The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Estimation of kinematic history of the faults by fault geometry, composite planar fabric, and stress inversion analysis: Case study of the Yunodake Fault that was active during the 2011 Fukushima-ken Hamadori earthquake
Toru Sakai
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2024 Volume 130 Issue 1 Pages 469-492

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Abstract

The Yunodake Fault is one of two normal faults that ruptured during the 11 April 2011 MW 6.7 Fukushima-ken Hamadori earthquake, which was part of the aftershock sequence of the 11 March 2011 MW 9.0 Tohoku Earthquake. The Yunodake Fault system has a complex slip history recorded by shear planes with various attitudes in the fault zone. The shear sense on each plane can be estimated from the composite planar fabrics developed around the plane. However, even if several planes show the same shear sense, it is impossible to distinguish whether those planes were formed during the same activity (stress) stage. To better constrain the kinematic and stress history of the Yunodake Fault, I combined observations of fault geometry and composite planar fabrics within the fault zone with stress inversion analysis. The datasets reveal three slip stages: Stage I (normal fault system), Stage II (reverse fault system), and Stage III (normal fault system). These stages are consistent with the results of previous studies that inferred the stress state in the area of the Yunodake Fault from dike swarms, veins, faults, and other structures. Generally, the number of activity (stress) stages decreases from the central part of the fault to the tips. The slip history of the Yunodake Fault is well preserved in the fault zone structure, allowing for accurate reconstruction of the kinematic and stress history.

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