Evolution of the Vp/Vs ratio during deformation and implications for fault mechanics

• Vp/Vs > 2.2 arises when high differential stress combines with low effective pressure. • The transition from compaction to dilation produces the largest change in Vp/Vs. • Differential stress curves are influenced by both effective pressure and initial porosity. • The results constrain the spatial and temporal evolution of fault zone Vp/Vs signatures. High ratios of compressional (P)-wave velocity to shear (S)-wave velocity (Vp/Vs) in seismogenic zones are commonly interpreted as evidence of near-lithostatic fluid pressures, yet active faults and associated damage zones also experience significant tectonic stress, which can strongly influence seismic velocity ratios. Despite this, few laboratory studies have explored how fluid pressurization and tectonic stress act together to control Vp/Vs evolution during deformation. We addressed this gap by performing triaxial experiments on Fontainebleau sandstone, measuring ultrasonic P- and S-wave velocities in real time. Each test started with a hydrostatic stress phase during which fluid pressure was increased up to near-lithostatic values. This phase was followed by a non-hydrostatic one, during which we simulated tectonic loading by increasing the differential stress until failure. Under hydrostatic conditions, fluid pressurization produced only a modest increase in Vp/Vs (from ∼1.6 to ∼1.7), consistent with elastic crack reopening. When differential stress was applied, however, Vp/Vs increased sharply at the onset of inelastic deformation (from ∼1.7 to ∼1.9) and reached extreme values during dilatant failure (∼2.2 to ∼2.4), especially under near-lithostatic fluid pressure. These results show that elevated Vp/Vs values arise when differential stress acts together with low effective pressure, conditions present near overpressurized and critically stressed fault zones. This mechanism provides a physical explanation for the extreme Vp/Vs anomalies observed in seismically active and overpressured fault structures, highlighting the diagnostic potential of monitoring the Vp/Vs evolution for assessing fault stability.