Passive seismic monitoring of a jointed rock mass at Acuto Field Lab (Italy)

Identifying potentially unstable sites and analyzing possible triggering factors are essential for the monitoring and mitigation of natural hazards related to rock slope instability. The challenge posed by the reduced predictability of rock failures has led to a growing demand for advanced monitoring techniques. In this framework, the integration of multi-sensor monitoring systems with numerical modelling represent a promising approach to investigate rock mass behaviour and understand the influence of external physical factors. In particular, passive seismic techniques play a key role in characterising and detecting precursory signals of rock mass failures, providing valuable insights into their internal structure in view of detecting the progress of preparatory processes, such as the rock mass damaging, which anticipates the failure. The passive seismic data screening over a one-month period is here reported for a 140 m3 prone-to-fall protruding rock tower at the Acuto Field Laboratory (Central Italy), coupled with a preliminary numerical model of the unstable volume to validate the seismic observations. Analysis of ambient seismic noise and microseismicity were performed on continuous seismic recordings to assess the stability of the unstable tower and investigate potential changes driven by meteorological stressors. The resulting seismic parameters (i.e., natural resonance frequency of the unstable block, microseismic events) exhibited daily variations without any irreversible changes over the monitored period. However, fluctuations in daily rock temperature were found to control resonance frequency values and microseismic activity. This was interpreted as the results of rock mass thermal expansion and contraction, as further confirmed by displacement measurements on extensometers. Ground motion orientation analysis and 3D numerical modeling of the unstable compartment provided additional support for these conclusions.