Mechanism of deep earthquakes from spectrums of isolated body-wave signals: 1. The Banda Sea earthquake of March 21, 1964

Abstract

Spectrums of teleseismic body waves have been obtained for the period range 20–100 seconds from the records of a group of USCGS standardized seismograph stations around the source of a deep-focus earthquake in the Banda Sea. Corrections were applied for all the factors which took part in shaping the waveform of the signal during the course of propagation, and the spectrums were extrapolated to the vicinity of the source according to the equalization procedure described in a previous paper [Ben-Menahem et al., 1965]. Radiation patterns drawn from the corrected P wave spectrums over the above period range show that the spatial factor of the source function is independent of frequency up to 0.1 cps. Through a comparison of the empirical radiation patterns with those calculated from an assumed double-couple point source, it was possible to determine the source parameters. These are in good agreement with the solution furnished by first motions. Similar results from S waves confirm the double-couple assumption. All the corrected spectrums confirm the theory that the gross structure of the source time function is of the form (1 − e-t/τ)H(t), with 0 ≤ τ ≤ 10 seconds. From the equalized amplitudes, a quantity L0ds (source displacement times the area of a virtual fault plane), is found to be 0.125 km3. Further, by the concept of equivalent source theory, a virtual moment of a volume source can be deduced which leads in a simple way to the total energy of the source.