GPS連続観測による日本列島上下地殻変動とその意義

抄録

To evaluate a reliability of GPS vertical data we mapped vertical crustal deformation field of Japan using continuous GPS measurements with a nationwide dense network (GEONET) of the Geographical Survey Institute during the period from 1996 to 2003. We confirmed that the GPS vertical deformation field reasonably agrees with those derived from leveling, tidal record and geomorphologic analysis. The conformity with the other data sets suggests a dependability of the GPS vertical results. The GPS data illustrate details of a spatial pattern of the vertical deformation field and manifest their usefulness when applied to constrain tectonic models. A subsidence along the pacific coast of the southeastern Hokkaido propounds a possibility of a downdip extension of a plate coupling reaching to the depth of about 80km. Uplift around Hidaka mountains in the central Hokkaido suggests a present-day mountain building process at least during the interseismic period. An apparent subsidence found in the central mountainous region of the central Honshu island contrasts sharply with the presumed uplift through Quaternary inferred from geomorphologic analyses. Vertical deformations along the Nankai trough in the southwestern Japan can be attributed to an elastic deformation due to a dragging of the subducting Philippine Sea plate. The GPS result confirms a coupling of plates and a resultant strain accumulation in the Tokai region. Those results demonstrate the usefulness of the GPS vertical data and encourage us in further applications in the studies to understand ongoing tectonic processes in Japan.