Along-Strike Variation and Migration of Long-Term Slow Slip Events in the Western Nankai Subduction Zone, Japan

We detected 24 long-term slow slip events (SSEs) in the western Nankai subduction zone, Japan, from 1996 to 2017 using a newly developed method: grid-based determination of slow slip events (GriD-SSEs). GriD-SSE systematically detects long-term SSEs and determines their fault parameters by fitting modeled displacement time series to observed Global Navigation Satellite System data. The detected SSEs included 11 previously undocumented events. The results suggest an along-strike segmentation of long-term SSEs. The along-strike variations in SSE recurrence intervals and total slip amounts appear to correlate with the variation of updip interplate locking. Long-term SSEs are less frequent in the downdip extensions of the Nankai megathrust (M-W 8-9) and Hyuga-nada earthquakes (M-W 7.5), with similar to 6-year recurrence intervals and total slip amounts that account for only 10-20% of the plate convergence. However, long-term SSEs do occur frequently downdip of the Hyuga-nada stable sliding zone and of the gap between the Nankai and Hyuga-nada earthquakes, with 2- to 3-year recurrence intervals and total slip amounts that account for 50-60% of the plate convergence. This correlation suggests that the long-term SSE activity is controlled by the updip locked areas owing to the stress shadowing effect. We also observed that the long-term SSEs repeatedly migrated in the downdip regions from the southwestern creeping zone to the northeastern Nankai megathrust locked zone. This SSE migration may represent the current state of stress accumulation and release processes that are controlled by the along-strike variations in interplate coupling along the entire western Nankai subduction zone.