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

Abstract

Broadband 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 ω2 at high frequencies and a nonlinear inversion method, we simultaneously inverted for Lg seismic moments (M0), corner frequencies (fc), and path‐variable values of Q0 and η (QLg at 1 Hz and its power law frequency dependence). The logarithms of M0 obtained using Lg correlate linearly with logarithms of fc, 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 fc values in this study are systematically lower than those found for explosions at the same M0 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. The mb 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 mb 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 QLg 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.