Abstract
The method proposed a few years ago by Ekström et al. (1986) to retrieve the focal-mechanism parameters from a set of Love and Rayleigh waves recorded on a single station is extended to regional intermediate-size earthquakes. This method is based on the inversion of the source phase and amplitude spectra in terms of the seismic moment-tensor components. For large teleseismic events, the success of the method relies mainly on the possibility to correct the observed surface-wave spectra for propagation effects by using globally averaged phase velocities. For moderate-size earthquakes (Ms ≈ 4.5 to 5.5), the periods of interest are shorter and it is no longer possible to apply this technique, due to large regional variations of the propagation parameters. In the method we propose here, a dispersion analysis of the regional Love- and Rayleigh-wave data is first performed in the period range 30 to 100 sec. The phase velocities obtained by integration of the apparent group-velocities observed on the seismograms themselves and a standard Q value are used to correct for the propagation effects. The validity of the method is tested on synthetic seismograms and an application to long-period records observed at different stations are then presented. Earthquakes in the Pacific Ocean, the Atlantic Ocean and the Mediterranean regions are used for this purpose. The feasibility of the method is also tested with an historical record made on Wiechert and Galitzin seismometers for an earthquake that occurred in Peru in 1946. Finally, an example of rapid computation of fault plane solution made after the Armenia earthquake of 7 December 1988 is presented.
