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in Italy, identifying the potential sites for moderate to large size earthquakes. Ad hoc passive seismic
experiments performed in these areas provide information to focus on the location and geometry of the
active faults more closely. This information is relevant for assessing seismic hazard and for accurately constraining
possible ground shaking scenarios. The area around the Città di Castello Basin, in the Northern
Apennines (Central Italy), is characterized by the absence of instrumental seismicity (M > 2.5), it is adjacent
to faults ruptured by recent and historical earthquakes. To better understand the tectonics of the area, we installed
a dense network of seismic stations equipped with broadband and short period seismometers collecting
data continuously for 8 months (October 2000-May 2001). The processing of ~ 900 Gbyte of data revealed
a consistent background seismicity consisting of very low magnitude earthquakes (ML < 3.2). Preliminary
locations of about 2200 local earthquakes show that the area can be divided into two regions with
different seismic behaviour: an area to the NW, in between Sansepolcro and Città di Castello, where seismicity
is not present. An area toward the SE, in between Città di Castello, Umbertide and Gubbio, where we
detected a high microseismicity activity. These findings suggest a probable different mechanical behaviour
of the two regions. In the latter area, the seismicity is confined between 0 and 8 km of depth revealing a
rather well defined east-dipping, low angle fault 35 km wide that cuts through the entire upper crust down
to 12-15 km depth. Beside an apparent structural complexity, fault plane solutions of background seismicity
reveal a homogeneous pattern of deformation with a clear NE-SW extension.
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