Detailed spatial slip distribution for short-term slow slip events and the spatial relation to megathrusts along the Nankai subduction zone, southwest Japan

Abstract:

Recent seismic and geodetic observations in subduction zones have revealed that slow earthquakes have preceded some large earthquakes. Characterization of slow earthquakes and their relation to megathrust events provide important clues to constrain a wide spectrum of slip rates on tectonic faults. In the Nankai subduction zone, southwest Japan, a bunch of slow earthquakes have intensively occurred especially at the transition zone at depths of ~ 35 km. Among a wide spectrum of slow earthquakes, short-term slow slip events (S-SSEs) with a duration of days to weeks have been comprehensively detected by dense geodetic observations such as GNSS and tiltmeters. Because crustal deformation due to a single S-SSE is small, they assumed the source fault as 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 Nankai subduction zone, we adopted a stacking approach of GNSS data by referencing the numbers of low-frequency earthquakes which are assumed to be accompanied by S-SSEs. We have successfully extracted cumulative displacements due to a series of S-SSEs occurred from 2004 to 2009, showing coherent signals that are almost opposite to the direction of plate subduction. Then, we inverted these surface displacements to fault slip along the plate interface. The estimated slip indicates significant slip patches laterally elongated along the transition zone at the depths of ~30–35 km, and small patches on the shallow portion of ~15 km depth in Tokai and Shikoku. While the Tokai small slip was located on the shallower side of the anticipated source area of a large earthquake, the two shallow patches in Shikoku were located on the downdip edge of the coseismic slip area of the 1946 Nankai earthquake. These transient slip in Shikoku were excited during S-SSEs at the transition zone. Through this triggering, the frequent occurrence of the deep S-SSEs invades the strongly locked megathrust zone and may intermittently increase the probability of large earthquakes in the Nankai subduction zone.