Thermo‐Hydro‐Mechanical Model and Caprock Deformation Explain the Onset of an Ongoing Seismo‐Volcanic Unrest

Abstract

Modeling seismicity at volcanoes remains challenging as the processes that control seismic energy release due to fluid transport, heat flow, and rock deformation are firmly coupled in complex geological media. Here, we couple fluid‐flow and mechanical (deformation) simulators (TOUGHREACT–FLAC3D) to reproduce fluid‐induced seismicity at Campi Flegrei caldera (southern Italy) in isothermal (HM) and nonisothermal (THM) conditions. The unique ability of the Campi Flegrei caprock to withstand stress induced by hot‐water injections is included in the model parametrization. After pore pressure accumulation is guided by a combination of thermal and hydromechanical interactions, contrasting compressive and extensional forces act on the basal and top parts of the caprock, respectively. Then, pressure perturbation and caprock deformation induce fractures that allow hot fluids uprising to pressurize the overlying fault, driving it toward failure and triggering seismicity. Under similar mechanical boundary conditions, the induced thermal effects prompt seismic slip earlier but with higher seismic magnitudes when (1) thermal equilibrium is preserved and (2) the thermal contrast is enhanced due to increased fluid injection temperatures. The results indicate that numerical models of volcano seismicity must consider the influence of rock‐sealing formations to obtain more robust, accurate, and realistic seismic predictions at volcanoes. The proposed models satisfactorily reproduce the magnitude–depth distribution of the swarm (October 5, 2019), preceding the two strongest earthquakes recorded in 35 years at the caldera (3.1 and 3.3—on December 6, 2019, and April 26, 2020, respectively) using hot‐water injection from depth.

Fluid migrations and volcanic earthquakes from depolarized ambient noise

“…9). Similar maps are obtained in a shorter time interval (one to three days) around the Md3.3 to account for the increase in pore pressure following the inter-seismic period 47 (Fig. 3b, Extended Data Fig.…”

“…6, 01/12/2019), the R increases at all the stations around the location of the Md3.1 (Fig. 3b, pre-seismic), in a manner that is consistent with an increase in compression preceding earthquakes 47 . After the Md3.1, the unpolarized anomaly east of Solfatara expands toward the east (Fig.…”

“…2c, the transfer structure crosses the capped resistive plume that stores steam and gas, feeding fumaroles. Here, injections of uids from depth [6][7][8][9][10][11][23][24][25][26]47 coupled with meteoric recharge 27,28,43 produce stress 10 and out ow eastern liquidbearing sediments 11 . Gas-bearing uids over-pressurized the eastern caldera between 2011 and 2013 19 due to concurrent lateral expansion 10 of the source region and saturation of the reservoirs [6][7][8] .…”

“…Once informed by thermo-hydro-mechanical simulations 41 , polarization parameters show a quasi-real-time monitoring potential. Recent thermo-hydro-mechanical modelling 47 shows that uids are injected at the base of faults in the east caldera between three and ve days before the Md3.1, depending on injection volumes. Fig 3b and Extended Data Fig.…”

Numerical investigation of the influence of discontinuity orientations on fault permeability evolution and slip displacement

“…Also, the magnitude of permeability enhancement after the onset of slip is significantly influenced by the fault orientations, as fault permeability was enhanced by one order of magnitude in angle 110°(from 10 -15 to 10 -14 ), two orders of magnitude in the case of angle 45°(from 10 -15 to 10 -13 ), and over four orders of magnitude in angle 60°(from 10 -15 to 10 -10 ). From the work of Akande et al (2021) that involved the same permeability evolution model, 60°fault dip angle yielded the most interesting result for their study after comparing results from other fault angles. This is obviously the same with our study, where fault dip angle 60°resulted in the highest permeability when compared with the other two fault angles.…”