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Abstract:
We estimate the source parameters of small-magnitude earthquakes that occurred during
2008–2020 in the Irpinia faults area (southern Italy). We apply a spectral decomposition
approach to isolate the source contribution from propagation and site effects for
∼3000 earthquakes in the local magnitude range between ML 0 and 4.2. We develop
our analyses in three steps. First, we fit the Brune (1970) model to the nonparametric
source spectra to estimate corner frequency and seismic moment, and we map the spatial
distribution of stress drop across the Irpinia area. We found stress drops in the range 0.4–
8.1 MPa, with earthquakes deeper than 7 km characterized by higher average stress drop
(i.e., 3.2 MPa). Second, assuming a simple stress-release model (Kanamori and Heaton,
2000), we derive fracture energy and critical slip-weakening distance. The spatial variability
of stress drop and fracture energy allows us to image the present stress conditions of
fault segments activated during the 23 November 1980 Ms 6.9 earthquake. The variability
of the source parameters shows clear patterns of the fault mechanical properties, suggesting
that the Irpinia fault system can be divided into three main sectors, with the
northern and southern ones showing different properties fromthe central one. Our results
agree with previous studies indicating the presence of fluids with different composition in
the different sectors of the Irpinia fault system. In the third step, we compare the time
evolution of source parameters with a time series of geodetic displacement recorded near
the fault system. Temporal trends in the correlation between geodetic displacement and
different source parameters indicate that the poroelastic deformation perturbation generated
by the karst aquifer recharge is modulating not only the occurrence rate of microseismicity
(D’Agostino et al., 2018) but may lead to rupture asperities with different sizes
and characteristics.
