The 1997 Umbria-Marche (Italy) earthquake sequence: analysis of the data recorded by the local and temporary networks

We present some preliminary results obtained from the joined analysis of the data collected by the permanent and the temporary networks operating in the area of the earthquake sequence that followed (and anticipated) the 26 September, Central Italy, main shocks. In particular, these earthquake data have allowed us to determine a well constrained wave-velocity model (both P and S) with station corrections which demonstrated to produce robust hypocentral locations. These velocity models with station corrections have been used for re-locating the whole September 1997-July 1998 subset of data of the permanent network, and the previous background seismicity, starting from May 1996. The focal mechanisms of the largest events were also obtained from an analysis of the first-motion polarities. Our results indicate that 1) the seismic activity aligns on a SE-NW trend for a total length of about 50 km of extension; 2) the focal depth of these events is restricted to the range 0-9 km; 3) most events can be related to sub-parallel SW dipping fault planes; 4) focal mechanisms of the largest shocks (M-L > 4) show a coherent behaviour, with normal fault solution on SSE-NNW striking, SW dipping planes; 5) the space-time evolution of the activity displays a discontinuous mode of energy release, with different episodes of activation and an apparent clustering of aftershocks at the edges of the areas which presumably ruptured in the main shocks.