GPS連続観測による長町-利府断層帯およびその周辺 (東北地方中部) の地殻変動

Abstract

We constructed eight continuous GPS stations across the Nagamachi-Rifu fault zone, northeastern Japan to reveal the detailed spatial pattern of crustal deformation. High strain rate around the fault zone observed by the GEONET and the occurrence of the MJMA 5.2 Sendai earthquake on September 15, 1998, at the deep edge of the seismogenic part of the fault motivated us to confirm the aseismic sliding of the fault in or below the seismogenic depth from the contemporary deformation. We analyzed the GPS data of new stations as well as existing stations, totaling up to 50 stations in central Tohoku, using GIPSY 2.6.1 software and estimated GPS site velocity and strain rate by a least square method. One component of the principal strain rate shows distinctive compression in the E-W or ESE-WNW direction in the whole region. In the forearc region, the other component of the principal strain rate are almost zero in the south of 38°15′, while NNE-SSW extension in the north of 38°15′. Strain rate distribution is spatially heterogeneous in the region. We found a high strain rate zone west of the Nagamachi-Rifu fault zone and that between Honshu and Awashima in the eastern margin of the Sea of Japan. Low strain rate zones exist in the west of the high strain region of the Nagamachi-Rifu fault zone and in the western half of Yamagata Prefecture. Strain rate distribution pattern on the hanging wall side of the Nagamachi-Rifu fault can be modeled by aseismic sliding of 10mm/yr on a horizontal detachment fault connecting to the Nagamachi-Rifu fault at a depth of about 15km beneath the Ou backbone range. However, it is difficult to constrain a fault model for the strain rate distribution from the present dataset uniquely.