Chemistry in earthquake: the active chemical role of liquid and supercritical waters in microfracturing at depth

Abstract

The role of water in the nucleation process of an earthquake and its contribution to the mainshock is ascertained by many models in its physical part, as a factor capable of altering the balance of pressures and thus influencing the effective pressure. Assuming that water is present at depth, starting from the observation of its molecular structure at various crustal pressure and temperature conditions, the present paper analyses water’s chemical role in relation with the rock matrix, and its response during microfracturing. The creation of a network of new void spaces produces a decrease of the water pressure. Water may respond at molecular scale differently, depending on its aggregation state. Effectively depressurisation has a limited influence on the liquid water, only if it does not cause the transition to the vapour phase. Conversely, depressurisation causes an instantaneous variation in the intermolecular structure of supercritical water (SCW). Specifically, the nearly total disappearance of its ionic characteristics: that means the severe drop of solubility constants. At the same time, the already low viscosity decreases too: SCW intrudes easily into new fissures. When the microcracks tend to close, SCW reacquires adequate ionic characteristics for the rise in density (isothermal pressurisation); hence, an intense water rock interaction starts with freshly opened surfaces. This process influences actively the subcritical crack growth too, again with differences between liquid and SCW: last one participates only when reacquires density. Summarising, it is likely that water plays a fundamental and active role in determining the rock weakening, once earthquake preparation process begin with the development of microcracks are forming, perhaps playing an active role in determining the main rupture. With different modalities according to its aggregation state.