Offshore geodetic data conducive to the estimation of the afterslip distribution following the 2003 Tokachi-oki earthquake

A cable-based seafloor observatory located offshore of Hokkaido, Japan, provides the worldʹs first direct observation of offshore post-seismic displacement. After removing thermal noise, this data is used to constrain the distribution of offshore afterslip from a great interplate earthquake (Mw 8.0) that occurred during 2003 along the southern Kuril Trench. A checkerboard resolution test showed a great contribution of the offshore data to improving the spatial resolution on the plate interface slip near the trench. The 1-yr afterslip is distributed in a U-shaped pattern encircling its co-seismic slip area. This result indicates that the stick-slip frictional properties of the plate interface are distributed in a patch-like pattern as opposed to a banding pattern as previously assumed in young subduction zones.