Excitation of Regional Waves and Magnitude Dependence of Phase Spectral Ratios in Central Asia

摘要

Multiple station recordings of Pn and Lg spectra from a large (mb = 6.6) nuclear explosion, and several recent chemical explosions (mb less than or equal to 3. 7) in central Asia are used to invert for source spectral parameters and path variable Q models. It is confirmed that a simplified, modified Mueller-Murphy source model fits the source spectra of nuclear explosions well. For chemical explosions, the model fits source spectra reasonably well, although the observed Pn source spectra tend to be complicated by event-variable, random fluctuations. The estimated source seismic moments (M0) and corner frequencies (fc) are used together with a previous data set to derive source spectral scalings for underground explosions. The new scalings essentially extend the validity of the previous scalings to a wider mb range of between about 3.7 and 6.6. specifically, the new scalings have the previously reported features of (1) linear correlation between log M0 and mb, (2) larger Pn M0 than Lg M0 for the same explosions, (3) quarter-root scaling between fc and M0, and (4) higher Pn fc than Lg fc by factors of 4-5. A necessary consequence of the scaling relationships is that the Pn/Lg spectral ratio from an explosion should grow rapidly in an intermediate frequency range roughly between the fc of Pn and Lg. Owing to the scaling of fc with event size, this frequency range shifts higher for smaller events. This magnitude dependence of the Pn/Lg spectral ratio is directly confirmed by the observation of the above explosions. Further, multiple-event stacking of the Pn/Lg ratios observed at a single station (WMQ), from many Soviet explosions with similar mb values of about 6.0 are compared with the respective ratio from events with mb of about 5.0.