A coupled hydrothermal and mechanical model for estimating thermo-poroelastic deformation, gravity and magnetic changes in calderas

Abstract:

Increasing integration of satellite and ground-based geophysical observations in volcanic areas has dramatically enhanced our ability to detect and track complex processes that can be difficult to reconcile with models of elastic mechanical behavior of upper crust. This study uses a thermo-poroelastic model to evaluate ground deformation, magnetic and gravity changes caused by hydrothermal fluid circulation and pressurization of magma chambers, in which surface topography, crustal heterogeneities and the presence of faults are taken into account. We develop a numerical framework for more realistic assessment of geophysical observations associated with volcanic processes, with particular focus on calderas. The numerical model is fully coupled with TOUGH2, a commercial software simulating multi-phase and multi-component fluid flow and heat transfer. The two-way coupling is performed through: (i) the concept of effective stress, which is controlled by pore pressure and thermal expansion, and (ii) empirical expressions for porosity, permeability, and capillary pressure, which are dependent on the effective stress. The model is applied to Campi Flegrei to simulate a generic unrest period caused by a deep injection of hot water and CO₂. Vertical uplift reaches 12 cm in 3 years, with associated thermomagnetic variations of c. 14 nT and gravity changes of c. 210 μGal, comparable to those observed during the 1982-84 bradyseism. Gravity and magnetic signals continuously increase for 2 years and approach steady state after 2.5 years. The contribution of thermal effects to total ground deformation is almost negligible in the first 3 years, but reaches half of the total after 15 years, and is dominant after 35 years. We also simulated the presence of faults and the contribution of a deeper magma chamber pressurization, which affect considerably the sub-surface circulation, and consequently the geophysical changes at the surface.