Geometry and mechanism of faulting of the 1980 El Asnam, Algeria, earthquake from inversion of teleseismic body waves and comparison with field observations

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Abstract
The El Asnam earthquake of October 10, 1980 (Ms = 7.3), provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models. The event produced extensive surface faulting. In addition to the main tectonic deformation, which is clearly a thrust, widespread secondary normal faulting was observed at the surface. In the southwest region the surface break of a thrust fault could be traced for 24 km. In the northeast part of the fault zone, normal faults with throws of several meters were observed. Although no clear thrust type surface breaks were observed in this region, geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation. In this paper the rupture process of the El Asnam earthquake is investigated by inversion of teleseismic P and SH waves. The effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood, north‐eastern segment of the fault zone. During the earthquake, rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes. The seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters: seismic moment 3.9×1019 N m, depth 6 km, strike 220°, dip 46°, and rake 72°. The duration of rupture on this fault segment was approximately 10 s which, together with the observed fault length of 24 km, indicates a rupture velocity of 2.4 km/s. Unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region. The normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region. Guided by the evidence from aftershocks and geodetic measurements, a solution with a thrust‐type mechanism was also sought. Using a priori constraints based on the field observations (the observed strike and slip angle), we obtain the following thrust source mechanism for the northeast segment: seismic moment 3.6×1019 N m, depth 6 km, strike 230°, dip 20°, and rake 91°. This solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest, suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth. Inversions are nonunique with respect to the seismic moment associated with normal faulting; the ambiguity being caused by the limited bandwidth of the observed body waves. The moment estimated from the surface waves at periods about 300 s, however, favors thrust faulting to be dominant in the northeast region.