Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of ML

Abstract

Starting January 2008, local magnitudes M_L for southern California are determined by a new calibration that provides various improvements for determining M_L for small earthquakes. Magnitudes for the previous years are being recalculated and the catalog continuously updated, with the first year of overlapping data now being available. Recalibrating a magnitude scale can cause a break in homogeneity of reporting and often produces artifacts in the catalog statistics that can influence a wide range of seismicity studies. To search for such a break, we compare the old M_L and the new M_L catalogs for 2007. We find (1) the two magnitude values differ for 96% of the M_L events, and hand-determined magnitudes are also revised; (2) the magnitude differences are irregular from magnitude increases of up to 1.5 units to reductions by as much as 2.3 units, with an average change of -0.13 units; (3) the number of events above M 1.8 decreases by 32% for the new magnitude scale; (4) the completeness magnitude apparently drops by 0.3 units from 1.6 to 1.3; (5) the b-value reduces by approximately 0.2 units, dropping from 1.16 to 0.95; (6) the new magnitude calibration produces a more stable b-value estimate and can therefore be regarded as the better scaling. We document selected examples of how the change in magnitude calibration may affect seismicity- and hazard-related analyses that are based on the Southern California Seismic Network (SCSN) catalog. Especially the change of the b-value from ~1.1 to ~0.9 has potentially major implications for hazard related applications.