High frequency seismic source spectra from earthquakes and explosions

Abstract

A three part investigation of the seismic source spectra of earthquakes and explosions is described. First, two of the most widely used simple P wave spectral models of earthquakes and two leading models of explosions are reviewed to assess their implications for high frequency (>lHz) seismic discrimination of underground nuclear explosions from earthquakes. The models are then compared with North American events and a limited data set of Soviet Central Asian events. The earthquakes are consistent with a constant stress drop model which decays as cb~2 at high frequencies. The deeper (>700 m) Nevada Test Site (NTS) explosions also show a of2 fall off between 10 and 30 Hz, in contrast with the shallower explosion which have a steeper decay. Near regional recordings of the September 14, 1988 Soviet JVE explosion show a higher comer frequency and lower 1 to 4 Hz P wave spectral ratios than predicted by either explosion model, and then predicted for similarly sized earthquakes. This high corner frequency may be influenced by source complications such as spall and tectonic release. The second part describes some simple forward modeling to quantify both the moment and the mechanism of the tectonic release of the Soviet JYE. Using 10-20 second Love and Rayleigh wave data we find the tectonic release moment to be from one fifth to one tenth of the explosion moment. To resolve the ambiguity in tectonic mechanism, synthetics were computed to compare the relative amplitudes of the SH and SV parts of the SmS phase. Although the character of the SmS phases are not completely matched, the strike slip model comes closer to matching the observed relative amplitudes than the reverse faulting mechanism. The final part describes a spectral model for the seismic radiation of a circular crack failing in tension. The model shows smaller S/P spectral ratios than are expected for pure shear crack sources. This model may be a useful first approximation for modeling seismic sources with some volumetric component, such as magma injection or slip on a non-planar fault.