Non-linear elasticity, earthquake triggering and seasonal hydrological forcing along the Irpinia fault, Southern Italy

Natural oscillatory stress sources can be exploited as ‘pump’ to ‘probe’ the time-dependent response of active fault zones to stress variations at various temporal and spatial scales and investigate time-dependent variations of their elastic properties (Delorey et al., 2021). Recently, we carried out a multidisciplinary study along the Irpinia Fault System (IFS, Southern Apennines) to investigate the response of the crust to hydrological forcing associated with phases of recharge/discharge of karst aquifers in terms of time-dependent variations of its elastic and hydraulic properties.

Charge/discharge phases of the karst aquifers in the Apennines cause significant seasonal and multi-annual strain transients (Silverii et al., 2016), that modulate the secular, tectonic deformation (~3mm/yr extension across the Apennines). These seasonal and multi-annual transients correlate with the seismicity rate (D’Agostino et al., 2018) and seismic velocity variations (De Landro et al., 2022; Poli et al., 2020). Previous studies (D’Agostino et al., 2018; Silverii et al., 2016) showed the high sensitivity of the IFS volume to hydrological stresses reflected in a complex, time-dependent response of deformation and seismicity.

Within this framework, we performed a natural analogue to a quasi-static laboratory ‘pump-probe’ experiment to assess the non-linear behaviour of the seismogenic volumes in response to non-tectonic deformations. We used the seasonal horizontal strains associated with discharge/recharge of karst aquifers as the ‘pump’. We computed continuous in-time seismic velocity variations δv/v using empirical Green's functions (the ‘probe’) reconstructed by autocorrelation on continuous 14-year-long time series of ambient-noise (Shapiro & Campillo, 2004). We initially analyzed two different sites (co-located GPS and seismic stations), near and afar the IFS. We found that δv/v are significant (∼0.2%) nearby IFS (shallow carbonate rocks), rather than far away from it.

We compared  for the site near IFS with the time series of Caposele spring discharge, strain and seismicity-rate. Our observations are coherent at seasonal and multi-annual scales and can be explained with the same mechanism. During periods of maximum hydraulic head within the aquifer, hydrologically related extensions are correlated with a decrease in the seismic wave velocity. During these episodes, the occurrence of microearthquakes is favored within the extensionally deforming belt along the Apennines thanks to the contribution of hydrological forcing. The non-linear elasticity suggested the presence of a multi-fractured and damaged crust subject to periodic seasonal phases of weakening/healing, potentially affecting earthquake nucleation processes.

Then, we extended our seismological analysis computing δv/v within the Campania-Lucania region. The other sites around IFS behave similarly, with a decrease in the seismic wave velocity commonly related to hydrologically extensions within the shallow carbonate rocks. Our observations confirm the significance of the hydrological forcing as a source of changes in elastic properties at a regional scale, a characteristic likely shared by the volumes surrounding the largest karst aquifers in the Apennines. For faults in a critical state, cyclical softening, g.e. caused by external forcing, may lead to failure and seasonal seismicity.