Geometry of Active Faults and Their Rupture Process.

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  • Nakata Takashi
    Department of Geography, Faculty of Letters, Hiroshima University
  • Suzuki Yasuhiro
    Laboratory of Geoscience, Information Science and Technology, Aichi Prefectural University

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  • 活断層の分布形態と破壊過程
  • カツダンソウ ノ ブンプ ケイタイ ト ハカイ カテイ

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Abstract

Physical characteristics are often used for fault segmentation, when historical and/or paleoseismological data are not sufficient to reveal coseismic behavior of faults. This paper proposes a method to identify the directivity of rupture propagation based on the branching features of active fault traces. An interdependent correlation between the branching direction of the surface ruptures and the direction of their propagation is often observed for recent earthquake fault ruptures. Geographical patterns of active faults are the results of repeated earthquake faulting, and therefore branching of active faults leads us to suggest that the direction of rupture propagation is predictable before the active faults generate earthquakes in the future. We propose that branching of faults and characteristic patterns of vertical-slip distribution along strikeslip faults can be used to identify fault segmentation. In a case where the branches of the two faults face each other, we may suspect the presence of a segmentation boundary and a change in the direction of fault ruptures between these branches. Regarding the characteristic pattern of vertical-slip distribution along strike-slip faults, the upthrown sides along strike-slip faults are, in general, located on the fault blocks in the direction of relative strike-slip. For example, along an E-Wtrending right-lateral strike-slip fault, upthrown sides are located on the north in the eastern section and on the south in the western section. Therefore, a fault segment may be identified based on a set of the vertical slip distribution.<br>Cumulative fault slip distributions along fault traces are also good indicators of the past rupture process. Most of the existing active faults in Japan show the characteristic earthquake pattern in Fig. 3-A; and only the Kochien fault in Hokkaido shows the pattern in Fig. 3-B, indicating that this fault was generated after formation of the Middle terrace. None of the active faults has the pattern in Fig. 3-C, and this indicates that active faults have not much expanded their extension by repeated movements.

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