The 2016 Kumamoto earthquake sequence started with a M J (Japan Meteorological Agency magnitude) 6.5 event on April 14, and culminated in a M J 7.3 event on April 16. Associated with the sequence, approximately 34-km-long surface ruptures appeared along the eastern part of the Futagawa fault zone and the northernmost part of the Hinagu fault zone. We carried out an urgent field investigation soon after the earthquake to map the extent and displacement of surface ruptures with the following results. (1) The rupture zone generally consisted of a series of left-stepping en echelon arrays of discontinuous fault traces of various lengths. (2) Slip exceeding 100 cm occurred on previously unrecognized fault traces in the alluvial lowland of the Kiyama plain and on the western rim of the Aso volcano caldera. (3) Large slip with maximum dextral slip of 220 cm was measured throughout the central section of the rupture zone along the Futagawa segment, and the slip gradually decreased bilaterally on the adjoining northeastern and southwestern sections. (4) The surface rupture mostly occurred along fault traces mapped in previous active fault investigations. (5) Most of the surface ruptures were produced by the mainshock, and significant postseismic slip occurred after the mainshock.
The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220• C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.
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