Horizontal Crustal Deformation in Southeastern Part of Shikoku, Southwest Japan, Deduced from GPS Surveys

Abstract

Observation of crustal deformation using Global Positioning System (GPS) has been conducted in Shikoku region, Southwest Japan, to monitor the interaction between the Eurasian and the Philippine Sea plates along the Nankai trough. Four repeated surveys conducted in 1990 to 1994 have revealed that the stations located at the coast line are displaced to the northwest to west-northwest direction relative to the inland station. This direction is parallel to the moving direction of the Philippine Sea plate and opposite to the coseismic displacement at the time of the 1946 Nankai earthquake (M=8.0). During the period of four years, the maximum rate of 35mm/yr is obtained at Muroto, the station nearest to the Nankai trough and located in the source region of the 1946 earthquake. This displacement rate amounts to three quarters of the relative velocity between two plates estimated from seismological data. This implies that the present interplate coupling along the Nankai trough off Shikoku region is strong and causes considerable deformation of the plate-convergence region. As a whole the southeastern part of Shikoku is contracted with a constant rate of about 3×10^-7/yr. Strain pattern obtained in this study is very similar to the long-term triangulation and trilateration results.