  Interplate megathrust earthquakes have inflicted catastrophic damage on human
society. An extremely hazardous megathrust earthquake is predicted to occur
along the Nankai Trough off southwestern Japan, an economically active and
densely populated area with historical records of megathrust earthquakes.
Megathrust earthquakes are the result of a plate subduction mechanism and occur
at interplate slip-deficit (or coupling) regions. Many past studies have
attempted to capture slip-deficit rate (SDR) distributions for assessing future
earthquake disasters. However, they could not capture a total view of the
megathrust earthquake source region because they had no seafloor geodetic data.
The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD)
has been developing a highly precise and sustainable seafloor geodetic
observation network deployed in this subduction zone to broadly obtain direct
information related to offshore SDR. Here, we present seafloor geodetic
observation data and an offshore interplate SDR distribution model. Our data
suggests that most offshore regions in this subduction zone have positive SDRs.
Specifically, our observations indicate previously unknown high-SDR regions
that are important for tsunami disaster mitigation and low-SDR regions that are
consistent with distributions of shallow slow earthquakes and subducting
seamounts. This is the first direct evidence suggesting that coupling
conditions are related to these seismological and geological phenomena. These
observations provide new fundamental information for inferring megathrust
earthquake scenarios and interpreting research on the Nankai Trough subduction
zone.
