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Shallow slow slip events accompanied by seismicity has been increasingly reported in subduction zones during the last decade; however, the relationship between shallow slip events and seismicity is unclear. We report episodic slow slip events and seismicity at the southernmost Ryukyu Trench. The inversion results from GPS-derived cumulative displacements indicate that the slow slip events occur offshore in northeastern Taiwan on the shallow subduction interface, where high seismic VP/VS ratios applies. Seismicity is observed above the subduction interface for locations of significant variations in the VP/VS ratio and does not follow the migration of peak slip along the subduction interface. We calculated the temporal evolution between the seismicity and the propagation of slow slip events and determined that seismicity is not primarily driven by slip propagation. The spatiotemporal relationship, coupled with the VP/VS ratio, suggests that high-pressure fluid activity may be crucial to episodes of slow slip events and seismicity.

Plain Language Summary Slow slip events are a member of the slow-earthquake family observed by geodetic measurements in global subduction zones in the past 15 years. Shallow slow slip events are considered to be important for the megathrust earthquake cycle of the shallowest locked zone. Recently, shallow slow slip events accompanied by different seismic behaviors has been increasingly reported. This paper reports episodic, shallow slow slip events accompanied by overlying plate seismicity at the southernmost Ryukyu Trench and analyzes their relationship. We discover that the spatiotemporal relationship between the propagation of slow slip and seismicity is weak, which infers that they may not have a causal relationship. We suggest that the initiation of shallow slow slip events and subsequent seismicity was probably caused by high-pressure fluid activity. The interpretation may reveal the cause of many shallow slow slip events as a broad phenomenon.