3-D crustal structure of the extensional Granada Basin in the convergent boundary between the Eurasian and African plates

Three-dimensional P and S wave velocity models of the crust under the Granada Basin in Southern Spain are obtained with a spatial resolution of 5 km in the horizontal direction and 2 to 4 km in depth. We used a total of 15407 P and 13704 S wave highquality arrival times from 2889 local earthquakes recorded by both permanent seismic networks and portable stations deployed in the area. The computed P and S wave velocities were used to obtain three-dimensional distributions of Poisson’s ratio (r) and the porosity parameter (Vp Vs). The 3-D velocity images show strong lateral heterogeneities in the region. Significant velocity variations up to ± 7% in P and S velocities are revealed in the crust below the Granada Basin. At shallow depth, high-velocity anomalies are generally associated with Mesozoic basement, while the low-velocity anomalies are related to the neogene sedimentary rocks. The south–southeastern part of the Granada Basin exhibits high r values in the shallowest layers, which may be associated with saturated and unconsolidated sediments. In the same area, Vp Vs is high outside the basin, indicating low porosity of the mesozoic basement. A low-velocity zone at 18-km depth is found and interpreted as a weak–ductile crust transition that is related to the cut-off depth of the seismic activity. In the lower crust, at 34-km depth, a clear slow Vp and Vs anomalous zone may indicate variations in lithology and/or with the rigidity of the lower crust rocks. D 2002 Elsevier Science B.V. All rights reserved.