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Episodic tremor and slow slip (ETS) occurs in the transition zone between the locked seismogenic zone and the deeper, stably sliding zone. Actual mechanisms of ETS are enigmatic, caused by lack of geological observations and limited spatial resolution of geophysical information from the ETS source. We report that quartz-filled, crack-seal shear and extension veins in subduction melange record repeated low-angle thrust-sense frictional sliding and tensile fracturing at near-lithostatic fluid pressures. Crack-seal veins were coeval with viscous shear zones that accommodated deformation by pressure solution creep. The minimum time interval between thrusting events, determined from a kinetic model of quartz precipitation in shear veins, was less than a few years. This short recurrence time of low-angle brittle thrusting at near-lithostatic fluid overpressures within viscous shear zones may be explained by frequent release of accumulated strain by ETS.

Plain Language Summary Slow earthquakes have been detected from seismologic and geodetic observations. Episodic transfer of stress from slow earthquake source region to locked seismogenic zone is thought to trigger huge megathrust earthquakes. Despite these important findings and suggestions for slow earthquakes, actual conditions and underlying deformation mechanisms of slow earthquakes are still lacking. This paper reports that low-angle brittle thrusting at very small shear strength under near-lithostatic fluid overpressures repeatedly occurred at the time scale of slow earthquakes within viscous shear zones. This report is based on detailed geological study of the subduction melange exposed in the Shimanto accretionary complex, southwest Japan, which is thought to be the on-land analogue of the Nankai Trough subduction zone. In the paper, we suggest that crack-seal shear and extension veins and localization of viscous shear are geological manifestation of episodic tremor and slow slip and represent frequent release of accumulated strain in the transition zone between the locked seismogenic zone and deeper, stably sliding zones. This will provide fundamental geological evidence for the mechanics and kinematics at the slow earthquake source and the possibility that this strain release could cause increased probability of triggering megathrust earthquakes in the seismogenic zone updip of this frequent loading.