Fault (re)activation and fluid-induced seismicity: an example from the Val d'Agri intermontane basin (southern Italy)

The occurrence of fluid-induced, moderate-to-large earthquakes in several locations around the globe sparked interest in the relationships between fluids and seismicity over the last few years. Several studies suggest variations of the stress state of rocks, due to the increase or drop of the pore fluid pressure, can be a mechanism that can trigger earthquakes in the presence of fluid phases. In this scenario, the Val d’Agri represents a precious case study where the effect of fluids on seismic activity can be observed. In this region, wastewater reinjection reactivated the Costa Molina blind thrust in the eastern sector of the Val d’Agri, where present-day seismicity was almost absent. A few kilometers SW from this cluster, seasonal water loading from the artificial Pertusillo reservoir generates further seismic activity within the buried carbonatic platform. The formation and evolution of the faults generating seismicity are still a matter of debate, especially in the compressional/extensional tectonic setting that characterizes the southern Apennines geological history. Consequently, the distribution of the seismic potential in the region is largely unconstrained. We built up a numerical, thermo-mechanical model to identify the principal mechanisms that generated the present-day tectonic setting observed in the Val d’Agri and the surrounding region, and to assess the seismic hazard characterizing this area. We suggest the presence of a major dècollement layer that decouples deformation between the sedimentary cover and the crystalline basement, represented by the Triassic Burano Formation. Our model quantifies the stress field and estimates Coulomb stress values in the Val d’Agri crust, allowing us to assess the potential of the rocks to generate earthquakes. We suggest that Coulomb stress values are positive in a large part of the crust, and therefore that fluid injection may be particularly effective for the reactivation of buried structures, especially within the carbonatic platform at a depth between 2 and 6 km.