GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.2 105 strongest local tectonic events, including a Mw 4.9 earthquake, suggesting that this gas has a common origin as CH 4 (i.e., mixing between microbial and thermogenic gas), but it is released only when tectonic stress is applied for sufficiently long periods as to cause H 2 S oversaturation in the hydrothermal aquifer. Water temperature decreases were also observed immediately after the two strongest earthquakes in the area, which helped us produce a comprehensive model to explain the observed geochemical variations. We model the geothermal system feeding the SVP thermal springs as a reservoir of fluids almost saturated in reduced gases (CH 4 and H 2 S) and very sensitive to tectonic stresses acting on it. When tectonic forces change the aquifer permeability, the geothermal reservoir becomes saturated in deep gases (CH 4 and H 2 S) and therefore it acts as a pressure valve that releases those gases depending on the rate and intensity of crustal stress acting on it. In this case, the relative proportions between CH 4 and H 2 S depend on the stress rate and its influence on the different solubility of the two gases in water. Volcanic activity seemingly did not play a direct role in changing the chemical composition of SVP gases, although it likely induced tectonic movements at the scale of the volcano that in turn influenced the chemistry of emitted gases. Methane is clearly derived from mixing between a thermogenic component and one originated from bacterial reduction of marine sulfates, as shown by its carbon isotopes analysis, thus ruling out any magmatic source of this gas. Slow stress accumulation could be the cause of long-term increases in CH 4 and maybe of CO 2 concentrations in the free gas phase, but not of H 2 S because of its fast dissolution/oxidation in the thermal reservoir. Conversely, more impulsive stress changes, possibly associated either with strong earthquakes or with accelerated spreading of Mt. Etna flanks, could induce fast release both of CH 4 and of H 2 S, thus revealing the common deep origin of the two gases. Carbon dioxide seems not to be affected in a similar significant way by such type of stress, thus highlighting its different, and shallower, origin. In any case, CH 4 is a proxy for the deep gas component and H 2 S is an indicator of very strong and fast stress change, thus being a potential precursor of impending strong release of seismic energy (at least at the SVP site). Actually, anomalous H 2 S gas pulses were seemingly correlated with shallow earthquakes that occurred along faults of the southern and southeastern flank of Mt. Etna, which affect in some ways the SVP site (Firetto Carlino et al. , 2019), whereas no correlation was found with the earthquakes magnitude. Fig. 3.