SEISMIC MULTIPLETS ANALYSIS AND ITS IMPLICATION IN SEISMOTECTONICS

Doublets analysis techniques suitable to obtain very accurate determination of arrival times of seismic phases on different stations are presented. These methodologies represent a very powerful tool to recognize the seismotectonic structures and to study the propagation characteristics of seismic waves. The signals similarity analysis, performed in the frequency and time domain, has been applied to increase the precision in arrival times definition of seismic phases generated by the same sources and having the same path. Tests on synthetic events have shown how it is possible to recognize the time delay between digital traces with a sensitivity less than the sample rate. In particular, in the time-domain analysis the results obtained using different interpolation functions (parabolic and sine for the cross-correlation operator) are compared. The analysis slightly suffers from a random noise component added to the signal, allowing the phase picking of highly noised traces. On the contrary, the frequency domain analysis offers the best results in case of high signal-to-noise ratio, while it appears less 'robust' with respect to high noise influence. The application of relative location techniques on the obtained arrival times allows to define with very high detail the geometrical shape of seismogenetic structures. As an example, a case of spatial doublets, recorded by the seismic network of Northwestern Italy, that occurred during two days in an area of the Maritime Alps (Italy), has been considered. In spite of the low magnitude of the events, an accurate reconstruction of the spatial distribution of the sources was obtained, allowing to discriminate the fault plane from the auxiliary one in the focal mechanism solution.