Foreshock sequence of September 26th, 1997 Umbria-Marche earthquakes

On September 3(rd) (22:07 UTC), 1997 a small earthquake with Mw=4.54 starte d the foreshocks sequence (approximate to 1500 events with M-L <3.1) of the September 26(th) seismic sequence. Two days after, three seismic stations of the University of Camerino were installed around the macroseismic epicen ter of the foreshock. We present in this paper the location of foreshocks ( with 2.1 <M-L<3.3) which occurred between September 3(r)d and 26(t)h. Fores hocks location, with horizontal (ERH) and vertical (ERZ) error less than 1. 5 km, define an area approximate to4 km large. Foreshocks have been localiz ed between the epicenters of the two major events of September 26(t)h, whic h occurred at 00:33 UTC with Mw=5.6 and at 09:40 UTC with Mw=6.0 (Amato et al., 1998; Ekstrom et al., 1998). In a vertical cross-section, hypocenters show a low angle (approximate to 30 degrees) structure with SW dip-directio n. Focal mechanisms for three of the major events show dip-slip fault solut ions with strike direction of about N130, in agreement with the CMT solutio ns of September 3(r)d and September 26(t)h earthquakes (Ekstrom et al., 199 8). Data recorded at two stations Popola (POP) e Capodacqua (CPQ) located o n the rupture area of the September 26(t)h faults, allowed us to calculate a mean Vp/Vs ratio of 1.84 +/-0.03 for the foreshock. This value is lower t han the Vp/Vs ratio of 1.89 +/-0.02 calculated for the aftershock sequence occurred in the same area. Besides, the Vp/Vs ratio during the foreshocks s equence is not stable in time but it seems to increase approaching Septembe r 26(t)h. After September 26(t)h mainshocks, this value tends to stabilize around a higher value of 1.89. Following the dilatancy model, we suggest th at the relative low Vp/Vs ratio before the main shocks could indicate the p resence of fluid in the focal volume. The presence of fluids could have inc reased the effective stress on the fault plane and could be responsible for the long foreshock activity before the two main earthquakes of September 2 6(t)h. Therefore, we suggest that this foreshock activity could have also c ontributed to reduce the friction along the September 26(t)h fault plane, b reaking the active structure in two smaller segments. In this hypothesis, f oreshock activity could have drastically contributed to mitigate the seismi c potential of the Colfiorito's active structure.