The El Asnam, Algeria, earthquake of 10 October 1980: Multiple-source mechanism determined from long-period records

Abstract

abstract We present a study of the El Asnam, Algeria, earthquake of 10 October 1980 from a large collection of long-period surface and body wave records. The focal mechanism of the main shock is well constrained by the P-wave first motions at teleseismic distances and field observations: it was a thrust event on a plane striking N45°E, and a dip angle of 54° to the north. It had a small component of left-lateral motion (λ = 83°) (Ouyed et al., 1981; Gaudemer et al., 1981). This earthquake was very well recorded on WWSSN stations and on GDSN and IDA digital stations, with a good azimuthal distribution. From these records, we confirm the focal solution and obtain a seismic moment Mo = 5 × 1026 dyne-cm. The P-wave seismograms indicate a complex source. We show that it is not sufficient to model the source by a multiple event, but it is also necessary to include a propagation effect in order to explain accurately the waveform. With assumptions based on field observation of the surface breaks, we model the P waves, including two discontinuities of the propagation, along the fault plane and obtain a good agreement of the waveform.