Electrical resistivity variation connected to volcanic earthquake in the Campi Flegrei, Italy

In this study we show an 2D Electrical Resistivity Tomography (ERT) survey acquired in Agnano site pre (Dec 5^th, 2019) and post (Dec 12^th, 2019) earthquake events occurred in Pisciarelli-Solfatara areas. This earthquake swarm consisted of sequence of 34 earthquakes with Magnitude (Md) -1.1≤Md≤2.8 at depths between 0.9 and 2.3 km. In particular, the earthquake of Dec 06^th, 2019 at 00:17 UTC with Md = 2.8 (depth 2 km) was the maximum recorded event since bradyseismic crisis began in 2005.

The ERT survey allow us to identify the main structural boundaries (and their associated fluid circulations) defining the shallow architecture of the Agnano volcano. The hydrothermal system is identified by very low values of the electrical resistivity (<20 Ω m). Its downwards extension is clearly limited by the lava and pyroclastic fragments, which are relatively resistive (>100 Ω m). The resistivity values are increased after the main shock. This increase in resistivity may have been caused by a change in the state of stress and a decrease in pore pressure (subsequent depressurization). Previously to the earthquake, an increase in pressurized fluids has been observed which have reduced the resistivity values. The present observation suggests that the temporal variation of the resistivity values is related to the variation of the pore fluid pressure in the source area of the swarm, facilitated by earthquake and the subsequent fluid diffusion. The combination of these qualitative results with structural analysis leads to a synthetic model of magmatic and hydrothermal fluids circulation inside the Agnano area, which may be useful for the assessment of potential hazards associated with a renewal of fluid pressurization, and a possibly associated partial flank-failure.