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Magnitudes and moments of duration

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

Coda-duration at 42 of the stations in the U.S. Geological Survey's central California seismic network (CALNET) for earthquakes in five source regions of central California—the Parkfield and San Juan Bautista sections of the San Andreas fault, the Sargent fault, the Coyote Lake section of the Calaveras fault, and the Livermore area—are used to obtain empirical formulas relating local magnitude M_L and seismic moment M₀ to and epicentral distance . Models with log² fit the data better than those assuming a log dependence. For 55 earthquakes with 1.1. M_L 5.3, M_L = 0.92 + 0.607 (±0.005)log² + 0.00268(± 0.00012). These M_L assume a Wood-Anderson seismograph magnification of 2800; 0.15 should be subtracted from these M_L for continuity with magnitudes obtained from or calibrated against typical (magnification 2000) Wood-Anderson seismographs. For 53 earthquakes with 18.4 log M₀ 22.3, log M₀ = 17.97 + 0.719(± 0.0007)log² + 0.00319(±0.00013). These relations provide unbiased estimates of M_L for 1.5 M_L 5.3 and 19 log M₀ 22.3. Station corrections can significantly improve the accuracy and precision of M_L and log M₀ estimates, particularly if from a small number of stations are used. Regional variations in station corrections reflect an increase in coda duration toward the south within the CALNET.

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