Slip distribution of the 1973 Nemuro-oki earthquake estimated from the re-examined geodetic data

Geodetic data, including leveling, tide-gauge, triangulation/trilateration, and repeated EDM data, from eastern Hokkaido, Japan, were re-examined to clarify the crustal deformation associated with the 1973 Nemuro-oki earthquake. We inverted the geodetic data to estimate the slip distribution on the interface of the subducting Pacific plate. The estimated coseismic slip, potentially including afterslip, showed a patch of large slip (i.e., an asperity) near the epicenter of the mainshock. The moment magnitude of the Nemuro-oki earthquake was estimated to be 8.0 from the geodetic data, which is comparable to the 2003 M_w = 8.0 Tokachi-oki earthquake. The estimated slip distribution suggests a 50 km-long gap in the coseismic slip between the 1973 Nemuro-oki and the 2003 Tokachi-oki earthquakes. The slip area of the 2004 M_w = 7.0 Kushiro-oki earthquake, estimated from GPS data, was located at the northwestern edge of the Nemuro-oki earthquake, which implies that the area may have acted as a barrier during the Nemuro-oki earthquake. The postseismic deformation observed by leveling and tide-gauge measurements suggests that the afterslip of the Nemuro-oki earthquake occurred at least in a western and northern (i.e., deeper) extension of the asperity on the plate interface.