Can Directional Resonances Be Used to Map Intensely Deformed Fault Zones of Mt. Etna Volcano ?

Abstract

Mt. Etna is characterized by flank instability of the eastern to south-western portions of the volcanic edifice, producing down-slope movements with rates up to several decimeters in a month during eruptive events of the 2002-2003 activity. The unstable sector is bounded to the North by a E-W transtensive fault (the Pernicana fault system), extending from the NE Rift to the coastline of the Ionian Sea for a length of >18 km. The western portion of the Pernicana fault system (close to Piano Pernicana area) is characterized by the most intense deformation. In this area we have performed volcanic tremor measurements on a dense grid along and across the fault zone. Ambient vibration measurements are also performed along a second fault (Tremestieri fault) which confines the slip of the eastern flank to the south-east. The analysis using both microtremors and local earthquakes recorded in these faults shows persistent polarization of ground motion. Horizontal-to-vertical spectral ratios (HVSR) show large directional resonances of horizontal components within the damaged fault zones. The resonance occurs around 1Hz at Piano Pernicana, and around 4 Hz in the Tremestieri fault zone. The resonance amplitude in the HVSRs seems to be fairly well correlated to soil gas anomalous concentrations (in particular, radon and CO2 both considered tracer gases of major crustal discontinuity) in the two fault zones, suggesting that both the effects are linked to local fracturing conditions. According to previous results on velocity anisotropy in the shallow crust, we believe that a role on polarization could be played by stress-induced anisotropy and micro-fracture orientation in the near-surface lavas. The occurrence of directional resonances, if confirmed in other faults, can be a powerful tool to map buried damaged fault zones on the Mt. Etna volcano.