INSIGHTS INTO THE STRUCTURE OF THE LITHOSPHERE BENEATH SOUTHWESTERN EUROPE AND NORTHWESTERN AFRICA OBTAINED FROM LOCAL, REGIONAL AND TELESEISMIC EVENTS

Seismic activity in the southern Iberian Peninsula and northern Morocco is associated with the interaction between the Eurasian and African plates. The spatial distribution of the earthquakes is consistent with a well-defined plate boundary on the western side. In contrast, the seismic activity between the Iberian Peninsula and Algeria is dispersed and the plate boundary is less clear. The seismicity zone reaches a maximum width of 300 km with earthquakes in this area reaching magnitudes of Mw = 6.0. The main aim of this study is to analyze in detail the features of the crust and upper mantle below Spain, Portugal and northwestern Africa from the results of seismic travel time tomography. We present the results of a detailed three-dimensional P- wave velocity structure of this region to 150 km depth. We used P-wave arrival times from more than 15,000 local and regional earthquakes and a set of teleseismic events recorded by several seismic networks situated in Spain and Morocco and some seismic stations situated in Algeria and Portugal. For teleseismic events we handpicked P-wave arrival times from high-quality original seismograms from the years 2000 to 2005, belonging to the Andalusian Seismic Network (Red Sísmica de Andalucía). We also handpicked data from seismic stations belonging to the GSN (Global Seismic Network) and monitored by IRIS. All events are located between 30° and 90° from the seismic networks. In this study we modified the original tomographic method of Zhao et al. (1992) to combine teleseismic residuals with local and regional earthquake arrival times in tomographic inversions. This method can deal with a general velocity model in which complex velocity discontinuities exist and velocity changes in three dimensions. To determine the 3-D P-wave velocity we set up a 3-D grid in the study area with a grid spacing of 50 km in the horizontal direction and from 5 to 100 km in depth. The preliminary results for southern Spain and northern Morocco reveal strong contrasts in the values at shallow layer-depths, suggesting the existence of strong crustal heterogeneities in the region. Several bodies of high P-wave velocity are located at 15 km depth and the magnetic and gravimetric data indicate superposition of bodies at different depths in this complex-structured zone. The high velocity in the upper crust may correspond to some basic rock bodies located under the Spanish Betic Cordillera and the Moroccan Rif. Large velocity variations are imaged in SW Portugal, suggesting the change from a continental to an oceanic crust in this area. This small region is characterized by a very high rate of seismic activity and the existence of a number of important faults. These results show how regions with high levels of seismicity are generally associated with higher P-wave velocities (Zhao and Kanamori, 1993).