M_L:M_0 as a regional seismic discriminant
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 M_0 (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 M_0, 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.
Additional Information© 1993, by the Seismological Society of America. Manuscript received 18 August 1992. We wish to thank Riley Geary for the use of explosion ML'S which he has compiled from the CIT catalog. This research was supported by the Defense Advanced Research Projects Agency (DOD), Nuclear Monitoring Research Office and was monitored by Air Force Geophysics Laboratory under Contract F29601-91-K-DB 14. It was also supported by Phillips Laboratory of the Air Force Systems Command under Contract F19628-90-K-0049. Contribution No. 5200, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
Published - 1167.full.pdf