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~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.

Although observations of microseisms excited by ocean swells were firmly established in the 1940s, the source locations remain difficult to track. Delineation of the source locations and energy partition of the seismic wave components are key to understanding the excitation mechanisms. Using a seismic array in Japan, we observed both P and S wave microseisms excited by a severe distant storm in the Atlantic Ocean. Although nonlinear forcing of an ocean swell with a one-dimensional Earth model can explain P waves and vertically polarized S waves (SV waves), it cannot explain horizontally polarized S waves (SH waves). The precise source locations may provide a new catalog for exploring Earth's interior.

We report on two small long‐term slow slip events (SSEs) within a gap between tremor and locked zones in the Nankai subduction zone, southwest Japan. The SSEs were detected by subtracting the steady state component and postseismic effects of large earthquakes from long‐term and high‐density Global Navigation Satellite System data. Both SSEs occurred in adjacent regions of the Bungo channel following long‐term SSEs in the Bungo channel in 2003 and 2010. The estimated slip was 1–5 cm/year that lasted at least 1–2 years after 2004 and 2011, partly accommodating plate convergence. As the low‐frequency tremor in the downdip region is activated at the same time as the Bungo channel long‐term SSE, a spatiotemporal correlation was observed between the detected SSEs and long‐term tremor activity in the downdip region. This correlation indicates along‐dip interaction of the slips on the subducting plate interface.