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Changes of reporting rates in the Southern Californian earthquake catalog, introduced by a new definition of ML

Thessa Thormann, Stefan Wiemer, & Egill Hauksson

Published 2010, SCEC Contribution #1408

Starting January 2008, local magnitudes ML for Southern California are determined by a new calibration, which provides various improvements for determining ML 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-ML’ and the ‘new-ML’ catalogs for 2007. We find (1) the two magnitude values differ for 96% of the ML events, 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 completeness, 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 SCSN catalog. Especially the change of the b-value from ~1.1 to ~0.9 has potentially major implications for hazard related applications.

Thormann, T., Wiemer, S., & Hauksson, E. (2010). Changes of reporting rates in the Southern Californian earthquake catalog, introduced by a new definition of ML. Bulletin of the Seismological Society of America, 100(4), 1733-1742. doi: 10.1785/0120090124.