活断層はどこまで割れるのか? : 横ずれ断層の分岐形態と縦ずれ分布に着目したセグメント区分モデル

New geometoric criteria for active fault segmentation : Fault branching and dip-slip distribution pattern along strike-slip faults

Nakata Takashi

Goto Hideaki

活断層研究

To estimate the size of future earthquakes generated from active faults, it is important to define the extent of faulting from existing fault traces. This paper proposes new criteria for identification of segments for active strike-slip fault systems based on geometric surface rupture heterogeneity.

Geometric and structural characteristics are often used for fault segmentation, in cases that historical and/or paleoseismological data are not sufficient to reveal coseismic behavior of faults.

We found an interdependent relation between the pattern of surface ruptures and the direction of their propagation based on an investigation of recent earthquake fault ruptures. If a fault is associated with a simple baranching at one end in a plane view, it is rather simple to deduce the direction of future ruptures from its branching feature. If a fault brances at two ends, we may define a fault segment which raptures bilaterally and moves independanly at future earthquakes. If branchings of two faults face each other, we may expect a segment boundary between these branchings as a change in the direction of fault ruptures propagation take place between them.

Regarding dip-slip distribution along a strike-slip fault segment, the upthrown sides is, in general, located in the area on the fault block in the relative strike-slip motion on a faulted block. For example, along an E-W trending right-lateral strike-slip fault, upthrown side is located toward the east end on the northern block and toward the west end on the southern block. Thus, a fault segment may be identified based on the pattern of dip-slip distribution.

We tested these criteria successfully on severarl recent surface fault ruptures and active faults, and applied them for segmentation of the active faults system of the Median Tectonic Line in Shikoku, Southwest Japan.

セグメンテーション

断層分岐形態

縦ずれパターン

横ずれ断層

中央構造線

fault segmentation

fault branching

dip-slip distribution pattern of active faulting

strike-slip fault

Median Tectonic Line

General geography. Description and travel [ 290 ]

jpn

活断層研究会

[ISSN] 0918-1024

[NCID] AN10157356

[URI] http://topo.earth.chiba-u.jp/afr/index.htm