Source Parameters Spectral Modelling of the Aftershock Sequence of the 25th April 2012 Beni Haoua MW 4.9 Earthquake and Its Relation with the El Asnam Fault, Northern Algeria

On 25th April 2012, a moderate Mw4.9 earthquake, followed by an aftershock sequence (0.6 ≤ ML ≤ 4.3), occurred in Beni Haoua, northern Algeria. To reveal the fault responsible for this earthquake and the likely associated subsurface deformation, a detailed study of the weak seismicity based on spectral modelling of source parameters has been performed. The earthquake affected the area located in the north-eastern extend of the El Asnam seismic zone and it appears likely related to the reactivation of a segment of the fault zone that caused the El Asnam Mw7.1 earthquake on 10th October 1980, one of the most destructive earthquakes recorded in northern Africa and western Mediterranean. The determination of the source parameters of these earthquakes is important because they are too small to be reported in a global catalogue. In this study, source parameters are determined for 35 associated aftershocks using spectral modelling of three components P and S waves, assuming the Madariaga 1976 model. This model is one of the most widely used for a singular crack, radially expanding at a constant rupture speed. In this study, we estimated the source parameters (seismic moment, size of the seismic source, source dislocation and stress drop) from data in the frequency domain and we show how the results depend on the model assumptions. The Q attenuation factors for P and S waves are estimated to be 97 (50–170) and 153 (73–242), respectively, with a ratio of Qs/Qp = 1.62. We analysed source spectra and stress drops of 35 micro-earthquakes and resolved significant variations in earthquake stress drop and apparent average source dimension Rp and Rs of about 17 and 75 m, respectively. From the local magnitude calculated for the studied aftershock sequence, we highlighted similar relationship between the moment magnitude and the local magnitude, Mwp = 0.62 ML + 0.86 and Mws = 0.63 ML + 0.81, for P and S waves, respectively. This type of study is very important since we have exploited low magnitude earthquakes to obtain information that can contribute to the seismotectonic analysis of active seismic zone.