Active Faults on Miura Peninsula, Southwest of Tokyo, and Their Holocene Activities.

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  • 三浦半島の活断層　完新世における活動史と問題点
  • ミウラ ハントウ ノ カツダンソウ カンシンセイ ニ オケル カツドウシ ト モンダイテン

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Abstract

Five subparallel active faults striking NWW-SEE truncate hilly Miura Peninsula, on the west side of Tokyo Bay. Right-lateral offset has predoiminated on these faults during the late Quaternary, associated with some vertical component. Seismic risk is a concern in Miura Peninsula, where the densely populated city of Yokosuka is built near or across these faults.<br>Eleven sites were drilled or excavated across three northern active faults, Kinugasa, Kitatake, and Takeyama faults, in order to obtain faulting history, including the time of faulting and the amount and nature of offset. This research has been carried out as official project of city or prefectural agcucies, supported by funding from the Science and Technology Agency, Japan, especiaally since the 1995 Kobe earthquake, as well as by personal research projects.<br>I summarize here the results of these projects. On Kitatake Fault, the latest activity is estimatetd to have occurred between 1, 200-1, 400yrs BP at seven sites. Repeat time is inferred to be 1, 500-2, 000 years, based on differential thickness of strata on the two sides of the fault. Three events during the last 5, 500 years were identified at Tsukui trench log along the Taleyama Fault. The latest event occurred 2, 000-2, 200yrs BP, certainly older than that of the Kitatake Fault., implying that these two faults activated independently. On the northernmost Kinugasa Fault Neogene rocks are intensively shattered and Holocene deposits are clearly deformed. This confirms that Kinugasa Fault is certainly an active fault which activated two times since ca. 13, 000 years BP.<br>Repeat time is estimated to be 1, 500-2, 000yrs for Kitatake Fault and to be average 2, 000 years along Takeyama Fault. But it may be shorter, because they probably converge in one major fault underground. In any case, these two faults are approaching the next period of activity.

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