Analysis of Soviet DSS Records for Propagation and Modeling of Lg Blockage across Major Crustal Features

Abstract

Abstract : Analysis of Quartz data resulted in two main conclusions directly relevant to the problem of nuclear test discrimination: (1) the uppermost mantle velocity and attenuation structure is significantly heterogeneous, and (2) the propagation of regional phases, including Lg is highly sensitive to this heterogeneity, and to the variation in crustal structure. The uppermost mantle is significantly heterogeneous, with vertical and horizontal velocity contrasts, high-and low-velocity zones, Moho topography, and two apparently regional reflecting boundaries present at 60-90 and 120-140 km depth. Such a complex structure would cause variations of amplitudes of Pn and Sn phases that cannot be accounted for by using traditional simplified 1-D models of the uppermost mantle. The Lg phase from PNE 323 is strong at distances below 500-700 km and at frequencies below 1.5-2.0 Hz, but is quickly attenuated with distance and nearly disappears between 1200 - 1400 km from the PNE. Thus in its amplitude-offset dependence, Lg propagation along the West Siberian Basin differs significantly from the propagation within the Baltic Shield where the absolute value of Lg amplitude appears to be substantially stronger, and no indications of its relative attenuation are found, even though the shot point is in a sedimentary basin. Compared to Lg, the S-wave from PNE 323 propagates effectively to regional ranges of about 2000 km, after which it becomes much weaker at teleseismic distances, although it still can be distinguished in the records. Important conclusions can be made from a comparison of Lg and S-wave amplitude. Lg quickly decreases in amplitude (faster than S-wave) during its propagation across the West Siberian Basin, whereas Lg from a PNE in a sedimentary basin effectively propagates in the Baltic Shield region.