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

Abstract

We present a study of regional earthquakes in the western Mediterranean geared toward the development of methodologies and path calibrations for source characterization using regional broadband stations. The results of this study are useful for the monitoring and discrimination of seismic events under a comprehensive test ban treaty, as well as the routine analysis of seismicity and seismic hazard using a sparse array of stations. The area consists of several contrasting geological provinces with distinct seismic properties, which complicates the modeling of seismic wave propagation. We started by analyzing surface wave group velocities throughout the region and developed a preliminary model for each of the major geological provinces. We found 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 oceanic crust of the western Mediterranean Sea, which is consistent with earlier works. Throughout most of the region, the upper mantle velocities are low which is typical for tectonically active regions. The most complex areas in terms of wave propagation are the Betic Cordillera in southern Spain and its north African counterparts, the Rif and Tell Atlas mountains, as well as the Alboran Sea, between Spain and Morocco. The complexity of the wave propagation in these regions is probably due to the sharp velocity contrasts between the oceanic and continental regions as well as the the existence of deep sedimentary basins that have a very strong influence on the surface wave dispersion. We used this preliminary regionalized velocity model to correct the surface wave source spectra for propagation effects which we then inverted 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 stations are available. In order to study the events in the region using very few broadband stations or even a single station, we developed a hybrid inversion method which combines Pnl waveforms synthesized with the traditional body wave methods, with surface waves that are computed using normal modes. 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 depths that we have obtained from regional waveforms are consistent with observed teleseismic depth phases, as far as they are available. We also show that the particular upper mantle structure under the region causes the various Pn and Sn phases to be impulsive, which makes them a useful tool for depth determination as well. Thus we conclude that with proper calibration of the seismic structure in the region and high‐quality broadband data, it is now possible to characterize and study events in this region, both with respect to mechanism and depth, with a limited distribution of regional broadband stations.