Influence of Frictional Melt on the Seismic Cycle: Insights From Experiments on Rock Analog Material

The formation of frictional melt likely impacts the coseismic and, when solidified (pseudotachylyte), the interseismic strength of faults. Here we investigate these effects through experiments using a new energy-controlled rotary shear machine (ECoR) on simulated faults made of a transparent rock analog material (polymethyl-methacrylate). As in nature, ECoR allows (a) elastic strain energy to accumulate at different loading rates and (b) the spontaneous nucleation of slip events. ECoR is equipped with a high-speed camera, thermocouples, and transducers to monitor the surface, temperature, and acoustic emissions (AEs), respectively. We perform experiments at normal stresses of ∼3.5 MPa across loading rates from 0.15 MPa/s, phase A, to 2.5 MPa/s, phases B-C-D. In phase A, the temperature remains constant, and slip events occur without visible melting every 3.3–6.4 s with 0.5–0.7 MPa stress drops and 3–7 mm displacements. In phases B-C, slip events occur in the presence of melts every 0.5–0.9 s, and the bulk temperature increases progressively. Melt solidification increases static friction yielding slip events with stress drops up to 5 MPa and displacements up to 3 cm. Samples produce high-frequency AEs during slip acceleration and deceleration. Once the bulk temperature reaches ∼110°C, a “final” and silent long displacement event occurs in the presence of melts (phase D). Experimental observations suggest that melt formation modulates the coseismic (flash melting, melt lubrication, and viscous braking) and interseismic (welding) stages. Furthermore, AEs associated with coseismic fault weakening and strengthening may have their natural equivalent and could be observed in seismograms through near-fault instrumentation.