Frictional Melting can Terminate Seismic Slips: Experimental Results of Stick-slips

Whether frictionally melted layers are weak or strong is a question in issue. We conducted stick-slip experiments for granite samples at 150 MPa confining pressure using a tri-axial apparatus. The pre-cut surfaces were mirror finished. In order to detect the exact time of melting, we set sensors inside the pressure vessel; two strain gauges for measurement of axial stress and fault slip, two electrodes on a pre-cut surface to measure tribo-electromotive force, and a troidal coil for monitoring the current which flows along the slip zone. From the electrode potential and the potential induced in the coil we calculate the resistance of the slip zone which is expected to decrease by several orders of magnitude once the slip zone is melted. The signals from these sensors were recorded synchronously at 2 MHz sampling rate. A moderately large stick-slip event was analyzed in detail. The fault slip, stress drop, rise time and maximum slip velocity were 0.32 mm, 230 MPa, 23 μ s and 40 m/s. The sensors detected precisely the time point when the slip zone melted. This occurred only 2 μ s after the slip velocity reached the maximum, and at the same time the friction coefficient reached a minimum (0.3). Immediately thereafter, it recovered promptly and remarkably, and the slip stopped eventually. Our SEM and EPMA observations ascertained the melting of the slip zone that was evidenced by a glass layer a few μ m thick in the experimented sample. The early half of the slip event is assumed to have been governed by solid interface friction, because carrot-shaped grooved and blobs of scratched debris were well developed in other experimented samples which experienced small events with ca. 0.1 mm slip. Our numerical simulations for frictional melting using observed time-shear stress and/or time-slip velocity data successfully reproduced the temperature and thickness of the melt layer, validating our experimental result at least phenomenologically. Therefore, we conclude that frictional melting is of the potential to stop seismic slips. We conducted the numerical simulations also for the spring-slider model with viscous friction. They were associated with a long tail of dumped slip velocity, while the tail was obviously shortened in our experiments. Our stick-slip experiments and numerical simulations demand a mechanism that melt layers are enforced to be cooled and strengthened. This mechanism is discussed in Otsuki and Koizumi (next presentation).