Earthquake Source Dynamics from Farfield Amplitude and Phase Spectra of Body Waves

Abstract

Summary Farfield amplitude and phase spectra for a longitudinal shear fault with constant or linearly varying dislocation is considered. The spectral comer frequencies are explained in terms of two characteristic source times, related to the displacement time function Tr, and rupture time, Tr., The relative position of the latter comer frequency for P and SH waves are shown to be azimuth dependent, the P wave corner frequency being lower, in the half space toward which rupture front moves, and higher in the opposite direction. The structural features of the phase spectrum are shown to contain information relevant to the dynamics of faulting at the source. For unilateral faulting, the general trend of the phase spectrum exhibits as much sensitivity to Tr, as the position of the related corner frequency in the amplitude spectrum does. Assuming a cosine ramp displacement time function, the farfield amplitude and phase spectra are computed for trial source models of two shallow continental earthquakes in Turkey and Iran, and the results are compared with observations. It is concluded that simultaneous utilization of observed amplitude and phase spectra imposes additional constraints on the adopted source models, and may serve to discriminate between a predominantly unilateral and bilateral advancement of failure at the source.