Earthquake focal mechanisms and stress inversion in the Irpinia Region (southern Italy)

The goal of this study was to estimate
the stress field acting in the Irpinia Region, an
area of southern Italy that has been struck in the
past by destructive earthquakes and that is now
characterized by low to moderate seismicity. The
dataset are records of 2,352 aftershocks following
the last strong event: the 23 November 1980 earthquake
(M 6.9). The earthquakes were recorded at
seven seismic stations, on average, and have been
located using a three-dimensional (3D) P-wave
velocity model and a probabilistic, non-linear,
global search technique. The use of a 3D velocity
model yielded amore stable estimation of take-off
angles, a crucial parameter for focal mechanism
computation. The earthquake focal mechanisms
were computed from the P-wave first-motion polarity
data using the FPFIT algorithm. Fault plane
solutions show mostly normal component faulting
(pure normal fault and normal fault with a strikeslip
component). Only some fault plane solutions
show strike-slip and reverse faulting. The stress
field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by
inverting selected focal mechanisms, and the results
show that the Irpinia Region is subjected to a
NE–SW extension with horizontal σ3 (plunge 0◦,
trend 230◦) and subvertical σ1 (plunge 80◦, trend
320◦), in agreement with the results derived from
other stress indicators.

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