Anomalous Vp/Vs ratios at seismic frequencies might evidence highly damaged rocks in subduction zones

Unusually high compressional (P) to shear (S) wave velocity ratios (V-p/V-s) were measured at different subduction zones and interpreted as fluid-pressurized regions. Because no laboratory data reported such high values in isotropic rocks, mineralogical or anisotropic constrains were assumed. However, fluid-saturated rocks' V-p/V-s is a frequency-dependent property so that standard laboratory measurements cannot be directly upscaled to the field. Using a new methodology, we measured the property in the elastic regime relevant to field measurements for diverse lithologies. We obtained extreme V-p/V-s values, consistent with those reported at seismic frequency in the field. Consistently with a model, it shows that if high fluid pressure is a key factor, anomalous V-p/V-s values could evidence intense degrees of microfracturation in isotropic rocks, whichever its mineralogical content. The permeability of these regions could be larger than 10(-16)m(2). Plain Language Summary Anomalous seismic properties measured at subduction zones across the globe have been linked to the occurrence of earthquakes or tremors. However, different physical causes were postulated to interpret such seismic properties, and none of the laboratory measurements quantitatively fitted with those anomalous values. From laboratory measurements dedicated to link with field ones, we report the first data set that fits with field measurements observed at subduction zones. We show that such values might evidence a large degree of microfracturing, opened by high pore fluid pressure, in isotropic rocks of any mineralogy.