From thermal pressurization (TP) to dilatant strengthening (DS) during stick-slip ruptures on saturated saw-cut thermally cracked westerly granite

In fluid-rich faults, thermal pressurization (TP) is theoretically predicted to induce rapid fault weakening and facilitate large earthquakes, whereas dilatant strengthening (DS) can counteract this process through pore-space expansion. This study experimentally investigates the relative efficiency and condition of TP and DS using triaxial stick-slip tests with on-fault pore pressure (Pp) measurements. Dynamic stick-slip events were generated on four saturated, saw-cut, thermally cracked Westerly granite samples under varying effective confining (30–60 MPa) and pore pressures (25–45 MPa). Results reveal a systematic rupture transition from co-seismic Pp rise (TP-type) at low shear stress to Pp drop (DS-type) at higher shear stress, accompanied by a coupled fast–slow spectrum: fast TP → slow TP → slow DS → fast DS. Fast events reach slip velocities up to three orders of magnitude higher (0.5–10 mm s^-¹) than slow ones (0.001–0.5 mm s^-¹). The TP model under undrained, adiabatic conditions reproduces the measured Pp evolution, indicating a progressive shear-zone widening (0.05–0.5 mm, as overestimated values) that reduces TP efficiency and promotes slow events. For DS sequences, an increasing dilatancy coefficient is inferred, consistent with enhanced Pp drops. Breakdown energy shows no clear difference between TP and DS events, suggesting similar rupture energetics despite opposite pore-pressure evolution. Overall, this study provides the first direct experimental evidence of TP–DS transitions, demonstrating that TP governs early-stage weakening but diminishes as the shear zone widens, allowing DS to dominate. These results imply that in mature fault zones, after several seismic cycles, fault weakening may be mainly governed by co-seismic dilatancy, although strong, fast ruptures can still occur when the dilatant strengthening is not sufficient to stop the on-going rupture.