Abstract
The 1997 Umbria Marche is probably the best ever monitored normal
faulting seismic sequence. Seismicity migration and multiple main
shocks characterize the activation of a 40-km-long system of contiguous
fault segments, as documented by seismological data. Many authors
as indicative of fault weakening by fluids migration have interpreted
this behaviour. In this study, we create a new catalogue of high
quality P- and S-wave arrival times merging data recorded by permanent
and temporary stations to improve the resolution of velocity and
attenuation models and earthquake locations. We show that the relocated
earthquakes and the joint interpretation of P- and S-wave velocity
and attenuation models help in understanding the faulting processes,
revealing new details of the geometry of the main faults and physical
state of fluids within the crustal volume. We observe that large
aftershocks occur on the top and within the Triassic evaporitic layer,
a rock volume locally characterised by fluid over-pressured, as evidenced
by high Vp/Vs and low Qp/Qs anomalies. Velocity and attenuation heterogeneities
are evidence that the migration of fluid pressure along the fault
system is the driving mechanism of the prolonged earthquake sequence.
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