Mechanisms of local earthquakes in 3-D inhomogeneous media determined by waveform inversion

Abstract

SUMMARY A method of waveform inversion for determination of a point source mechanism and its time function is developed. It allows the use of amplitudes for relocalization of the event and in a simplistic way the optimization of the structural model as well. This is accomplished by minimization of the residual norm during modelling of the data in a phase space comprising both hypocentral coordinates and structural interpolation parameters. The ray method, used to solve the forward problem, allows consideration of generally 3-D inhomogeneous isotropic media with arbitrarily curved interfaces. Seismograms generated by sources buried in such media can be inverted provided that identification of individual body-wave phases in the observed records can be made. The point source approximation restricts application of the method to weak events only. The moment tensor description advantageously allows us to investigate local seismicity, for which it is reasonable to suppose that the foci in general have a non-DC mechanism. The method includes an error analysis consisting of transformation of the data noise variance into confidence regions for volumetric, DC and CLVD parts of the retrieved moment tensor and for its principal axes (T, N and P) as well. Tests of the method were first performed on synthetic data generated for a configuration of a local seismic network near Dobra Voda, Slovakia. In the next step, the real data from the region were inverted to obtain the mechanism of a small earthquake of local magnitude of about 2.