Seismic source and structure estimation in the western Mediterranean usinga sparse broadband network

We present a study of regional earthquakes in the western Mediterranean gea red toward the development of methodologies and path calibrations for sourc e characterization using regional broadband stations. The results of this s tudy are useful for the monitoring and discrimination of seismic events und er a comprehensive test ban treaty, as well as the routine analysis of seis micity and seismic hazard using a sparse array of stations. The area consis ts of several contrasting geological provinces with distinct seismic proper ties, which complicates the modeling of seismic wave propagation. We starte d by analyzing surface wave group velocities throughout the region and deve loped a preliminary model for each of the major geological provinces. We fo und variations of crustal thickness ranging from 45 km under the Atlas and Betic mountains and 37 km under the Saharan shield, to 20 km for the oceani c crust of the western Mediterranean Sea, which is consistent with earlier works. Throughout most of the region, the upper mantle velocities are low w hich is typical for tectonically active regions. The most complex areas in terms of wave propagation are the Betic Cordillera in southern Spain and it s north African counterparts, the Rif and Tell Atlas mountains, as well as the Alboran Sea, between Spain and Morocco. The complexity of the wave prop agation in these regions is probably due to the sharp velocity contrasts be tween the oceanic and continental regions as well as the the existence of d eep sedimentary basins that have a very strong influence on the surface wav e dispersion. We used this preliminary regionalized velocity model to corre ct the surface wave source spectra for propagation effects which we then in verted for source mechanism. We found that this method, which is in use in many parts of the world, works very well, provided that data from several s tations are available. In order to study the events in the region using ver y few broadband stations or even a single station, we developed a hybrid in version method which combines P-nl waveforms synthesized with the tradition al body wave methods, with surface waves that are computed using normal mod es. This procedure facilitates the inclusion of laterally varying structure in the Green's functions for the surface waves and allows us to determine source mechanisms for many of the larger earthquakes (M > 4) throughout the region with just one station. We compared our results with those available from other methods and found that they agree quite well. The epicentral de pths that we have obtained from regional waveforms are consistent with obse rved teleseismic depth phases, as far as they are available. We also show t hat the particular upper mantle structure under the region causes the vario us P-n and S-n phases to be impulsive, which makes them a useful tool for d epth determination as well. Thus we conclude that with proper calibration o f the seismic structure in the region and high-quality broadband data, it i s now possible to characterize and study events in this region, both with r espect to mechanism and depth, with a limited distribution of regional broa dband stations.