Amplitude spectra moment tensor inversion of shallow earthquakes in Spain

The study of focal mechanisms for shallow earthquakes of low-to-moderate magnitude at regional distances requires a proper inversion technique. One of the major problems is the correct separation of path effects from source effects. 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. 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 has been tested with synthetic data to define instabilities that are due to mismodelling of several parameters. The inversion technique is applied to broad-band seismic data of shallow (10 km depth) earthquakes in Spain. 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. 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. 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.