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

Abstract

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. 606M. Ouyed et al.

The Tadjena Earthquake (Mw = 5.0) of December 16, 2006 in the Cheliff Region (Northern Algeria): Waveform Modelling, Regional Stresses, and Relation with the Boukadir Fault

“…Northern Algeria lies along the Eurasian-African plate boundary and can be affected by strong shallow earthquakes as demonstrated by the events of Algiers in 1365 and 1716 (Io = IX-X) (MOKRANE et al, 1994), Orléanville in 1954 (Ms = 6.7) (DEWEY, 1990), El Asnam (formerly Orléanville) on October 10, 1980 (Ms = 7.3) (OUYED et al, 1981, 1983, BENHALLOU, 1985YIELDING et al, 1989) and Boumerdes on May 21, 2003 (Mw = 6.8) (AYADI et al, , 2008BELABBES et al, 2009;BOUNIF et al, 2004;DELOUIS et al, 2004;MEGHRAOUI et al, 2004;SEMMANE et al, 2005;YELLES-CHAOUCHE et al, 2004). The CRAAG (Centre de Recherche en Astronomie Astrophysique et Géophysique) recently installed (2006)(2007) a new digital seismic network (Algerian Digital Seismic Network: ADSN) composed of eight broadband (BBVS-60: http://www.geodevice.cn) and two very broadband (GCZ-NT: Institute of seismology China Earthquake Agency.…”

Equations for the Estimation of Strong Ground Motions from Shallow Crustal Earthquakes Using Data from Europe and the Middle East: Horizontal Peak Ground Acceleration and Spectral Acceleration

“…These assumptions will sometimes lead to incorrectly classifying earthquakes. For example, Ouyed et al (1983) compute well-constrained focal mechanisms for 81 aftershocks of the thrust faulting 10th October 1980 El Asnam (Algeria) earthquake using an array of 28 portable seismic stations. They find that aftershocks mainly displayed thrust mechanisms but a significant proportion showed strikeslip mechanisms and two aftershocks even had normal faulting.…”

Consequences of the El-Asnam earthquakes: Turbidity currents and slumps on the Algerian margin (Western Mediterranean)

“…For example, the Malta-Gibraltar cables located ideally to detect the 1954 turbidity currents are out of service (Table I). The major earthquake of E1-Asnam was followed by many aftershocks [4]. In fact, all of the northern Algerian margin is affected, mainly in the Tellian Ranges, by a large and almost continuous seismicity (80 earthquakes from 1950 to 1970) [14].…”

“…It was not possible to evaluate the main stress orientation accurately, but there is some geologic evidence of a NW-SE shortening in conjunction with reverse faulting [21. On October 10, 1980, another major seismic event took place in the same area as the 1954 quake, which has been well documented by geological and geophysical observations [3][4][5].…”