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Calibration of m_b(P_n), m_b(L_g) scales and transportability of the M₀:m_b discriminant to new tectonic regions

Department of Earth Sciences Bullard Laboratories Cambridge University, Cambridge C83 DEZ, United Kingdom
Environmental Programs Directorate Lawrence Livermore National Laboratory, Livermore, California 94551

Abstract

We have developed M₀:m_b relationships for earthquakes and explosions in central Asia using regional waveform data from the Chinese broadband, digital station, WMQ. Regional magnitude scales, m_b(P_n) and m_b(L_g), were established by (1) determining the attenuation rates of P_n and L_g waves, (2) calibrating the m_b(P_n) against the teleseismic m_b, and (3) applying Nuttli's method to determine m_b from the L_g 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:m_b(P_n) relationships for central Asian earthquakes and explosions are in excellent agreement with the relationships for the western United States. This demonstrates that the M₀:m_b discriminant is transportable to other regions and supports theoretical claims (Patton and Walter, 1993) that M₀:m_b are directly comparable for explosions detonated in different emplacement media. Successful application depends upon correcting the m_b(P_n) for the magnitude bias known to exist between the Shagan River test site and the Nevada Test Site (NTS). In general, region-specific m_b bias must be accounted for whenever regional magnitude scales are calibrated against teleseismic m_b, as pointed out by Douglas and Marshall (1996). Transporting M₀:m_b(L_g) can be problematic because attenuation rates based on coda waves or L_g waves from earthquakes may not necessarily apply to L_g 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 m_b(L_g) 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 L_g 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 L_g 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.

Footnotes

^* Present address: Geophysics Group, EES3, Los Alamos National Laboratory, Los Alamos, New Mexico 87545.

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