Calibration of mb(Pn), mb(Lg) scales and transportability of the M0:mb discriminant to new tectonic regions

Abstract

Abstract We have developed M0:mb relationships for earthquakes and explosions in central Asia using regional waveform data from the Chinese broadband, digital station, WMQ. Regional magnitude scales, mb(Pn) and mb(Lg), were established by (1) determining the attenuation rates of Pn and Lg waves, (2) calibrating the mb(Pn) against the teleseismic mb, and (3) applying Nuttli's method to determine mb from the Lg waves. Seismic moments for earthquakes and explosions were obtained by applying the Bolt and Herraiz (1983) method calibrated using CMT moments for earthquakes and regional moment estimates of the Soviet Joint Verification Experiment (JVE) explosion. The results show that M0:mb(Pn) relationships for central Asian earthquakes and explosions are in excellent agreement with the relationships for the western United States. This demonstrates that the M0:mb discriminant is transportable to other regions and supports theoretical claims (Patton and Walter, 1993) that M0:mb are directly comparable for explosions detonated in different emplacement media. Successful application depends upon correcting the mb(Pn) for the magnitude bias known to exist between the Shagan River test site and the Nevada Test Site (NTS). In general, region-specific mb bias must be accounted for whenever regional magnitude scales are calibrated against teleseismic mb, as pointed out by Douglas and Marshall (1996). Transporting M0:mb(Lg) can be problematic because attenuation rates based on coda waves or Lg waves from earthquakes may not necessarily apply to Lg waves from explosions, and vice versa. This is demonstrated for the western United States where the coda Q measured from explosion data is much lower than the coda Q from earthquake data. We show that mb(Lg) for earthquakes located close to NTS is greatly overestimated using the Q values determined from the coda of NTS explosions. This is apparently not a problem for Lg calibration in central Asia, perhaps because of differences in velocity and/or Q structures for the two regions. Other important considerations are the character of Lg propagation on short paths (<400 km) for NTS explosions versus long paths (1000 km) for East Kazakh explosions and the properties of coda-wave spreading as a function of distance and velocity structures for different tectonic regions.