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M_L:M₀ as a regional seismic discriminant

SEISMOLOGICAL LABORATORY 252-21, CALIFORNIA INSTITUTE OF TECHNOLOGY

Abstract

The m_b:M_S ratio determined by teleseismic observations has proven to be an effective discriminant, for explosive sources tend to be significantly richer in short-period energy than are earthquakes. Unfortunately, this method is limited by the detection threshold of teleseismic surface waves. However, recent advances in instrumentation allowing low amplitude surface wave measurements coupled with new analytical techniques make it feasible to use regional waveform data to determine the long-period source excitation level of low magnitude events. We propose using the ratio of M_L (local magnitude) to M₀ (scalar seismic moment) as an analogous regional discriminant. We applied this criterion to a data set of 299 earthquakes and 178 explosions and found that this ratio seems to be diagnostic of source type. For a given M₀, the M_L of an explosion is more than 0.5 magnitude units larger than that of an earthquake. This separation of populations with respect to source type can be attributed to the fact that M_L is a short-period (1 Hz) energy measurement, whereas seismic moment is determined from long-period body wave phases (period > 4 s) and surface waves (10 to 40 sec). Using regional stations with sources 200 to 600 km away, the effective threshold for magnitude measurements for this discriminant is found to be M_L = 3.1 for earthquakes and M_L = 3.6 for explosions. This method does require the determination of regional crustal models and path calibrations from master events or by other means.

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