Isotropic and nonisotropic components of earthquakes and nuclear explosions on the Lop Nor test site, China

We test the hypothesis that the existence of an observable non-zero isotropic component of seismic moment can be used as a discriminant to distinguish nuclear explosions from shallow earthquakes. We do this by applying the method described herein to a small set of data recorded between 1990 and 1996 following events (seven nuclear explosions, three earthquakes) that occurred on the Lop Nor test site in Western China. We represent each source as a sum of an isotropic component at the surface and a nonisotropic, double-couple component at an estimated depth. The explosions all possess a significant nonzero isotropic component and the estimated depth of the double-couple component of the moment tensor, presumably the result of tectonic release, lies between about 0 and 3 km. For the earthquakes studied, the isotropic component is indistinguishable from zero and the depths of the sources are estimated at 3, 17 and 31 km. The data set we have studied, although still very small, suggests that certain source characteristics (namely, double-couple depth and the ratio of the isotropic to nonisotropic components of seismic moment) may prove useful in discriminating explosions from shallow earthquakes, Further work is needed to determine whether these observations hold for explosions at other test sites, to investigate a much larger set of shallow earthquakes located in regions of interest, and to study the robustness of the estimated source parameters as source magnitude and the number of observing stations decrease.