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Regional bias in estimates of earthquake M_S due to surface-wave path effects

Southern California Earthquake Center Department of Geological Sciences University of Southern California, Los Angeles, California 90089-0740

Abstract

Continental earthquakes have long been known to have anomalously high surface-wave magnitudes relative to their seismic moments. A recent global study of shallow earthquakes by Ekström and Dziewonski (1988) confirmed this and found other regional, systematic anomalies in the M_S-M₀ relationship. It is important to determine the source of these anomalies in order to understand the controls on earthquake-source radiation and to obtain accurate estimates of historical seismic strain rates. In this study the magnitudes of 82 earthquakes from eight different tectonic regions are recalculated using a simple surface-wave path correction to determine whether path effects are responsible for the observed anomalies. The magnitudes of continental earthquakes are reduced by an average of 0.2 magnitude units, an improvement in fit to the global average significant at the 98% level. Surface-wave path effects are clearly responsible for the high M_S observed in continental areas. There is a small decrease in scatter in the other areas, but lateral refraction of the surface waves at plate boundaries prevents the simple correction from having a significant effect. There is no evidence in the observed anomalies, however, for any dependence of earthquake-source type on tectonic setting. It is clear that to obtain reliable, unbiased estimates of regional seismic strain rate and hazard, a local moment-magnitude relationship should be preferred to a global one.

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