WGCEP historical California earthquake catalog

Abstract

This appendix provides an earthquake catalog for California and the surrounding area. Our goal is to provide a listing for all known M > 5.5 earthquakes that occurred from 1850-1932 and all known M > 4.0 earthquakes that occurred from 1932-2006 within the region of 31.0 to 43.0 degrees North and -126.0 to -114.0 degrees West. Some pre-1932 earthquakes 4 < M < 5.5 are also listed. The completeness of the catalog varies strongly in time and space, in accordance with the locations of population centers and seismic instrumentation. The issue of catalog completeness is covered in depth in Appendix I, where seismicity rates are calculated for the state. The state of California was sparsely populated until the gold rush began attracting waves of migrants in 1848. The regular publication of newspapers did not begin in many locations until 1849 and 1850. Consequently, while the historic occurrence of earthquakes in California has been documented back to the 1700s, the pre-1850 catalog is quite incomplete and most of the earthquakes are poorly constrained. Thus we list the pre-1850 earthquakes in this catalog, but for all seismicity rate calculations performed in Appendix I only the 1850-2006 part of the catalog is used. One known and significant pre-1850 earthquake that is not listed in this catalog is the MW9.0 Cascadia subduction zone earthquake of January 26, 1700 (Satake et al. 2003). This earthquake was primarily off the coast of Oregon and Washington, and we do not list it here because its epicenter may very well have been outside of the California region. Yet the southern tip of this earthquake did extend to Eureka. A repeat of this event could create both a substantial shaking and tsunami hazard in California. Other significant pre-1850 earthquakes, which are listed in the catalog below, include two earthquakes in December of 1812 in Southern California of roughly M 7; at least the first of the pair occurred on the San Andreas Fault (Toppozada et al. 2002), and a June 1838 earthquake that occurred on the San Andreas near San Francisco. Data on the 1838 earthquake is particularily sparse ? we do not even know the day on which it occurred - and magnitude estimates range from M 6.8 (Bakun 1999) to M 7.4 (Toppozada et al. 2002). Earthquake listings from 1850-1932 are generally solved for from historical data sources, with magnitudes and locations determined from felt and damage reports rather than from instrumentation. Most earthquake listings from 1932-2006 are based on instrumental recordings, at least in Southern California. The Northern California seismic network did not produce a full instrumental catalog until 1942 and did not begin routine calculation of magnitudes until late in the 1940s (Uhrhammer et al. 1996). Southern California processed some Northern California events, however, particularly with M > 5, before the Northern California network was online. Some earthquakes from 1900-1932, and particularly from 1910-1932 are also based on instrumental readings, but the quality of the instrumental record and the resulting analysis are much less precise than for later listings. A partial exception is for some of the largest earthquakes, such as the San Francisco earthquake of April 18, 1906, for which global teleseismic records (Wald et al. 1993) and geodetic measurements (Thatcher et al. 1906) have been used to help determine magnitudes.