Source moment tensors of the earthquake swarm in Abu-Dabbab area, south-east Egypt

Seismic sources of a big swarm occurred in August 2004 have been studied by investigation of the moment tensors of 15 earthquakes with magnitudes range from 1.7 to 3.7. For each selected event, two sets of moment tensor inversions were carried out. The first one with fixed epicentral coordinates and the second one with variable epicentral coordinates. The data fit between observed and synthetic seismograms was computed for an elastic layered media, and minimized using a least squares algorithm. The obtained fault plane solution for each event is in agreement with the few reliable first arrival polarities. The resulting source time functions, characterized by one or two peaks, indicate that even small events might be associated with complex sources. The resulting focal mechanisms of the selected events represent different styles of faulting (normal and reverse with strike slip faulting mechanisms). The normal faulting events are characterized by focal depths larger than 7 km and the reverse ones are shallower with focal depths less than 6 km. A non-double-couple component, compensated linear-vector dipole (CLVD) ratio up to 35% is obtained for some events. The presence of some events with high CLVD ratio, besides the initiation of a high-level seismic activity, in the Abu-Dabbab area, without a large seismic main shock, led us to suggest that the August 2004 swarm might be due to igneous activity, increasing of fluid pressure and gases being consistent with the relatively high heat flow of the area.