SEISMIC MOMENT TENSOR RESOLUTION BY WAVE-FORM INVERSION OF A FEW LOCAL NOISY RECORDS .1. SYNTHETIC TESTS

A theory is described that takes into account the effects of the rando m noise contained in seismic records, the mislocation of the hypocentr e and the inaccurate knowledge of the velocity structure, while estima ting the error affecting the mechanism and the source time function de termined by waveform inversion of noisy records. Assuming Gaussian err ors, the variance of the forward model error is defined and then trans formed into confidence regions of eigenvalues and eigenvectors of the moment tensor and into error bars of the source time function. The met hod allows us to treat point sources, composed of a few subevents, eit her acting simultaneously or delayed in time. Therefore, in a series o f synthetic tests, we investigate separately the effects of the noise level, the mislocation of the hypocentre and the inaccurate knowledge of the medium on the resolution of the two parts of a focus, composed of a double-couple (DC) and a volumetric (V) source, a general case th at we find in volcanic areas. The crucial effect of the recording netw ork geometry on the mechanism resolution is confirmed and quantitative ly estimated for both the vertical component and the three-component c onfiguration of the network. Our goal is to apply this method to the i nvestigation of the reliability of the retrieval of the focal mechanis m and the time function associated with events of low magnitude (M(d)< 4) recorded by local stations, both in tectonic and in volcanic areas. Special attention is devoted to volcanic areas, where the non-double- couple components of the source may reveal the existence of complex fr acture processes, which are due to fluid movements in the crust.