Source processes of the 1965 New Hebrides Islands earthquakes inferred from teleseismic waveforms

Abstract

This study is a detailed analysis of two foreshocks (FS‐1, M=7 and FS‐2, M= 6½), the main shock (M, M= 7½) and two aftershocks (A‐1, M= 6¾ and A‐2, M=1) from 1965 August 1 New Hebrides Islands earthquake sequence. Focal mechanisms, depths, moments, time function durations and directions of rupture (where they could be inferred) for the events have been found using time domain synthetic seismograms of the far‐field body waves and surface waves. All of the events except A‐2 were thrust events whose focal mechanisms are consistent with faulting on the interface between the subducting Indian plate and the overriding Pacific plate. A‐2 seems to have been an event on a steeply dipping fault plane which ruptured into the underthrusting plate. M and A‐2 are inferred to have been events with two sources where, in both cases, the second source was larger than the first. The relative depths and focal mechanisms of the two sources indicate a downward rupture for A‐2 and a downdip rupture for M. The observed radiation patterns for the Rayleigh and Love waves from these events are consistent with the results of the body wave analysis. Furthermore, an analysis of the radiation pattern for FS‐2 suggests that this event ruptured downdip, and a comparison of the amplitudes of R2, R3 and R4 at GSC indicates an updip rupture for M. The theoretical static vertical surface displacements computed from the teleseismic source model for M are found to be much smaller than the observed coastal uplift. The seismicity during the sequence is noted to have migrated first from north‐west to south‐east and then toward the south‐west and north‐east. The locations of the large events suggest that there are three barriers to the propagation of seismic ruptures in the crust under the New Hebrides, one under northern Santo, one between Santo and Mallikolo and one under central Mallikolo. Copyright © 1980, Wiley Blackwell. All rights reserved