An aftershock study of the El Asnam (Algeria) earthquake of 1980 October 10

Geophys. J . R . astr. Soc. (1983) 7 3 , 605-639 An aftershock study of the El Asnam (Algeria) earthquake of 1980 October 10 M. Ouyed G. Yielding Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Rise, Madingley Road, Cambridge CB3 OEZ D. Hatz feld Laboratoire de Geophysique Interne, IRIGM, Universite Scientifique et Medicale de Grenoble, BP No. 53X, 38041 Grenoble Cedex, France G. c. P. King Bullard Laboratories?Department of Earth Sciences, University of Cambridge, Madingley Rise, Madingley Road, Cambridge CB3 OEZ Received 1982 November 15; in original form 1982 May 14 Summary. An array of 28 portable seismic stations was operated in the region of El Asnam following the magnitude 7.3 (M,)thrust earthquake of 1980 October 10. Locations of 494 events are presented in this paper and provide an indication of the overall form of the aftershock distribution. Tests to establish location accuracy (particularly depth) reduce this set to 277 events which, it is argued, are well constrained. P-waves alone are used in this study as a consequence of a debate about the reliability of reading S-phases. From the reduced set of 277 events, 81 events provide well-constrained focal mechanisms. The locations are presented in the form of maps and cross-sections, and discussed in relation to information already derived from field mapping of surface breaks and teleseismic studies of the waveforms of the main event. The zone of surface faulting (including secondary normal faulting) extended for 35 km but the aftershock distribution extends for twice this distance. Along the part of the fault which experienced substantial displacement in the main shock, the fault plane itself appears to be devoid of aftershocks,although many lie in the footwall beneath the fault. At junctions between segments of thrust faulting, strike-slip motion occurs. This is apparent in the aftershock focal mechanisms, and in the surface ruptures in one place. The large number of aftershocks in the north-east area appears to be due to the reactivation of a fan-like system of smaller reverse faults associated with surface folding. Activity at the south-west end is considerably less than that in the north-east, and is not obviously associated with recognizable geological or morphological features. Laboratoire de Geophysique Interne, IRIGM, (Iniversite Scientifique et Medicale de Grenoble, BP No. 53X, 38041 Grenoble Cedex, France ind Centre National d 'Astronomie, d 'Astrophysique et de Geophysique, BP 15-1 6, Algiers Bouzareah, Algeria M. Ouyed et al. 606 1 Introduction secandary normal f a u l t / edge of modern d l u v i a l basins k0 ,",,"::: 36 3' N 16 2' N 361° N 13OE 1 LOE 15'E 16"E 17OE Figure 1. (a) Simplified map of the surface breaks produced by the El Asnam earthquakc of 1980 October 10, based on Philip & Meghraoui (1983) and Yielding et al. (1981). The fault o n Kef el Mes (marked as a bold line) is interpreted differently: Philip & Meghraoui interpret it as a normal fault which curves at shallow depths to become the main thrust fault (northern segment); Yielding ef al. interpret it as a normal fault caused by bedding-plane slip in the hanging-wall of the main fault (northern segment). The focal mechanism of the main shock (after Ouyed et al. 1981) is shown, and also the location of the main shock by different authors: (1) USGS; ( 2 ) CSEM; (3) Cisternas el al. (1982); (4) Yielding et al. (1981). The lake which formed soon after the earthquake is shown stippled. (b) Topographic map of the same area as (a). The El Asnam (Algeria) earthquake occurred at 12.25 GMT 1980 October 10, and had a surface-wave magnitude of 7.3 (USGS). It was one of the largest earthquakes in the Mediterranean region in recent years, and the largest ever instrumentally recorded in the Atlas belt of north-west Africa. Immediately following the main shock, scientists from a number of countries began studies in the epicentral region. Results published so far include descriptions of the surface faulting and its structural setting, relocation of the main shock and major aftershocks, focal mechanism and waveform modelling of the main shock, and preliminary locations of locally recorded aftershocks (Ouyed et al. 1980, 1981 ; Lepvrier 1981; King & Vita-Finzi 1981; Ambraseys 1981; Yielding et al. 1981). This paper presents locations for almost 500 locally recorded aftershocks, and more than 80 individual focal mechanisms. These results are used to interpret the tectonic environment of the aftershock sequence. El Asnam aftershock study 607 The main shock of 1980 October 10 occurred between the lower and Middle Chelif Basins, on a NE-striking reverse fault which dips NW beneath a range of low hills (see Fig. 1). Surface ruptures on the reverse fault extended for 30 km, with an average displacement of 2-3 m. Major offsets and changes of trend divide the reverse fault into southern, central and northern segments; the southern and central segments were each about 1 2 k m long, the northern segment only 5 km. In addition to the reverse faulting, a large amount of normal faulting was also observed at the surface. Much of this was caused by tension on anticlines whose amplitude increased during the earthquake as a consequence of motion at depth on the main and related faults (King & Vita-Finzi 1981). A large arcuate system of faults in the Beni Rached area (see Fig. 1) is less well understood and has been interpreted as a tensional mechanism similar to elsewhere (Yielding et al. 1981; Cisternas, Dorel & Gaulon 1982) or as a landslide (Ouyed et al. 1981 ; Ambraseys 1981 ; Yielding et al. 1981). Epicentral locations for the main shock, by various authors (see Fig. l), suggest that the rupture initiated near the south-west end of the observed reverse fault, near the city of El Asnam. The depth of the focus has been calculated as 10 ? 2 km using waveform modelling (Yielding et el. 1981). The focal mechanism (see Fig. 1) is well constrained with a dip of 52-58" for the NW-dipping nodal plane which the surface fault break indicates is the fault plane. Major aftershocks (occurring within a few hours of the main shock) were predominantly NE of the main shock, near the central and northern fault segments (Ouyed et al. 1981; Yielding et al. 1981; Ouyed 1981). The largest aftershock (15.39 GMT, 1980 October 10, mb = 6.3) may have contributed t o the observed displacement on the central fault segment. 608 M. Ouyed et al. 2 Operation of array, and data reduction The first portable seismic stations were installed in the epicentral area within 48 hr of the main shock. These were progressively augmented to give a maximum array of 28 stations (22 smoked-drum recorders with vertical-component seismometers, and six 3-component analogue magnetic tape recorders). Fig. 2 shows the distribution of stations, and Table 1 lists the type and operating period of each station. Because of the high level of (...truncated)