The effect of an initial hypocentral stress upon the radiation patterns of P and S waves

Abstract

The effect of an initial hypocentral deviatoric stress upon the radiation patterns of radiated P and S waves is explicitly described for the case of an infinitesimal, nonpropagating seismic dislocation. A nonzero hypocentral stress deviator produces two small changes in the familiar quadrupole radiation pattern; it gives rise to a small additional explosion-like component, and it acts to skew slightly the quadrupole component relative to the fault plane and auxiliary plane. The latter phenomenon is not of sufficient magnitude to give rise to any serious uncertainties in the interpretation of fault-plane solutions; in fact, both phenomena are so small that they will be exceedingly difficult ever to detect. The recent measurements of P-wave amplitudes on the focal sphere by Randall and Knopoff (1970) cannot be explained by these results.