A dense GPS network was established in 1997 around Ou Backbone Range (OBR), northeastern Japan, by deploying 9 new continuous GPS stations so as to complement the sparse portion of GEONET (GPS Earth Observation Network) operated by Geographical Survey Institute (GSI), Japan. This new observation network is aimed to investigate the present deformation and to understand the relationship between the occurrence of intraplate earthquakes and the deformation process of an island-arc crust.
GPS data are analyzed using a precise point positioning strategy of GIPSY/OASIS-II. Coordinates of the GEONET stations in daily SINEX files have been supplied by GSI. A map of the intraplate deformation field derived by combining the two data sets and by subtracting the motion of the North American Plate demonstrates displacements toward WNW along the Pacific coast. This is caused by the interplate coupling between the Pacific and North American Plates. Displacements along the Japan Sea coast, on the other hand, are considerably small in magnitude, and directed to NW and N in the southern and northern area, respectively. OBR lies halfway between the two coasts and shows the transitional pattern of both deformation characteristics.
By producing grid data of displacement velocities and taking spatial derivatives, we obtained a map of strain rate distribution. We found that the region between 38.8°N and 39.8°N in OBR demonstrates notable concentration of EW contraction. The region coincides with the area of active seismicity and includes the focal areas of 1896 (M7.2), 1962 (M6.5), 1970 (M6.2), and 1998 (M6.1) events. The observed strain is larger than can be explained by the total moment release of earthquakes that occurred in the same period of this study. Possible sources of the strain concentration may be viscoelastic deformation due to the large earthquakes and/or aseismic slip along the deeper extension of active faults.
