Surface wave radiation patterns for underground nuclear explosions and small-magnitude earthquakes

The surface wave radiation patterns (amplitude and initial phase) were determined for several of the larger underground nuclear explosions in Nevada. Initial phases of surface waves from explosions and earthquakes were obtained by performing a Fourier analysis of the record and equalizing the phase to the source, using long-period records from the temporary long-range seismic measurement stations installed by the Geotechnical Corporation. Rayleigh waves from explosions in tuff and alluvium appear to have nearly the same initial phase at all azimuths, indicating an explosive force acting as a step function in time as the probable approximate source mechanism. However, the presence of Love waves, along with some asymmetry in Rayleigh wave amplitudes, indicates that some asymmetrical forces are also acting at the origin. The collapses following these explosions generate Rayleigh waves with polarities apparently reversed from those of the explosions. The collapses generate much weaker Love waves relative to Rayleigh waves than the explosions, and therefore the Love waves must be generated at or very near the source. The one explosion in granite (Hardhat) gave a surface wave radiation pattern with double-couple symmetry in both phase and amplitude, a symmetry very different from that found for the explosions in tuff and alluvium. Such a radiation pattern cannot be explained by a simple, symmetric explosive force. Several explanations of this double-couple symmetry are considered, including the possibilities that the surface waves were generated by cracking or forced motion along pre-existing cracks, tectonic strain release resulting from cavity formation, or, more probably, triggering of tectonic motion. The latter hypothesis would explain the radiation pattern, and it is supported by the occurrence of an aftershock sequence, outside the cavity zone, similar to that common for earthquakes. Analysis of surface waves from earthquakes indicates that, in many cases, the earthquake sources are more complicated than the explosion sources. The information about source mechanisms obtained in this study is of fundamental importance to the problem of the identification of explosions and earthquakes.