Mechanism of shallow slow slip events in the Nankai Trough: Insights from laboratory friction experiments and numerical modeling

Shallow slow slip events (SSEs) have attracted considerable attention due to their mutual interaction with earthquake processes, and are known to occur in the Nankai Trough, southwest Japan. To examine their underlying mechanisms, we conducted velocity-stepping friction experiments on intact core samples retrieved from IODP Site C0023 at the toe of Nankai accretionary prism and performed numerical modeling based on rate-and- state friction (RSF) laws. Our measurements show that fault zone samples transition from velocity weakening to velocity strengthening as slip velocities increase, and that some RSF parameters show a dependence on sliding velocity. Numerical models using velocity-dependent RSF parameters, constrained by our experimental data, successfully replicate SSEs comparable to those observed in the Nankai Trough, whereas models based on non- transitional frictional behavior (constant RSF parameters) or near-neutral stability (constant RSF parameters with extremely small velocity weakening) cannot. We propose that the transitional frictional behavior with increasing slip velocity is a key mechanism of shallow SSEs in this region. Furthermore, our findings document a weak thrust fault with potentially unstable slip behavior.