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), which has been reported upon in previous annual reports. 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.

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 15.1) 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).

Figure 15.1: 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 were transcribed and analyzed as a part of the San Francisco Bay Region: Historical Earthquake Re-analysis Project (HERP).
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The lack of a homogeneous catalog of earthquake for the SFBR that 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

To begin and expedite the transcription process, we first converted all relevant available data from the online NCEDC event catalogs and the in-house phase data to a flat transcription file format for the years 1951 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 for the subset of Berkeley stations that were contributed to the ISC. The CDROM also contains an 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 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.

Current Effort

During the past year, five students worked on the process of transcribing and verifying the transcription of the data from the original BSL reading/analysis sheets to computer readable form. The verification process had two components: the first step is perform a detailed syntactical check of the transcribed information format using an ad hoc syntactical algorithm, and the second is to obtain an initial location for the event and check for anomalous observations. Syntactical errors were subsequently corrected, and anomalous observations were verified and corrected if necessary. To date, they have transcribed and verified data for over 5200 earthquakes that occurred in NCC between 1951 and 1983. They started by verifying the transcription of the original reading/analysis sheets and locating the earthquakes starting in 1983, and they worked back in time. More detailed felt reports and comments were added when available. Owing largely to the computer readable infomation that was availble from the ISC, processing the 1964 to 1983 files took the least amount of time. Since none of the data on the pre-1964 reading/analysis sheets existed in a computer readable form, all data has to be transcribed, and, consequently, it takes more time to transcribe each event. We have now completed the transcription and verification process for over 5200 NCC ML 2.9+ earthquakes that occurred between 1951 and 1983.

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 will have to be retrieved from the archive and the amplitudes read. This quite labor intensive component of the project is planned for the coming year.

Acknowledgements

UC Berkeley students Vicky Chi Sum Chan, Rose Li, Sean Tsai, Noli Valera, and visiting U Pennsylvania student Carmen Rodriguez participated in this project during the past year, and we thank them for their efforts.

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

References

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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