Structural Analysis of a Previously Unknown Active Fault That Triggered the 2013 M w 5.8 Awajishima Earthquake, Southwest Japan

Abstract

The Mw 5.8 Awajishima earthquake occurred on 13 April 2013 in southwest Awaji Island, ∼25 km southwest of the epicenter of the 1996 Mw 6.8 Kobe earthquake, southwest Japan. Analyses of aerial photographs and 3D perspective images, field investigations, and structural analysis of fault rocks reveal that: (1) a previously undocumented fault, here called the Yamada fault, strikes northwest–southeast and dips southwest at 86° along a topographic lineament at the geological boundary between Mesozoic granitic rocks and the late Pliocene–Quaternary Osaka Group composed of interbedded sandstone and mudstone; (2) the main shear zone of the Yamada fault consists of a fault core with a <10 cm wide zone of fault gouge (generally 1–5 cm), a fault breccia zone of <100 cm wide, and a damage zone of 10–50 m wide, composed of cataclastic rocks and fractures; (3) foliations characterized by S‐C fabrics that have developed in the shear zone indicate a dominantly thrust fault sense, consistent with that revealed by the focal mechanism; and (4) coseismic surface ruptures occurred locally along the main trace of the Yamada fault, consisting of numerous short fissures ranging in length from centimeters to several meters and concentrated in a zone <5 m wide. Our findings show that the newly identified Yamada fault is an active fault and that it is probably the fault on which the 2013 Mw 5.8 Awajishima earthquake occurred. Therefore, it is necessary to construct a fault model to better understand the deformation characteristics of the seismogenic source fault and for reassessing the seismic hazards on the densely populated Awaji Island of southwest Japan.