Intermediate and deep earthquakes in Spain have been studied in more detail ( Buforn 1997 Coca 1999), while the analysis of shallow events has been approached only recently for Ibero-Maghreb region ( Stich 2003 Rueda & Mézcua 2005). Owing to the small average magnitude of the earthquakes, focal mechanism solutions are rarely available from regional or global moment tensor projects.

The Pyrenees region is characterized by small earthquakes at shallow depth ( Souriau & Pauchet 1998 Goula 1999). Intermediate and deep earthquakes also occur in this region ( Hatzfeld 1978 Buforn 1991, 1997, 2004 Morales 1999). The most active seismic regions are located in southern Spain and Alboran Sea, mainly related to NW–SE convergence along the plate boundary between Eurasia and Africa ( Udías 1976 Buforn 1988, 1995, 2004 Calvert 2000 Jiménez-Munt & Negredo 2003). The seismicity of Spain is characterized by the occurrence of mostly shallow ( h < 35 km) earthquakes of low-to-moderate magnitude ( M w < 5.0). The proposed method can be implemented in fast moment tensor inversion when a Green functions database has been created in advance for the studied area.Įarthquake-source mechanism, inversion, seismic moment, seismology 1 Introduction A combined fit of amplitude spectra, first motion polarities and double-couple component is introduced as a method which resolves better the non-uniqueness of solution. Solutions are obtained for a range of depths and compared with solutions obtained with different methods, such as first motion polarities and moment tensor inversion in the time domain. The inversion technique is applied to broad-band seismic data of shallow (10 km depth) earthquakes in Spain. The method has been tested with synthetic data to define instabilities that are due to mismodelling of several parameters. In this study a method of frequency domain moment tensor inversion to determine deviatoric moment tensor solutions from body wave amplitude spectra is described. The method used should take into account problems arising from errors in the definition of crustal structure and inclusion of the high-frequency content of the waveforms. One of the major problems is the correct separation of path effects from source effects. The study of focal mechanisms for shallow earthquakes of low-to-moderate magnitude at regional distances requires a proper inversion technique.

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