Effects of episodic slow slip on seismicity and stress near a subduction-zone megathrust

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab under Kii Peninsula, Japan, spanning nearly 50 major deep slow slip and tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of inslab seismicity are temporally associated with the slow slip episodes. Furthermore, although stress orientations in the slab below these slow slips may rotate slightly, inslab orientations 20 to 50 km updip from there rotate significantly, suggesting previously-unrecognized transient slow slip occurs on the plate interface updip. We infer that fluid migrates from slab to interface, promoting episodes of slow slip, which break mineral seals, letting fluid migrate 10’s of km further updip along the interface where it promotes transient slow slips. The proposed methodology, based primarily on inslab seismicity, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes.