Historical Earthquake Re-analysis Project

Robert A. Uhrhammer

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 fundamental 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 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.

Our approach is to systematically re-analyze the data acquired from the archive to develop a homogeneous SFBR catalog of earthquake location, local magnitude ($M_{L}$), moment magnitude ($M_{w}$), 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 $M_{L}$ which spans 1927 to the present and is complete at the $M_{L}$ $\sim $3 threshold, and of $M_{w}$ which spans 1911 to the present and which is complete at the $M_{w}$ $\sim $4.5 threshold.

Previous 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 Wood-Anderson maximum trace amplitude data used in the determination of $M_{L}$ were not registered on the original reading/analysis sheets kept in the BSL archive, so we read the maximum trace amplitudes recorded by the Wood-Anderson seismograms in order to calculate $M_{L}$ and its uncertainty. 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, which began with the 1950 Wood-Anderson records and worked backward in time and finished with the earliest Wood-Anderson records (circa 1927), was completed in June 2001. At the same time we also completed the process of transcribing the data from the original reading/analysis sheets to computer readable flat files bank through 1927.

We finished transcribing the 1910-1927 data (pre-Wood-Anderson instrument) data from the original reading/analysis sheets to computer readable flat files in December 2001. Prior to the advent of the Wood-Anderson seismographs, there were only two seismic stations operating in the Berkeley network, namely at the Student Observatory on the Berkeley Campus (BRK) and at the Lick Observatory at Mt. Hamilton (MHC). During the pre-1927 era, the primary seismic instrumentation at BKS and MHC consisted of Bosch-Omori and Wiechert seismographs which operated at low magnification (100x) and recorded on smoked paper.

We began the time consuming task of scanning and digitizing selected pre-1932 seismograms in September 2001. Obtaining digital representations of the Bosch-Omori and Wiechert smoked paper seismograms is crucial particularly because the earliest smoked paper records, kept on store in the Berkeley Seismogram Archive, are becoming quite brittle and difficult to handle. We have been scanning these seismograms mostly on a flat bed scanner because some of the records could be damaged if they are passed through the rollers in the large format scanner. In February, 2002, the project received a six-month no-cost extension to complete this labor intensive scanning and digitizing effort.

See previous Annual Reports for more detailed information.

Progress During the Past Year

The task of scanning and digitizing the pre-1933 seismograms was completed in August 2002. Figure 15.1 compares the E-component signals recorded by co-sited Bosch-Omori, Galitzin, and Wood-Anderson seismometers located at BRK on the Berkeley campus. The quality of these images is fairly typical of the types of images that can be obtained by scanning the old paper seismograms kept on store in the BSL seismogram archive. A comparison of the Bosch-Omori, Galitzin, and Wood-Anderson records for ten SFBR earthquakes which occurred in 1931 and 1932, when all three types of instruments were on operation at Berkeley, was used to empirically calibrate the magnitude estimates for SFBR events which occurred prior to the installation of the Wood-Anderson seismographs.

We found that the existing earthquake location algorithms do not provide robust solutions when using the potentially imprecise data available from the sparse four-station pre-1960 SFBR seismic network. Consequently, a fuzzy logic based algorithm was developed to facilitate the determination of robust earthquake locations (Uhrhammer, 2001). The algorithm inherently has a high tolerance for imprecision in the observed data and it can yield robust sparse network solutions without requiring that the problematic observed data be either identified, down-weighted, or removed. This characteristic also renders the algorithm ideally suited for use in automated systems, such as the REDI Project, which provide rapid earthquake information.

The ultimate goal of this project has been the development of a uniform and internally consistent catalog of SFBR seismicity for instrumentally recorded earthquakes which have occurred during the past century in the region and to make the data and the results of this study available on-line via the Northern California Earthquake Data Center at http://quake.geo.berkeley.edu.

As a part of this goal, we also developed a calibration procedure for obtaining robust earthquake locations throughout a time when the number of SFBR seismic stations evolved from the initial two stations (BRK and MHC) at the turn of the last century to the more than 100 seismic stations at present (Uhrhammer, 2003). The complex geology and faulting observed in the SFBR results in seismic wave propagation times which scatter significantly over differing propagation paths in the region.

The final effort to place all the raw and processed data on the NCEDC is underway. The goal is to have all the raw data and the results in a searchable format and to be available on-line circa September 2003.

Figure 15.1: Example of scanned seismograms. Shown are the BRK E-component Bosch-Omori (top), Galitzin (middle), and Wood-Anderson (bottom) seismograms for a $M_{L}$ 4.5 earthquake which occurred on June 14, 1932 at 09:44:13 UT. The epicenter is located along the San Andreas Fault approximately 2 km N of Castle Rock State Park and 71 km SSE of Berkeley. For reference, the distance between adjacent traces is 10 mm, 2.5 mm and 2.5 mm on the original Bosch-Omori, Galitzin, and Wood-Anderson seismograms, respectively.
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Acknowledgements

We thank William Bakun of the USGS who encouraged us to pursue this project. UC Berkeley students Tom Fournier and Gabe Trevis participated in this project during the past year and we thank them for their efforts.

This project was supported by the USGS, through the NEHRP External Grants Program.

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.

Uhrhammer, R. A., The San Francisco Bay Region Historical Earthquake Re-analysis Project: A progress report and locating pre-1960 instrumentally recorded earthquakes using a fuzzy-logic-based algorithm, Seism, Res, Lett., 72, 245, 2001.

Uhrhammer, R. A., Historical Earthquake Re-analysis Project: San Francisco Bay Region, Seis. Research Lett., 74, p. 217, 2003.

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