The Umbria-Marche (central Italy) earthquakes:: relation between rupture directivity and sequence evolution for the Mw > 5 shocks

We derive the relative moment rate function for the shocks with M-w > 5 in the 1997-98 central Italy seismic sequence, started with the M-w = 5.7, 09/26/97 00:33 event, by applying an empirical Green function method. By using three conveniently placed broadband MedNet stations we obtain apparent source time functions indicating clear directivity effects for all of the analysed earthquakes, with rupture propagation generally directed outward from the epicentral area of the 09/03/97 foreshock sequence. Taking a simple source model, forward modeling is utilized for determining fault parameters such as length, rupture propagation velocity and direction and, for the three major events, slip distribution. The effects of the break propagation directions on the sequence evolution is discussed. A possible inconsistency results for the shallow 04/03/98 shock, which could be avoided if a link with the deep 03/26/98 event is assumed. Our analysis evidences the importance of a quick determination for the fault parameters in order to image the possible induction of fracturing on nearby structures.