Regional phase excitation and propagation in the Lop Nor region of central Asia and implications for P/Lg discriminants

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Journal of Geophysical Research-Solid Earth
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Jan 10
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Pn and Lg spectra for Lop Nor explosions and nearby earthquakes were analyzed for source and propagation effects using a modified inversion method of Xie [1993]. Spectral analyses were performed on waveform data from Incorporated Research Institution for Seismology, Chinese Digital Seismic Network, Kazakhstan, and Krgyzstan networks. Pn amplitude variations of a factor of 20 are observed across the small-aperture (< 200 km) Kyrgyzstan network, and are probably related to three-dimensional Moho structure close to the network. Comparisons of Pn amplitudes and spectral ratios provide evidence for nonisotropic radiation pattern effects in the case of earthquakes and overshoot in explosion time histories. Complex source effects are born out in our inversion results: (1) logarithm of seismic moment (M-0) from Pn. and Lg spectra scale linearly with body wave magnitude (m(b)) for explosions (5.0 less than or equal to mb less than or equal to 6.0) and for earthquakes (4.0 less than or equal to m(b) less than or equal to 5.5); (2) Pn M-0 estimates are similar to Lg M-0 for earthquakes but are higher than Lg Ma for explosions; (3) corner frequency f(c) scale as quarter root of M-0 for explosions and cube root for earthquakes; (4) at the same M-0 level, Pn f(c) are 4-5 times higher than Lg f(c) for earthquakes and explosions; and (5) Pn source spectra are characterized by significant overshoot for explosions. Our results suggest that the cause of maximum separation between earthquake and explosion populations on plots of P/Lg spectral ratios, which typically occurs for frequencies above explosion Pn f(c) (between 2 and 4 Hz for explosions in this study), may be related to overshoot in the reduced velocity potential.


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