Fig. 1

a Tectonic setting and the distribution of GEONET stations: The rectangle with a broken line indicates the region of b. The solid circles indicate the used GEONET stations, and the solid square indicates the reference station. The solid triangle indicates Mt. Usu volcano. The solid stars with year indicate the epicenters of large earthquakes that occurred around Hokkaido. The open star indicates the epicenter of the 2003 Tokachi-oki earthquake with its focal mechanism (Yamanaka and Kikuchi 2003). b The secular interseismic velocity and the epicenter of large earthquakes: The secular velocity of crustal deformation from March 1, 1999, to September 1, 2003, is plotted. The vector represents the horizontal velocity, and the color represents the vertical velocity. The rectangle indicates the coseismic fault model of the 2003 Tokachi-oki earthquake estimated by the Geospatial Information Authority of Japan (2003). The solid side of the rectangle indicates the up-dip edge of the fault. The depth of the plate interface is indicated as contours with an interval of 20 km (Miyazaki et al. 2004). The open and solid stars with year indicate the epicenters of large earthquakes