Detailed Spatial Slip Distribution for Short-term Slow Slip Events along
the Nankai Subduction Zone, Southwest Japan
Abstract
Short-term slow slip events (S-SSEs) intensively 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 to
little constraint on the spatial heterogeneity in amounts of fault 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 (GNSS) data
using low-frequency earthquakes as reference. We extracted cumulative
displacements due to a series of S-SSEs from 2004 to 2009; coherent
signals in almost opposite direction of plate subduction were obtained.
The inverted slip indicated significant slip patches laterally elongated
along the transition zone at ~30–35 km depth, and small
patches in the shallow portions at ~15–20 km and
~10–15 km depth in eastern Shikoku and in Tokai as well
as western Shikoku, respectively. The shallow patches in Shikoku were
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 were complementary to 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 even within the transition zone provides new insights
regarding the generation mechanism of slow earthquakes.