Modeling propagation effects from explosions in western China and India

The authors report on the results of finite-difference simulations of regional seismic wave propagation from a 1995 explosion at the Chinese test site at Lop Nor and from the recent Indian test. These simulations provide insight into variations in explosion seismograms recorded for these events. Previous modeling efforts by App et al., 1996, and Jones et al., 1998, have shown that many features in seismograms from explosions and earthquakes received at various stations surrounding Lop Nor can be reproduced with simple descriptions of the geologic structure along each path. However, differences in detail between simulations and the earthquake-like seismograms recorded at Station TLY in the Baikal Rift for the Lop Nor explosion have motivated further study of propagation effects. Differences between the TLY data and the simulations indicate that the use of homogeneous materials within the basins, crust, and mantle is too simplistic to capture some of the important features of the observed seismograms. Current efforts involve simulations in which 1-D and 2-D lithosphere models are merged to mimic gradients and 2-D structure, features which produce Rayleigh-wave dispersion and an improved agreement in group velocity curves. Source models have been modified to include a component of tectonic release during the Lop Nor explosion. The recent nuclear tests in India have been modeled in a similar fashion. The propagation path crosses the Moho step beneath the southern edge of the Tibetan Plateau before being received at station NIL. Similarly, the path from Lop Nor to NIL crosses a Moho step beneath the northern edge of the Tibetan Plateau and, in both cases, the effect of a step when combined with surface basin attenuation effectively block the Lg phase. Simulations show that both attenuation and crustal thickening are important when modeling propagation near the Tibetan Plateau