The Itoigawa-Shizuoka Tectonic Line active fault zone is one of the largest active faults in Japan, with a length of 150 km. The previous paleoseismological studies revealed a 14% possibility of a large earthquake in the next 30 years. The Japanese Ministry of Education, Culture, Sports, Science and Technology has initiated an intensive research project on this fault zone in 2005, in which we have conducted a tectonic geomorphological study on the northern part of the fault zone in order to predict its coseismic behavior. We have interpreted largescale aerial photographs taken not only in recent years but also in the 1940s and 1960s for examining faultrelated topography including those which have been already modified or destroyed, and then conducted a field survey to determine ages of geomorphic surfaces. We also carried out photogrammetric analyses which have resulted in our highly-dense offset data whose spatial intervals are of 500-1000 meters in average. Based on these surveys, we estimated a net-slip rate distribution along the northern part of the fault zone, and calculated coseismic slip distribution assuming that the fault zone follows the characteristic earthquake model. Largeroffset areas were identified in the Hakuba Village and the Ikeda Town. These peaks of surficial slip distribution would imply subsurface fault which generate strong ground motion. The maximum vertical offset during the last earthquake is estimated to be 5 - 6 meters. Therefore, the coseismic net slip would exeed 10 meters when we assume the fault-slip responds to the maximum shear stress. Our detailed coseismic slip distribution and the subsurface fault geometry predict a moment magnitude of 7.5 for the northern part of the fault zone.

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Geologic structure around the source area of the 2007 Chuetsu-oki earthquake and the Sado Basin was examined based on seismic profiles. The earthquake occurred in the fold belt of the Niigata basin, one of the largest Miocene rift basins in Japan. High-resolution multi-channel seismic profiles in and around the source area showed that there are active folds and inactive folds. One of the NE-SW trending active folds has been related to the SE dipping source fault of the 2007 earthquake, which continues from the shelf to the basin. The fold is parallel to the strike of the source fault of the 2007 earthquake, but is located about 5 km northwest of the source area. Other folds above and around the source area of the 2007 Chuetsu-oki earthquake are inactive. The active anticline accompanies 5 m vertical offset on the erosional surface during the last glacial age and the surface sediments covering the Sado Basin. The active anticline does not continue to the north of 37°38′N. The slope of the SE side of the Sado Basin is underlain by nearly horizontal sediments and the slope has been developed by progradation of shelf slope during glacial ages. These observations on seismic profiles indicated that the active fault and anticline related to the 2007 Chuetsu-oki earthquake does not extend to the northeast along the southeastern slope of the Sado Basin except in the southern most part.

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We show locations of active submarine faults near the focal area of the 2007 Chuetsu-oki earthquake in detail, and investigate fault structure based on tectonic landforms and sounding profiles obtained by Tokyo Electric Power Company. There are several active submarine faults in the area. The Plio-Pleistocene series are cumulatively deformed by reverse faulting. The Sado Basin lies between the Eastern-boundary fault dipping southeast and the Western-boundary fault dipping northwest. The former is an active reverse fault over 50-km long. Tectonic geomorphic features indicate that the Eastern-boundary fault branches upward into two faults at a certain depth with a ramp and flat structure next to the focal area. The Eastern-boundary fault B merges downward into the southwestern half of the Eastern-boundary fault A extending into the swarm of the aftershocks associated with the earthquake. The 2007 Chuetsu-oki earthquake has resulted from the reverse faulting in the southwestern half of the fault A. Although the magnitude of this recent quake was only 6.8, the Easternboundary fault A, over 50-km long reverse fault, is capable of generating a much greater earthquake. The largest nuclear power plant suffered considerable damage from the earthquake. This is because the active submarine faults have been slurred over. We should correct the inappropriate fault assessments in Japan's nuclear society.

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The γがんま-ray survey was executed as a technique for detecting the fault. From the result of detection of fracture width, the fracture width broadens by the fault length . The relationship of fracture width(FW) to fault length(L) can be expressed as follows: L(km)＝0.36FW(m)＋0.62. Activity level of the active fault can be evaluated according to the fracture width of short active fault. The short active fault distributed on northeast side of Kego fault is evaluated as C class active fault from the estimation using fracture width.

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This study clarifies the recurrence interval of earthquakes in the southern part of the Western Boundary Fault Zone of the Yamagata Basin by the formative age of fluvial terraces based on drilling survey on the terrace to obtain overlying loamy soil including wide-spread volcanic ashes, such as K-Tz (95 ka) and AT (26-29 ka). The late Quaternary fluvial terraces in this area are divided into Terraces I to V in descending order. K-Tz is contained at the lowest part of loamy soil on Terrace I and found in the fluvial deposits of Terrace II, AT is found at the middle part of loamy soil on Terrace III. From the ages and the vertical displacements of the fluvial terraces, the average vertical slip rates of the southern part o the Western Boundary Fault Zone of the Yamagata Basin are 0.22-0.28 mm/yr, and recurrence interval of the earthquakes is estimated to be 7100-13800 years.

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