Detailed Spatial Slip Distribution for Short-Term Slow Slip Events Along the Nankai Subduction Zone, Southwest Japan

Many short-term slow slip events (S-SSEs) occur at the transition zone along the Nankai subduction zone, southwest Japan. Because crustal deformation due to a single S-SSE is small, the source fault is often represented using a planar uniform single-fault slip model, resulting in little constraint on the spatial heterogeneity in amounts of slip. To comprehensively investigate the detailed cumulative spatial distribution of S-SSEs in the entire Nankai subduction zone, we adopted a stacking approach of Global Navigation Satellite System data by referencing low-frequency earthquakes. We extracted the cumulative displacements due to 12, 11, and 14 S-SSEs that occurred between 2004 and 2009 in eastern Shikoku, Kii, and Tokai; coherent displacements nearly opposite to the direction of plate subduction were obtained. The inverted slip indicated significant slip laterally elongated along the transition zone at similar to 30-35 km depth. Small slip patches were inferred in the shallow portions at similar to 15-20 km depth in eastern Shikoku and at similar to 10-15 km depth in Tokai, respectively. The shallow patch in eastern Shikoku was located on the downdip edge of the coseismic slip area of the 1946 Nankai earthquake, while the Tokai small slip was located on the shallower side of the anticipated source area of a large earthquake. Large slip patches of S-SSEs tend to avoid the spatially dense low-frequency earthquake areas; in major S-SSE areas, the number of low-frequency earthquakes is small. This spatial dependence of fault slip style within the transition zone provides new insights on the generation mechanism of slow earthquakes.