Analysis of a Century of Instrumentally Recorded Historical Earthquakes in the San Francisco Bay Region

The San Francisco Bay Region (SFBR) Historical Earthquake Re-analysis Project (HERP) objective is to characterize spatial and temporal evolution of SFBR seismicity during the initial part of the earthquake cycle as the SFBR emerges from the stress shadow of the great 1906 San Francisco earthquake. The problem is that the existing Berkeley Seismological Laboratory (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. Creation of a SFBR catalog of seismicity that is homogeneous, that spans as many years as possible, and that includes formal estimates of the uncertainty in the estimated parameters is a fundamental prerequisite for probabilistic studies of SFBR seismicity. The existence of the invaluable BSL seismological archive, containing the original seismograms as well as the original reading/analysis sheets, coupled with the BSL computational capabilities allows the application of modern analytical algorithms towards the problem of determining the source parameters, including formal uncertainties, of the historical SFBR earthquakes. 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 report on the progress to date.