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

LABORATOIRE D'ETUDE GÉOPHYSIQUE DES STRUCTURES PROFONDES INSTITUT DE PHYSIQUE DU GLOBE DE PARIS UNIVERSITÉ PIERRE ET MARIE CURIE, 75230 PARIS CEDEX 05, France
DÉPARTMENT DES SCIENCES PHYSIQUES DE LA TERRE UNIVERSITÉ DE PARIS VII, PARIS, France

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 ( = 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 M_o = 5 x 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.

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