GEOALGAR project: First results on seismicity and fault-plane solutions

The project GEOALGAR, initiated in May 2000, is devoted to the geodynamic monitoring and seismic characterization of the Algarve region. A brief description of the project goals, as well as the first results concerning the analysis of the recent seismic digital data, from 1999 and 2000, are presented. After simultaneous inversion of the seismic data and the velocity model parameters, the relocation of the hypocenters was performed for two selected areas. Twenty-five earthquakes were used for Area 1 and 125 earthquakes for Area 2, selected from the period 1999-2000 and with magnitude M-L greater than or equal to 2.0. The results show that there are two main regions where there are more events: the Monchique region (inland) and another one in the area of Guadalquivir Bank (ranging from 36.4degreesN, 08degreesW to 36.8degreesN, 7.2degreesW); for Area 1, the hypocentral corrections are relatively small, with the focus slightly deeper than that in the old solution; for the more regional events, the hypocenters corrections are bigger, with the focus becoming more shallow. Fault-plane solutions for the recent events were also estimated, showing that best solutions are dominated by strike-slip movement consistent with a stress controlled by a horizontal compression in the NW-SE to NNW-SSE direction, with two exceptions showing reverse mechanisms, with maximum horizontal stress orientation slightly rotated to a N-S direction. These results are also in agreement with those presented in previous studies performed by different authors. The new epicentral locations show a more organized spatial distribution that could indicate a possible correlation with some known tectonic features. However the fault-plane solutions are considerably more difficult to correlate with the neotectonic features.