High-frequency seismic radiation from a buried circular fault

Summary. We propose a simplified method for the calculation of near field accelerograms. It is based upon the hypothesis that, in the course of dynamic faulting, the dominating part of the seismic radiation is emitted by the rupture front. As the rupture moves smoothly it radiates continuously, generating the low-frequency part of the field. High-frequency waves are produced by jumps in the rupture velocity and abrupt changes in the stress intensity factor. The wave-front discontinuities created in this fashion are evaluated by asymptotic methods and may be propagated away from the source by ray theoretical methods. We apply our technique to the evaluation of asymptotic near field accelerograms for a circular fault buried in a half-space. The agreement with numerical accelerograms calculated by full-wave theory is very satisfactory. Two problems are given particular emphasis: (1) the phase shifts introduced by focusing and (2) a simpler method, based on dislocation theory, is proposed for the calculation of the radiation coefficients from a discontinuously moving rupture front.