It is very important to determine the initiation ages of active faulting in order to clarify the tectonic history of the Japanese Islands and to predict the evolution of the geological environment. In this study, we compiled existing information about the initiation ages of active faulting in the Japanese Islands; these ages were estimated by stratigraphic interpretation and calculation. We also included the initiation ages that we calculated from the total displacement and slip rate of active faults. The number of active faults, which started current style of activity, seems to have gradually increased from about 3 Ma. Since then, N-S trending reverse faulting has been observed in northeast and southwest Japan, and strike-slip faults in central Japan. The number of active faults increased drastically after 1.5 Ma, and peaked at around 0.5 Ma. Since 1.5 Ma, NE-SW trending reverse faulting occurred in the region extending from the southern part of northeast Japan to southwest Japan. Most of the active faulting in Kanto and Izu districts, which is attributed to reverse and strike-slip faults, started after 1 Ma. The trend of the initiation ages of active faulting almost correspond to the periods of the tectonic events that occurred near the Japanese Islands. After the peak of the initiation of active faulting around 0.5 Ma, the number of new active faults decreased. However, it is possible that the new active faults were undetectable because of their small displacements.

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The Chino fault consists a section of the Middle Itoigawa-Shizuoka tectonic line active fault system (ISTL), central Japan. The Chino fault is characterized by a high average slip rate reaching 10 mm/yr., which is one of the highest slip rates reported from active faults onshore Japan. Along the linear fault trace, channels and terrace risers are systematically offset by left-lateral slip and fault scarps by vertical component is clear on Late Quaternary terrace surfaces. Intensive studies on fault topography were conducted at Sakamuro. We carried out 1) air-photo interpretation, 2) LiDAR DEM analyses, 3) geomorphic mapping of terraces and faults, 4) pit excavation for stratigraphy and chronology, 5) measurement of strike-slip and dip-slip offsets, and 6) estimation of recurrence intervals and slip-per-event. The fluvial terraces are classified into eight surfaces from H to L3 in descending order, based on terrace deposits, overlaying tephra layers and radiocarbon ages (H: older than 120 ka, M1: 100 ka, M2a: 60 ka, M2b: 40 ka, L1a: 30 ka, L1b: 10 ka, L2: 5 ka, L3: 1-2 ka). Through these analyses, an average left-lateral slip rate of the fault is estimated to be 8.5 to 14.8 mm/yr. The last faulting event is inferred to have occurred after 1,000-2,000 yr B.P. when the L3 surface was formed. In addition, the left-lateral and vertical slip amounts per event are estimate as at 4.5 to 5 m and 2 m, respectively. The average recurrence interval calculated from the slip rate and slip-per-event is about 1,700-5,000 years.

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