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Large earthquakes occurred in the central part of Kumamoto Prefecture on April 14-16, 2016, causing severe damage to the northern segment of the Hinagu faults and the eastern segment of the Futagawa faults. Earthquake surface ruptures appeared along these faults and on the Aso volcanic edifice, which in turn generated landslides. We conducted landform change analysis of the central cones of Aso volcano by using satellite and aerial photographs. First, we categorized the topographical changes as surface scarps, arc-shaped cracks, and linear cracks. Field survey indicated that landslides caused the scarps and arc-shaped cracks, whereas faulting caused the linear cracks. We discovered a surface rupture concentration zone (RCZ) formed three ruptures bands with many surface ruptures and landslides extending from the west foot to the center of the Aso volcanic edifice. The magmatic volcanic vents that formed during the past 10,000 years are located along the north margin of the RCZ. Moreover, the distribution and dip of the core of rupture concentration zone correspond with the Nakadake craters. We conclude that a strong relationship exists between the volcanic vents and fault structures in the central cones of Aso volcano. [ABSTRACT FROM AUTHOR]

The ~30-km-long surface ruptures associated with the M 7.0 ( M 7.3) earthquake at 01:25 JST on April 16 in Kumamoto Prefecture appeared along the previously mapped ~100-km-long active fault called the Futagawa-Hinagu fault zone (FHFZ). The surface ruptures appeared to have extended further west out of the main FHFZ into the Kumamoto Plain. Although InSAR analysis by Geospatial Information Authority of Japan (GSI) indicated coseismic surface deformation in and around the downtown of Kumamoto City, the surface ruptures have not been clearly mapped in the central part of the Kumamoto Plain, and whether there are other active faults other than the Futagawa fault in the Kumamoto Plain remained unclear. We produced topographical stereo images (anaglyph) from 5-m-mesh digital elevation model of GSI, which was generated from light detection and ranging data. We interpreted them and identified that several SW-sloping river terraces formed after the deposition of the pyroclastic flow deposits related to the latest large eruption of the Aso caldera (86.8-87.3 ka) are cut and deformed by several NW-trending flexure scarps down to the southwest. These 5.4-km-long scarps that cut across downtown Kumamoto were identified for the first time, and we name them as the Suizenji fault zone. Surface deformation such as continuous cracks, tilts, and monoclinal folding associated with the main shock of the 2016 Kumamoto earthquake was observed in the field along the fault zone. The amount of vertical deformation (~0.1 m) along this fault associated with the 2016 Kumamoto earthquake was quite small compared to the empirically calculated coseismic slip (0.5 m) based on the fault length. We thus suggest that the slip on this fault zone was triggered by the Kumamoto earthquake, but the fault zone has potential to generate an earthquake with larger slip that poses a high seismic risk in downtown Kumamoto area. [ABSTRACT FROM AUTHOR]

An ENE-trending ~30-km-long surface rupture emerged during the Mw = 7.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu faults. This included a previously unknown 5-km-long fault within the Aso Caldera, central Kyushu. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic slip. One of the noteworthy features we observed in the field are ~10-km-long segmented normal fault scarps, dipping to the northwest, along the previously mapped Idenokuchi fault, 1.2-2.0 km south of and subparallel to the Futagawa fault. The maximum amount of coseismic throw on the Idenokuchi fault is ~2 m, which is nearly equivalent to the maximum slip on the strike-slip rupture. The locations and slip motions of the 2016 rupture are also manifested as interferogram fringe offsets in InSAR images. Together with geodetic and seismic inversions of subsurface fault slip, we present a schematic structural model where oblique motion occurred on a northwest-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. Our simple dislocation model demonstrates that this bifurcation into pure strike-slip and normal faults likely occurs for optimally oriented failure near the surface. The Kumamoto case, with detailed geological observations and geophysical models, would be the second significant slip-partitioned earthquake around the globe. It provides an important insight into scale- and depth-dependent stress heterogeneity and an implication to a proper estimate of seismic hazard in complex and broad multiple fault strands. [ABSTRACT FROM AUTHOR]

The 2011 Tohoku-Oki offshore subduction earthquake (Mw 9.0) triggered many normal-type earthquakes inland in northeastern Japan. Among these were two very similar normal-faulting earthquakes in 2011 (Mw 5.8) and 2016 (Mw 5.9), which created surface ruptures along the newly named Mochiyama fault within the southern Abukuma Mountains, northeastern Japan, where no active faults had been previously mapped by interpretation of aerial photographs. We conducted field surveys in this area immediately after both earthquakes, and we performed trench excavations and observations of fault fracture zones after the 2016 event. These activities were complemented by an interferometric synthetic aperture radar analysis that mapped the areas of deformation and locations of surface discontinuities for both events. The combined results document the coseismic behavior of the Mochiyama fault during both events. Subtle tectonic geomorphic features associated with the fault were evident in a lidar digital elevation model of the area, and layered structures of gouge were documented in the field. These lines of evidence indicate repeated activity at shallow crustal levels and the possibility of Quaternary activity. In addition, our trench excavations revealed at least one faulting event before 2011. Our comparison of paleoseismic records on this and two other normal faults in the Abukuma Mountains suggests that great earthquakes in the Japan Trench supercycle of 500–700 years do not consistently trigger ruptures on these faults, and the case of 2011, in which the Tohoku-Oki megathrust earthquake triggered all three faults, is a rare occurrence. [ABSTRACT FROM AUTHOR]