Northern California Seismicity Project

Robert A. Uhrhammer

Introduction

The Northern California Seismicity Project (NCSP) is a counterpart to the San Francisco Bay Region (SFBR) - Historical Earthquake Re-analysis Project (HERP) reported in the research section on HERP. The initial objective of this project which commenced in August, 2000, is to transcribe the pre-1984 data for M$_{L} \geq$ 2.8 earthquakes which have occurred in Northern and Central California (NCC) (outside of the SFBR covered by HERP), from the original reading/analysis sheets, kept on store in the Berkeley Seismological Archives, to a computer readable format.

As is the case with HERP, characterization of the spatial and temporal evolution of NCSP seismicity during the initial part of the earthquake cycle as the region emerges from the stress shadow of the great 1906 San Francisco earthquake is the long term goal. 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, NCC seismicity since 1906 is poorly understood.

Creation of a NCC seismicity catalog 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 NCC 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.

Background and Motivation

Although the 1910 to present BSL catalog of earthquakes for NCC 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 NCC 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 starting circa 1927 with the installation of the Wood-Anderson and Benioff seismometers at several sites in NCC (see Figure 16.1) and the resulting increase in sensitivity lowered the threshold for detection of NCC earthquakes by about 2 $M_{L}$ units. The most significant reporting change occurred circa 1942 when the BSL began determining $M_{L}$ for some earthquakes and by 1948 $M_{L}$ 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 16.1: Map of the NCC Region showing the 1951-1983 M$_L$ $\ge $ 2.9 seismicity (small dots). The triangles are the seismic stations, operated by Berkeley and adjacent networks between 1951 and 1983, for which data are available. Events occurring in the dashed inset box are being transcribed and analyzed via HERP (see the previous section).
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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.

Figure 16.2: Example of printer file of data for Berkeley network data extracted from ISC CDROM.
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Current Effort

To expedite the transcription process, we converted all relevant available data from the online NCEDC event catalogs and the in-house phase data to the flat transcription file format as shown in Figure 16.2 for the years 1978 through 1983. We also acquired a copy of the International Data Center (ISC) CDROM which contains events and associated station data published in the ISC Bulletins from January 1964 through December 1977 (Version 1.2). This ISC data set includes event and station data contributed by Berkeley and the CDROM also contains a algorithm to search the database and extract and translate the ISC coded phase Berkeley data to a readable print format for years 1963 through 1977. This enabled us to start with transcription files that contained approximately half of the data that is on the original reading/analysis sheets for the years 1964 through 1983. The primary data from the original reading/analysis sheets, that was not included in this process, was the Wood-Anderson maximum trace amplitude data that is crucial for the determination of local magnitude.

During the past year two students worked on the process of transcribing the data from the original BSL reading/analysis sheets to computer readable form. They started by transcribing the original reading/analysis sheets from 1963 and they have been working back in time. The data from 1952 through 1983 has now been transcribed.

We are currently transcribing reading/analysis sheet data from 1951. Since none of the data on the pre-1964 reading/analysis sheets exists in a computer readable form, all data has to be transcribed and consequently it takes more time to transcribe each event. Also, the pre-1951 reading/analysis sheets data do not contain Wood-Anderson maximum trace amplitude data which is used in the calculation of local magnitude so that the original Wood-Anderson seismograms with have to be retrieved from the archive and and the amplitudes read. This quite labor intensive component of the project is planned for the coming year.

Figure 16.3: Example of completed transcription file format for the event shown in Figure 16.2.
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Acknowledgements

UC Berkeley students Tom Fournier, Karin Spiller, Gabe Trevis, Maxwell Wilmarth, and Jennifer Epstein participated in this project and we thank them for their efforts.

This project was partially supported by the USGS funding of the Northern California Earthquake Data Center.

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.

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