COMPLETE MOMENT TENSOR RETRIEVAL FOR WEAK EVENTS - APPLICATION TO OROGENIC AND VOLCANIC AREAS

Aiming to study the mechanism and time history of weak focal events we invert the dominant part of high-frequency seismograms (S and surface waves) by using two methods which implement moment tensor description of the focus. The point-source approximation is applied since we assu me that the size of the focus with respect to the minimum wavelength o f the analyzed signals is relatively small. Various constraints of the moment tensor are applied to cover local events of different origin - both the tectonic earthquakes and seismic events induced by volcanic activity. In the former case the double-couple constraint is applied, in the latter one a full moment tensor is decomposed into a volumetric part (V), representing volume changes, a compensated linear vector-di pole part (CLVD), describing opening of a fluid-filled lenticular crac k, and a double couple part (DC), representing a shear slip. In the fu ll moment tensor inversion the hypocentral depth and structural model may vary within pre-defined intervals. In the orogenic area of Friuli, Northern Italy, both the method looking for a DC only and the procedu re implying the complete moment tensor arrive produce a DC mechanism, the orientation of which is consistent with the polarity readings. In the volcanic area of Phlegraean Fields, Southern Italy, the possible e xistence of fluid motion, which can be associated to volume changes an d crack openings has to be taken into account, therefore, we used only the full moment tensor description to analyze several events in the m agnitude range from 1.3 to 3. The obtained source durations vary from a few tenths of a second to about two seconds, suggesting that even sm all events may be characterized by relatively complex rupture history, although some of the retrieved complexities may be an artifact due to lateral inhomogeneities and other unmodelled structural effects. The percentage of the V component was found to be as large as 30% here, wh ile it was negligible in the orogenic area of Friuli. Therefore, the r etrieved V component, even though partly corrupted by imprecise struct ural modeling, can be used to distinguish between events with and with out significant volume changes in the source. This might lead to a bet ter understanding of the source processes in volcanic areas and to the use of source properties as possible precursors of explosive eruption s.