The objective of Northern California Seismicity Project is to characterize the spatial and temporal evolution of the northern and Central California seismicity during the initial part of the earthquake cycle as the region emerges from the stress shadow of the great 1906 San Francisco earthquake. Although the current BSL catalog of earthquakes for the region appears to be a simple list of events, one must remember that it really is a very complex data set. The existing catalog is inhomogeneous in that it suffers from the three types of man-made seismicity changes, namely, detection changes, reporting changes, and magnitude shifts. The inherent catalog inhomogeneity exists because the location and magnitude determination methodologies have changed as the instrumentation and computational capabilities improved over the past century. It is easy to misinterpret observed variations in seismicity if we do not understand these inherent limitations of the catalog. As a result, the northern and central California seismicity since 1906 is poorly understood.
Creation of a northern and central California 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 seismicity. The existence of the invaluable BSL seismological archive, containing the original seismograms as well as the original reading/analysis sheets allows the application of modern analytical algorithms towards the problem of determining the source parameters of the historical earthquakes.
Our approach is to systematically re-analyze the data acquired from the reading/analysis sheet archive to develop a homogeneous catalog of earthquake location and local magnitude (ML) 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 catalog of location and ML which spans 1930 to the present and is which complete at the ML 3 threshold.
The locations and available magnitude estimates for all northern and central California earthquake epicenters, for which geographic coordinates were available, were transcribed to a computer readable flat file in 1964. This list of events subsequently became the origins of the current BSL historical seismicity catalog. BSL began routine computer transcription of the reading/analysis sheet data in 1984. The first NCSP task is to transcribe the 1983 and earlier data from the original reading/analysis sheets for northern and central California earthquakes to a computer readable flat files. We thus started by transcribing the data for 1983, and we have been working back in time. Several students have been working on the project this past year, and we are currently transcribing data from the early 1950's. As we work back in time, the task has become more time consuming. Two significant epochs have affected our transcription effort to date. First, the BSL began transmittal of the BSL locations and phase data for a subset of BSL stations to the ISC via teletype in 1964. We recovered this data from the ISC CD-ROM's and generated the relevant files to facilitate and expedite the transcription effort. Starting in 1950, the BSL bulletins contained all relevant phase onset and amplitude data for the determination of location and local magnitude, and only the location data were available in computer readable form. Thus, we have to transcribe all the available phase and amplitude data on the reading/analysis sheets. To date, we have transcribed data for over 7,300 earthquakes with more than 132,000 phase readings and 31,000 amplitude readings.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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