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A note on discrepancies between local magnitude (M_L) and microearthquake magnitude scales^*

U.S. GEOLOGICAL SURVEY, MENLO PARK, CALIFORNIA 94025

Abstract

A necessary correction for relating local network magnitude scales to Richter's local magnitude (M_L) involves accounting for the shape of the far-field body-wave spectrum of the phases used for determining magnitude. When not corrected for, this effect causes errors of about one magnitude unit at M_L 3 for some southern California earthquakes. The discrepancy should be comparable for M_L > 3, but at smaller magnitudes will decrease with decreasing M_L. It may be corrected for either by direct comparison of network scales with magnitudes determined from Wood-Anderson seismograms, or by spectrum measurements over a range of magnitudes. The nature of the discrepancy and the corrections required to account for it are demonstrated by an example, the aftershocks of the 1968 Borrego Mountain, California earthquake.

Footnotes

^* Publication authorized by the Director, U.S. Geological Survey.

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