1990 Fiscal Year Final Research Report Summary
Reevaluation of the Activity of Quaternary Intraplate Thrust Faults Based on Surface Deformation and Fault Geometry at Shallow Depths
| Project/Area Number |
63460235
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
自然地理学
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| Research Institution | The University of Tokyo |
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Principal Investigator |
YONEKURA Nobuyuki The Univ. Tokyo, Fac. Science Professor, 理学部, 教授 (30011563)
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| Co-Investigator(Kenkyū-buntansha) |
IKEDA Yasutaka The Univ. Tokyo, Fac. Science Research Assoc., 理学部, 助手 (70134442)
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| Project Period (FY) |
1988 – 1990
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| Keywords | active fault / thrust fault / slip rate / fault geometry |
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| Research Abstract |
The activity of Quaternary thrust faults in Japan has been evaluated on the basis mainly of vertical slip rates. This method, however, would to underestimating the activity of a thrust fault, if has a low angle fault plane or it is covered with unconsolidated sediments. The overall objective of this study is to evaluate the activity of thrust faults on the basis of shallow fault geometry and associated surface deformation. Three Quaternary thrust zones in northeast and central Japan were selected as test fields ; the following results were obtained :
1. An exploratory trench was excavated across the boundary fault zone at the western margin of the Kitakami Plain. It is revealed that the main fault plane is listric ; the dip angle is as low as 20 deg at a 2 m depth. Detailed observations of deformed late Ouaternary alluvial fans suggest that the fault plane becomes flat with depth.
2. Kofu basin is bordered on the west and on the south by active fault zones. High spatial resolution gravity measurements were carried out along a profile across each boundary fault zone, in order to reveal the geometry of the fault zone at shallow depths. The western boundary fault zone is a low-angle thrust fault, along which Ouaternary basin fills are underthrust for at least several km, indicating that the rate of slip on the fault is quite high. The southern boundary fault zone also is a thrust fault probably with an extremely low dip angle. The total thickness of basin fills, however, decreases toward the southern boundary, whereas it increases toward the western boundary. This suggests that the sedimentation in the Kofu basin is mainly controlled by the activity of the western boundary fault zones.
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