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The Kofu basin is situated in the south Fassa Magna, and is bordered on the west and south by active thrust faults. Previous studies have suggested that these faults are low angle thrust, from qualitative evidence. The activity of these faults has been evaluated in terms only of vertical slip rates, in spite of the fact that a low angle thrust fault has significant component of horizontal shortening. We attempted to determine the rates of net slip on these faults using gravity and geomorphological data. First, the geometry of the faults to 2-3km depths was determined by using closely spaced gravity values across the faults. In the Sone Hills along the south margin of the Kofu basin, the master boundary fault (presently inactive), which has thrust basement rocks northward over basin fill sediments, dips 10south. The frontal active fault at the northern margin of the hills dips 5 south, and develops wholly within the basin fill sediments, indicating thrust-front migration. In the Ichinose Upland along the west margin of the basin, however, the boundary fault (dipping 18 west) between basement rocks and basin fill sediments lies in front of the foothills, not at the topographic range front; the active fault at the range front is likely to be a secondary or subsidiary fault. Second, the rate of net slip on these faults was determined by elastic dislocation models, using geomorphologically observed surface deformation and the fault geometry that were deduced from gravity anomaly. The net slip rates were found to be 1.3 mm/yr for the Sone Hills, and as high as 6.38.3mm/yr for the Ichinose Upland. These results indicate that the west margin of the Kofu basin is subjected to horizontal shortening in an E-W direction at an extremely high rate, whereas the south margin of the basin is subjected to horizontal shortening in a NW-SE direction at a lower rate.

Fault Geometry and Net Slip of Low Angle Thrust Faults in the Kofu Basin, Central Japan