Vp/Vs ratio and dehydration reactions in subduction zones

High Vp/Vs ratio is a commonly used diagnostic for elevated fluid pressure when interpreting seismological data. The physical basis for this interpretation comes from rock physical data and models of isotropic, cracked rocks. Here, we establish precise conditions under which this interpretation is correct, by using an effective medium approach for fluid-saturated rocks. While the usual result of an increasing Vp/Vs with increasing fluid-saturated porosity holds for crack-like pores, we find that Vp/Vs ratio is not always monotonically increasing with increasing fluid content if the porosity shape deviates from thin cracks, and if the initial Vp/Vs of the rock (without porosity) is already quite high. This is specifically the case of dehydrating rocks, where initial Vp/Vs may already be high (>1.9 for lizardite, for instance), and where the porosity created by the dehydration reaction may be in the form of elongated needles. The model predictions are supported by existing experimental data obtained during dehydration experiments in gypsum and lizardite, which both show a significant decrease in Vp/Vs as dehydration proceeds. Although no experimental data is yet availbale on antigorite, we make a prediction that antigorite dehydration may not lead to any strong increase in Vp/Vs ratio under typical subduction zone conditions. We present our theoretical results in the form of simple closed-form solution (valid asymptotically for a range of limiting cases), which should help guide the interpretation of Vp/Vs ratio from seismological data.