Illuminating active fault zones at Campi Flegrei Caldera (Southern Italy) from high-precision earthquake locations

The Campi Flegrei caldera, located to the west of the city of Naples, is one of the most active and urbanized volcanic areas in the world, also hosting an eruptive episode in historical times. This area periodically experiences notable unrest episodes which include ground deformations and seismic swarms, as in the recent 1982-1984 crisis. During the past decade, the central portion of Campi Flegrei caldera underwent a sustained and continuous ground uplift reaching rates of 15 mm/month, along with an increase in the rate, maximum magnitude and spatial extent of seismicity especially in the last two years, culminated with the occurrence of an Md 4.2 earthquake on 27^th September 2023.

In this study, we compute high-precision earthquake locations using multi-scale, source-specific station corrections and waveform coherence. We relocated ~8.3 k earthquakes between 2014 and 2023, resulting in hypocentral uncertainties less than ~ 100 meters and thus assessing the spatiotemporal evolution of the earthquakes during the current crisis. We show that the integration of the station corrections and the coherence-driven, cross-correlation weighted stack of the probabilistic locations for nearby events (< 2km) strongly improves the location accuracy for target earthquakes. Relocated hypocentres allow us to clearly show with unprecedented detail the complexity of the kilometric-size fault structures activated in response to the increasing rate of the ground uplift phenomenon. Most of the seismicity is clustered along identifiable segments concentrated in a shallow region around the Solfatara-Pisciarelli area, where epicentres define an ~1x1 km, horseshoe-shaped structure, opened and deepening toward the northeast.  In contrast, the deepest offshore seismicity, between 3-5 km depth, appears to fit and approximate the downward propagation of the previously identified south-western caldera inner ring fault. Relocated seismicity appears coherent with the fracture zones activated during the 1982-84 unrest episode. However new sectors of activity have been identified during the present unrest, including the one at the eastern boundary, which hosted the largest Md 4.2 event caused by a km-size rupture within the shallow (3 km) volcanic sedimentary layer. Given the size of the structures mapped in this study and the source parameters estimated for the main event, these faults can accommodate earthquakes of moment magnitude up to 5.0, both around the Solfatara and the offshore, south of Pozzuoli, significantly increasing the hazard in the area.