Seismotectonics of the Sierra Albarrana area (southern Spain). Constraints for a regional model of the Sierra Morena-Guadalquivir Basin limit

Abstract

A new approach to the present day stress field of the Sierra Albarrana area (southern Spain) has been inferred from the application of fault population analysis (FPA) to a set of fault/striae pairs and a short sample of earthquakes and from the simultaneous determination of stress tensor and fault plane solutions for this last set. The first population of data concerned 226 fault and striae pairs obtained in 8 field outcrops. In their turn, seismic data correspond to 45 polarities of P-wave arrival and take-off angles for a set of source-station pairs. These data were obtained in a microseismicity survey of 30 months carried out in an area of 50 × 40 km 2. Four FPA methods have been chosen: the slip model, the right dihedral method, a 3D random search of the stress tensor and the stress inversion method. In addition, fault planes have been selected by using the symmetries established in the slip model, and the methodology of FPA has also been applied to seismic data. In this way, it has been possible to take advantage of two sources of information too small to be considered independently. The use of complementary methods based on the FPA has allowed the possible solutions for the fault planes to be constrained and has yielded a picture of the tectonic evolution of the area. The analysis of the seismic information and the comparison of the results obtained independently with FPA methods provide a reliable interpretation of the present-day stress distribution in the zone. The regional stress tensor shows a compression close to uniaxial with its compressive axis, σ y (or σ Hmax), oriented N132E and plunging 0°–19°. Focal mechanisms correspond to either reverse or normal faults, some of them with a noticeable strike-slip component. The reverse faults show a NW dip and are deeper than the normal ones. A regional deformation model including a crustal flexure with axis parallel to Guadalquivir River has also been sketched.