三峠活断層系, 殿田断層世木林地区のトレンチ調査と最近の活動履歴 Trenching study on the Tonoda Fault of the Mitoke Active Fault System at Sekibayashi, Kyoto Prefecture, Southwest Japan
The Mitoke active fault system (MAFS) is about 50 km long with a NW-WNW strike in the western margin of the Tamba Mountains, and is generally characterized by an uplifting of the northeast side accompanied by a left-lateral slip. The MAFS is composed of many active faults and is one of the largest active fault system in the northern Kinki district. The Tonoda fault is situated in the central part of the MAFS and is about 15 km long. Based on a study of the fault topography, the Tonoda fault acted with an uplifting northeastern side and a left-lateral slip during the late Quaternary period. A paleoseismological study was requested due to the lack of evidence on which to base on evaluation of earthquake risk.<BR>To clarify the age of the latest event and intervals of events of the Tonoda fault during the late Quaternary, trenches were excavated across the fault scarplet at Sekibayashi where this fault cut river terraces with a NW trend.<BR>The results of this study can be summarized as follows:<BR>This fault is a reverse fault with northeastern side up-thrown and fault planes dips about 80° to the north. Based on differences of geologic section and fault striation, this is accompanied by a left lateral slip.<BR>1) The vertical displacements of depositional surfaces of Li and L2 terraces are 5 to 9m and 1.9-3m respectively, based on drilling and trenching data. This fault is characterized by a average vertical slip rate of 0.1-0.3m/1, 000 years during the late Quaternary period.<BR>2) At trench B, three faulting events were identified since 11 ka. The latest event was dated to be 1, 950-2, 310y. B. P.(A. D. 100-B.C. 395), that is from the Middle Yayoi to Latest Jomon period. The second and third latest events were dated to be 5, 500-7, 500y. B. P, and 8, 500-10, 710y. B. P. The average reccurence interval of faulting is estimated to be about 3, 740 years. A much older faulting event from 17, 000-22, 000y. B. P. was only identified at trench A because of the poor resolution for a paleoseismicity analysis.
地學雜誌 109(1), 73-86, 2000-02-25
Tokyo Geographical Society