Spatiotemporal Evolution of Microseismicity Seismic Source Properties at the Irpinia Near-Fault Observatory, Southern Italy

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 similar to 3000 earthquakes in the local magnitude range between M-L 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 M-s 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 from the 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 micro-seismicity ( D' Agostino et al., 2018) but may lead to rupture asperities with different sizes and characteristics.

Automated summary

This study estimates the source parameters of small-magnitude earthquakes in the Irpinia faults area in southern Italy from 2008 to 2020. By applying a spectral decomposition approach, the source contribution is isolated from propagation and site effects. The study reveals the spatial distribution of stress drop, fracture energy, and the relationship between source parameters and fault mechanical properties. The study also compares the time evolution of source parameters with geodetic displacement, indicating the influence of poroelastic deformation on earthquake occurrence and rupture characteristics.