The Bou Medfaa (Algeria) earthquake of 7 November 1959

Abstract

The Bou Medfaa earthquake of 7 November 1959 occurred at 2 h 32 min 7 s (GMT); it is one of the most destructive seismic events that central Algeria experienced this century. The main shock, which lasted 8 s in Bou Medfaa, caused only two injuries but made at least 500 homeless; it destroyed or heavily damaged more than 80% of the houses, farms and public buildings in Bou Medfaa and its immediate surroundings. Poor-quality constructions were the main cause of the damage. The total cost of damage was estimated at 300 million French francs. The earthquake was preceded by two slight foreshocks and followed by a series of lower intensity aftershocks. It was associated with slight surface ground fissures in Bou Medfaa. Compilation and detailed study of the contemporary source documents relative to this earthquake have led to the reconstruction of its macroseismic field and thus to the re-assessment of the strength of the ground shaking. Intensities were re-evaluated anew in many sites. Maximum intensity has been re-estimated at I 0 = VIII (MSK), assigned to Bou Medfaa, Hammam Righa and their close vicinities, an area about 8 km radius. The shock was felt as far as Dellys 150 km away with intensity III (MSK). From the intensity data, the macroseismic epicentre was located slightly north of Bou Medfaa at 36·41°N, 2·48°E, and an isoseismal map of the main shock has been constructed. The surface-wave magnitude has been calculated, without station corrections, at 4·90 (±0·40). The instrumental epicentre has been relocated, using the present location procedure of the ISC, at 36·38°N, 2·55°E. The analysis of destructive earthquakes provides a fundamental means for the reduction of future seismic catastrophes by suggesting new ways of improving local construction procedures, building materials, strengthening and properly repairing existing structures and implantation of new urban and rural settlements.