Study of the earthquake source mechanism from high frequency seismic radiation

Abstract

High frequency ( f > 1 Hz ) near source (distance < 30 km) seismic signals can be used for retrieving the main geometrical fault parameters of earthquakes, i.e. the two fault orientation angles and slip direction. In the framework of the far-field representation theory for a point-like source, amplitudes and polarizations of body waves recorded at the Earth surface constitute a valuable data set for source mechanism inversion problems. A double-couple system of forces is assumed to be a reference earthquake mechanism model which is described by a second order symmetric tensor (moment tensor). In this paper we describe two different approaches to the problem of inversion of body waveforms for obtaining moment tensor for a double-couple fault model. The first one is based on a weighted least squares technique by using P and S waves first pulse amplitudes. Relation between data and parameter is linearized and resolution on parameter is explored in the vicinity of the found minimum norm value. The second one is based on model probability estimate from S wave polarization data. In this case a global image of resolution on model parameter space can be achieved. The probability approach turns out to be very powerful in the case of highly nonlinear inversion problems when topography of the probability function is very irregular in the model parameter space and many secondary maxima may be present. Some applications to real cases are finally presented.