Structural Analysis of Coseismic Normal Fault Zones of the 2011 Mw 6.6 Fukushima Earthquake, Northeast Japan

Abstract

The 2011 M w 6.6 Fukushima earthquake occurred in Iwaki City, ∼250 km southwest of the epicenter of the 2011 M w 9.0 Tohoku (Japan) earthquake, and produced two subparallel ∼15–23‐km‐long surface rupture zones with distinct displacements along two pre‐existing normal faults: the Shionohira and Yunodake faults, which strike north‐northwest–south‐southeast and northwest–southeast (both dipping southwest), respectively. Field investigations and structural analysis of the coseismic shear zones developed within the Shionohira and Yunodake fault zones, reveal that (1) the coseismic shear zones consist of a fault core that includes a narrow fault gouge zone of <10 cm in width (generally 1–5 cm), a fault breccia zone of <50 cm in width, and a damage zone of 5–50 m in width that is composed of cataclastic rocks, fractures, and subsidiary faults; (2) the foliations characterized by S‐C fabrics developed in the shear zones indicate a dominantly normal fault‐slip sense, consistent with that observed along the coseismic surface ruptures; and (3) veinlet cataclastic rocks composed of unconsolidated fault gouges and fine‐grained materials occur as simple discrete veinlets and multiple, cross‐cutting, interconnected networks of veinlets. These newly documented structural characteristics for two coseismic fault/shear zones and associated cataclastic rocks indicate that the locations of coseismic slip zones associated with the 2011 Fukushima earthquake were controlled by pre‐existing fault/shear zones central to the Shionohira and Yunodake faults, which have repeatedly moved as seismogenic normal faults since the timing of formation of associated cataclastic rocks. Our results demonstrate that the structural analyses of coseismic shear/fault zones provide a powerful tool to study the nature and active faulting history of seismological faults.