Excitation and propagation of Lg from earthquakes in central Asia with implications for explosion/earthquake discrimination

L. Cong, J. Xie, B.J. Mitchell
Department of Earth & Atmospheric Sciences, Saint Louis University, 3507 Laclede Ave., St. Louis, MO 63103 U.S.A.

Abstract:

Broad-band Lg records from 52 shallow earthquakes that occurred near the Balapan and Lop Nor nuclear test sites have been used to study spectral characteristics of the excitation and propagation of the Lg phase in central Asia. Using a source model that falls off as at high frequencies, and a nonlinear inversion method, we simultaneously inverted for Lg seismic moments M, corner frequencies f, and path-variable values of Q and (Q at 1 Hz and its power law frequency dependence). The logarithms of M obtained using Lg correlate linearly with logarithms of f, the slope being about -3.56 when all data are used and -4.04 when data were sufficient that full inversions could be performed. These slopes differ only slightly from the slope of -3.83, obtained in our earlier studies using underground nuclear explosions. The f values in this study are systematically lower than those found for explosions at the same M level. A comparison of corner frequencies and Lg moment values for unknown events with the plots for known events of this study, therefore, provides means to discriminate between earthquakes and explosions. m values are systematically lower for earthquakes of this study than for previously studied explosions at the same seismic moment; moreover the logarithms of Lg moments obtained in this study correlate linearly, with a slope of 1.04, with m values when all data are used and 1.21 when only well-recorded events are used. These slopes are close to the slope of 1.19 obtained for explosions. The average Q values along the paths used in this study agree well with those obtained using explosion data in central Asia and with reported Lg coda Q values.

AGU Index Terms: 7219 Nuclear explosion seismology;
Keywords/Free Terms: Lg, source spectra, moment, corner frequency, Q.

JGR-Solid Earth 96JB02657
Vol. 101 , No. B12 , p. 27,779