Source parameters and three-dimensional attenuation structure from the inversion of microearthquake pulse width data: Method and synthetic tests

We propose a new method to determine source parameters and attenuation stru cture of a three-dimensional medium based on first P rise time and total pu lse width measurements from microearthquake data. The effects of fault fini teness on seismic radiation are taken into account by assuming the rupture model for a circular crack of Sato and Hirasawa (1973). Ray theory syntheti c seismograms in a constant Q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters (fault radius, orientation of the fault plane, and quality factor) to the p ulse width data (half and total duration of the P waveforms). The numerical ly built relationships are used to compute the direct problem in the framew ork of a nonlinear inversion scheme, based on the modified downhill Simplex method. The validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration o f the seismic passive experiment conducted in the Campi Flegrei caldera (so uthern Italy) during the last microearthquake crisis (1982-1984). Different heterogeneous Q models have been considered in order to assess the uncerta inty and resolution of source and attenuation parameters for the given acqu isition layout. The results of this simulation study indicate that first pu lse width data from a local network permit retrieval with sufficient accura cy of the heterogeneous Q structure and fault radii. A rather dense azimuth al coverage of the sources is instead needed to recover the angles (in part icular, the fault strike) which define the fault orientation.