Nature of short-periodP-wave signal variations at LASA

Abstract

Seismic events recorded by the individual subarrays at LASA were Fourier-analyzed. For any particular subarray the amplitude spectrum was divided by the average spectrum for all twenty-one subarrays. This process produced similar spectra for events with the same epicenter, suggesting that the average amplitude spectrum is that of a ‘primary’ signal and that each subarray has a distance and azimuth-dependent transfer function that operates on this primary signal to produce the amplitude spectrum of the subarray. The assumption is made that the phased summation of the whole array is representative of the primary signal. The time-domain transfer function associated with a subarray is then shown to consist of a series of time-delayed delta functions, each associated with an amplitude coefficient. In other words, the P-wave signal recorded at a particular subarray consists of a series of closely spaced individual arrivals. The interference of these multiple arrivals with one another causes the recorded signal variations between subarrays. It is suggested that deep crustal relief causes multipathing of a P wave incident from below.