Surface phenomena witnessed during and after the 2012 Emilia earthquakes: Fact or Fiction? A response from water and gas geochemistry

The occurrence of anomalous surface phenomena accompanying seismic events of particular intensity is well known for a long time. During the recent 2012 Emilia earthquakes a number of anomalous events were experienced by local population: uprising of salty waters with bubbling gas (11 cases); heating of soil, groundwater and lake water often followed by fish death (13 cases); fracturing and subsidence (6 cases) and appearance of sand boils (about 20 cases) (http://ambiente.regione.emilia-romagna.it/geologia/temi/geologia/fenomeni-geologici-particolari). During and after the seismic crisis, both satellite IR thermal sensors and direct in situ measurements have revealed the existence of surface thermal anomaly, whose origin is still matter of debate. With the exception of small areas where the structural highs of the carbonate rocks are closed to the surface, the measured geothermal gradient in the area is normally quite low (1°C/100 m), with temperature not exceeding 50 °C at 2000 m depth. Moreover, no evidence of uprising of thermal fluid has been documented. With the aim of shedding light on the real characteristics of this thermal phenomena and on its origin a particular area directly affected by the 2012 seismic crisis known as “Warm Earths of Medolla” (TCM; literally, “Terre Calde di Medolla”) has been investigated in detail. TCM is a farming area, located in the Po Plain about 30 km far from the town of Modena (Emilia-Romagna region, northern Italy), which has already been known by local population for the relatively high temperatures of the soil. A measurements campaign, carried out after the devastating 2012 Emilia earthquake, showed soil temperatures up to 50 °C, i.e. 25-30 °C above the background value, coupled with diffuse fluxes of CH4(0-2432 g×m-2×d-1) and CO2(0-1184 g×m-2×d-1). According to the lateral and vertical distributions of the measured temperatures and chemical and isotopic compositions of the soil gases, the most reliable explanation consists in an exothermic oxidation of diffuse biogenic methane seeping at very shallow depth (< 1 m). Such a process occurs in the presence of free oxygen and methanotrophic bacteria and can thus explain (i) the observed ground heating up, (ii) the diffuse emission from the soil of CO2 characterized by an extremely negative isotopic (13C/12C) signature, and (iii) the lack of diffuse and low CH4 fluxes. Numerical modelling of heat generation and gas propagation toward the surface confirmed that this mechanism can sustain the observed anomaly. This mechanism of heat generation due to a strain-induced variability of CH4 fluxes and possible oxidation at shallow depths could explain most of the observed surface anomalies apparently associated with the 2012 Emilia earthquake, such as ground and groundwater heating, increasing turbidity of superficial waters and fish death, uprising of deep salty waters, other than, of course, the sudden appearance of sand boils.