Deep Structure of Active Fault Systems Developed along the Itoigawa-Shizuoka Tectonic Line, Central Japan, from Recent Seismic Reflection/Refraction Surveys

The 250-km long Itoigawa-Shizuoka Tectonic Line (ISTL), running with NS direction in Central Japan, is a major tectonic boundary between NE and SW Japan. The northern part of the ISTL is defined as the western boundary fault of the sedimentary basin formed by the Miocene backarc spreading of the Sea of Japan. Under a compressive stress regime since the late Neogene, the northern segment of the ISTL has been reactivated as a reverse fault system with a large slip rates (4-9 mm/yr), and is ranked an earthquake fault with the highest risk. Detailed formation process of the southern part of the ISTL, on the other hand, remains unclarified although it is evident that the arc-arc collision between the Honshu arc and Izu-Bonin arc, which is continuing from the Miocene time, plays an important role. Since 2002, several seismic reflection/refraction surveys were conducted across the ISTL. The most intensive research activity is the “Integrated Research Project for Active Fault System along Itoigawa-Shizioka Tectonic Line” funded by MEXT, Japan in 2005-2009, which succeeded in elucidating remarkable structural variation along this tectonic line. In the northern part of the ISTL (north of the Suwa Lake), the fault shows a gentle eastward dipping geometry (10-30 degrees) dominated by the thin-skinned tectonics associated with the backarc spreading and the subsequent tectonic inversion. The pre-tertiary basement was well mapped both from the reflection and refraction data, from which we see no evidence of higher-angle fault. Such low-angle fault structure well explains older refraction/wide-angle reflection data acquired in 1980 and 1990’s. The southern part of the ISTL, on the other hand, has a westward dip of 15-30 degrees. The most important finding in this region is a high deformation rate of 7-10 mm/y estimated for its southern most part, which provides quite an important constraint for fault extent/size of future earthquake. An intensive reflection/refraction experiments were carried out in the middle part of the ISTL, crossing the Suwa Lake. In the northern part of the lake, the eastward dipping event is dominant, which is consistent with the fault geometry in the northern ISTL. In the middle part of the lake, on the other hand, we see clear image of westward dipping fault extending more than 5-km depth, as is seen in the southern ISTL. These results strongly indicate an existence of remarkable segment boundary near the Suwa Lake, which probably controls the rupture process of earthquake occurring along this tectonic line.