Slip-partitioning on the Itoigawa-Shizuoka Tectonic Line, central Japan

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The overall objective of this study is to clarify the partitioning of slip on low-angle, oblique-slip faults. The test field is the central and northern part of the Itoigawa-Shizuoka Tectonic Line(ISTL), central Japan. The following results are obtained :(1)High resolution S-wave seismic reflection profiling across the Kamishiro fault(northern part of ISTL)revealed that the fault sprays up-dip into several fault planes, the lowest of which is a blind thrust at a depth of 50-60 meters.Radiocarbon dating of deformed sediments indicates that the rate of net slip on the Kamishiro fault is about 6 mm/yr, which value is significantly larger than previous estimates without considering fault spraying(2)P-wave and S-wave seismic reflection profiling was carried out across the central part of ISTL at Fujimi. The P-wave profile revealed that a high-angle, East-dipping fault is developed on the hanging-wall side of a low-angle, west-dipping fault. Late Quaternary surface deformation indicates that the former is dominantly strike-slip, whereas the latter is dominantly thrust. Bouguer gravity anomalies suggest that the low-angle thrust is the material boundary between Quaternary-Pliocene sediments on the east and Miocene and older rocks of Akaishi range on the west. These lines of evidence indicates that oblique slip(thrust+left slip)on the deeper part of ISTL is partitioned at shallow depths into two fault planes ; the strike-slip component is accommodated on the high angle fault that develops in the low-angle thrust sheet.(3)Similar phenomenon is observed in the northern part of ISTL at Matumoto and Shiojiri, although the master fault(material boundary)here is not west-dipping but east-dipping.