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 13.23) 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 13.23: 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 HERP project.
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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.

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