Analysis of Hydrodynamic-Induced Stress in the Bossea Mountain Karst Aquifer through Passive Seismic Monitoring

Continuous passive seismic monitoring has provided insights into detecting and tracking the evolution of the rock mass responses under hydro-mechanical processes at multi-daily timescales. The Bossea Cave is a mountain karst aquifer located in the Ligurian Alps (Piedmont, NW Italy). It developed at the contact between a Permian impermeable meta-volcanic basement and an overlying Triassic meta-carbonate sequence. This upper unit controls the drainage of infiltrating water, mainly driven by rainfall and snowmelt processes occurring in the aquifer’s recharge area. The seismic signature of these water inflows was investigated using a three-component seismometer installed in the hypogeal environment. Ambient-noise Power Spectral Density (PSD) was computed on each component to identify high-energy frequency bands and examine their temporal evolution, comparing it with the discharge trends of the main drain (Mora Creek). A strong correlation was detected at approximately 33 Hz on the North, East, and vertical components, with a zero-lag response under ordinary recharge conditions. In contrast, during extreme inflow events (e.g., October 2020), increases in seismic-noise amplitude preceded the discharge peak by approximately 4 hours. These observations complement the results of a temporary external seismometer survey deployed on the slope above the cave in 2022, where precipitation-driven modulation was identified in the 50–90 Hz band and a 30-Hz amplification occurred about 4 hours before the peak discharge. This combined evidence provides a basis for investigating the triggering mechanisms of coupled thermo- and hydro-induced rock-mass deformation and for characterizing the spectral signatures of impulsive microseismic signals potentially associated with incipient fracturing.