Source mechanism of intermediate and deep earthquakes in southern Spain

Focal mechanisms of 10 intermediate-depth earthquakes (30<h<150 km) and one very deep (h congruent to 650 km) which occurred in southern Spain and the Alboran Sea are studied in this paper. Distribution of epicenters with foci at intermediate depth shows a N-S alignment with a geometry parallel to the east of the Are of Gibraltar. Focal mechanisms have been determined from first motion of P-wave and modeling wave forms of direct P arrivals. Most of the intermediate depth events present single source time functions with small time duration (smaller than 0.3 s) and only for the event of March 27, 1987 a complex source time function has been found. The very deep earthquake of March 8, 1990 has a complex focal mechanism with a long source time function (1.2 s) and two different fault-plane orientations. Scalar seismic moments and dimensions have been obtained from modeling and spectral analysis. The results are interpreted in terms of the seismotectonic framework of the region and suggest the existence of a vertical slab extending from 50 km to 150 km with strike N-S produced by a lithospheric delamination process. The existence of the very deep activity at 650 km of depth may be explained in terms of a block of lithospheric material that still cold and rigid, generates the very deep earthquakes.