographic Stations, University of California, Berkeley, iv + 567 pp., 1975.

Habermann, R. E., Man-made changes of seismicity rates, Bull. Seism. Soc. Am., 77, 141-159, 1987.

Romney, C. F. and J. E. Meeker, Bulletin of the Seismographic Stations, University of California Press, 18, pp. 89, 1949.

Uhrhammer, R. A., Microfilming of historical seismograms from the Mount Hamilton (Lick) seismographic station. Bull. Seismo. Soc. Am., 73, 1197-1202, 1983.


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