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Therefore hydrogeological phenomena occurring in active volcanic areas are probably the source of the observed electrical signals. This electrical field is the result of a polarization mechanism associated with the relative displacement between the solid grains and the pore fluid contained in the connected pore space of the porous material (see Ishido and Mizutani for a description of the underlying physics of this phenomenon). The flow of groundwater inside a porous material generates an electrical field of electrokinetic nature directed in the flow direction. Indeed, THM disturbances are responsible for forced groundwater flow in a volcanic system as shown recently by Matsumoto et al. The physical link between the two phenomena described above, that is between the electromagnetic theory (described by the Maxwell equations) and thermoporoelasticity (described by the Biot equations) can be found in the electrokinetic coupling. Other catastrophic hydrovolcanic phenomena have been described in the recent literature (see a short review by Fontaine et al. A set of observations made at Vulcano (Aeolian Islands, 1987–1989) and Campi Fregrei (Italy, 1982–1984) are qualitatively in agreement with the arrival of such hot and pressurized fluid fronts at the ground surface. When these shock waves reach the ground surface, they generate paroxysmal fluid emissions. The upsurge of pore water depends in turn on the pore fluid pressure gradient carried up with these waves. These “solitary” shock waves are associated with an upsurge of pore water yielding hydrothermal brecciation of the porous rock during their ascent. In such conditions, thermoporoelastic theory predicts the upsurge of hot and pressurized fluid fronts, which take the form of nonlinear thermomechanical and mechanical wave solutions of two Bürgers equations. investigated a model of rock fracturing in the subsurface of hydrothermal systems in response to temperature and pore fluid pressure perturbations. Several types of THM effects can coexist in a volcanic system. Active volcanic systems are also subject to thermohydromechanical (THM) disturbances in the preparation phase of a volcanic event. Among these signals, time and space electromagnetic disturbances of relatively high amplitudes (several tens of nanotesla for the magnetic field and several hundreds of millivolts for the electrical potential) have been clearly evidenced in a substantial number of field studies and correlated with volcanic activity. Early diagnosis of volcanic eruptions is a difficult task that has led geophysicists and volcanologists to measure various kinds of signals at the ground surface of active volcanoes. Tomography of the quasi-static electrical field is discussed and applied to self-potential profiles performed at the Piton de la Fournaise volcano during the preparation phase of the March 1998 eruption. Indeed, electromagnetic phenomena recorded at the ground surface of a volcanic system, once properly filtered to remove external contributions, provide a direct and quasi-instantaneous insight into the THM disturbances occurring in the heart of the volcanic structure prior and during a volcanic event. These signals can be used as precursors of a volcanic eruption. Our theoretical analysis predicts the diffusion of electromagnetic disturbances and quasi-static electrical signals. A new set of laboratory data shows that the electrokinetic coupling is very substantial in fractured basaltic and volcaniclastic materials, and in scoria with several hundreds of millivolts of electrical potential gradient produced per megapascal of pore fluid pressure variations. The nature of this coupling is electrokinetic, i.e., associated with water flow relative to the mineral framework and the drag of the excess of charge located in the vicinity of the pore water/mineral interface (the groundwater flow disturbance being related here to the THM disturbances in drained conditions). We couple electromagnetic theory (Maxwell equations) and thermoporoelasticity (Biot equations) to look at the ground surface electrical signature of these THM disturbances. The formation of a magmatic intrusion at depth is responsible for the formation of various thermohydromechanical (THM) disturbances including the upsurge of shock waves and diffusion of pressure fronts in the volcanic system.