Source of Moderate Size Events during the 1997 Umbria-Marche Seismic Crisis

F. Courboulex
UMR Géosciences Azur-CNRS, Valbonne, France

S.K. Singh
Instituto di Geofisica, UNAM, Mexico D.F

B. Hernandez
Institut de Protection et de Sûreté Nucléaire, Fontenay-aux-Roses, France
Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph Fourier, Grenoble, France

A. Deschamps
UMR Géosciences Azur-CNRS, Valbonne, France

B. Castello
UMR Géosciences Azur-CNRS, Valbonne, France
Istituto Nazionale di Geofisica, Roma, Italy

M. Cocco
Istituto Nazionale di Geofisica, Roma, Italy

Abstract. The temporal network that operated from Sept. 28th to Oct. 24th during the Umbria-Marche crisis recorded more than 2000 earthquakes on 3-component digital stations. We focused on the events of magnitude larger than 3.7 and applied different techniques to retrieve the main parameters of their source. We first considered a point-source approximation and applied a simple source inversion technique at short distances in an homogeneous medium. Theoretical seismograms used in the inversion included intermediate and near field contributions. A grid search was performed to determine the focal mechanism and the seismic moment which minimize the error between the observed and the synthetic seismograms. This procedure gave fast and quite robust estimation of source parameters when the point source approximation hypothesis could be considered as valid.
For the largest earthquakes, an extended source definition was necessary to fit the waveform recorded. We applied a trial and error approach to retrieve the main parameters of the rupture (directivity, fault dimensions). Green's functions were obtained by Empirical Green's function technique when possible, or were computed by a reflectivity method in an heterogeneous media.