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Deformations associated with the El Asnam earthquake of 10 October 1980: Geodetic determination of vertical and horizontal movements

LABORATOIRE D'ETUDE GÉOPHYSIQUE DES STRUCTURES PROFONDES ASSOCIÉ AU CNRS, NO. 195 INSTITUT DE PHYSIQUE DU GLOBE DE PARIS, 4 PLACE JUSSIEU, 75230 PARIS CEDEX 05, France
CENTRE NATIONAL D'ASTRONOMIE D'ASTROPHYSIQUE ET DE GÉOPHYSIQUE, ALGIERS, Algeria
INSTITUT GÉOGRAPHIQUE NATIONAL, 2 AVENUE PASTEUR, 94160 SAINT-MANDE, France

Abstract

The resurvey of both a geodetic network and a leveling net was carried out in June 1981 8 months after the M_S = 7.3 El Asnam earthquake of 10 October 1980. Previous seismological and neotectonic studies (Ouyed et al., 1981; Yielding et al., 1981) indicate that this event results from a northeast-trending overthrust complex fault of about 40-km length, which shows at least three principal segments with slightly different directions. Vertical movements, evaluated by means of a trigonometric leveling method, show an uplift of the northwestern side of the thrust fault of about 5 m and a depression of the southeastern edge of about 1 m. These movements are progressively attenuated away from the fault trace. Horizontal movements have been evaluated by a classical first-order triangulation method. The resulting mean strain tensors, calculated for different triangles of the geodetic network, indicate a shortening of about 2.50 m which is consistent with the SE-NW direction of compression determined from neotectonic evaluations and focal mechanisms. Dislocation models are used to explain and discuss the observed deformations in light of the seismological data and observed ground breakages. Five segments are required to explain both horizontal and vertical deformations. The magnitude of vertical displacements (about 6 m) at the junction between the southwest and the central segments of the fault argues for the breaking of this area during the main shock and for a slip vector of about 8 m, at least in the central segment.