The 1997 Umbria-Marche (Italy) earthquake sequence: Tomographic images obtained from data of the GNDT-SSN temporary network

We present some preliminary images of the 3-D P-wave velocity model and of the relocated seismicity obtained from the data collected by the GNDT-SSN temporary network installed in the epicentral area of the earthquake sequence that followed the 26 September, 1997, Central Italy main shock (M-w = 6.0). This network consisted of a total of 15 stations, was deployed in the southern part of the area affected by the earthquake sequence and operated for a total of 17 days starting on 10/18/97. Our results indicate that 1) the P-velocity structure displays a pattern of lateral variations consistent with the general NW-SE trend of the Apennines in the area; 2) the aftershock foci distribute, in the southern part of the sequence, on distinct and well defined SW dipping planes which surface intersections match previously recognized active normal faults; 3) a distinct zone of aftershock quiescence is observed in correspondence of the 10/12 (M-L = 5.3) and 10/14/97 (M-L = 5.7) hypocenters near Sellano; 4) the seismicity at the southern end is very shallow and it is unclear the relationship between the 1997 and the 1979 Norcia sequences.