Geomorphic features of surface ruptures associated with the 2016 Kumamoto earthquake in and around the downtown of Kumamoto City, and implications on triggered slip along active faults

Abstract

The ~30-km-long surface ruptures associated with the M w 7.0 (M j 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.

Coseismic Groundwater Drawdown Along Crustal Ruptures During the 2016 Mw 7.0 Kumamoto Earthquake

“…These faults were formed by large (>10 ‐5 ) extensional volumetric strain (Figure b). Numerous aftershocks occurred along these fault systems (see Fujiwara et al, , and Goto et al, , for their locations). In this study, special emphasis is placed on the co‐location between the new rupture systems and the coseismic groundwater changes (Figure ) to understand how the crustal deformation can induce the observed water‐level changes.…”

“…A high‐resolution groundwater monitoring network was in place at Kumamoto at the time of the earthquake (Figure a; see section ) with a spatial resolution that is more than 3 and 15 times finer than those documenting the effects of the 1999 Chi‐Chi (Manga & Wang, ) and the 2010 Canterbury (Cox et al, ) earthquakes, respectively. The data set thus provides an excellent opportunity to directly document groundwater responses along newly formed rupture systems (Figure b; Fujiwara et al, ; Goto et al, ).…”

Bayesian inference and interpretation of centroid moment tensors of the 2016 Kumamoto earthquake sequence, Kyushu, Japan

“…Field investigation by Toda et al (2016) shows that surface displacements along the previously mapped active fault traces of the Hinagu-Futagawa fault zone are dominated by right-lateral strike-slip surface displacement up to 2 m. A normal surface rupture zone of about 10 km in length dipping to northwest, which is parallel to the Futagawa fault outside the Aso caldera, was also reported by Toda et al (2016), and its maximum co-seismic displacement is also up to 2 m. The normal dip-slip aftershocks that occurred along the NW edge of the mainshock rupture had no clear relationship with co-seismic surface ruptures; however, minor surface ruptures in downtown of Kumamoto City have been mapped by InSAR (Himematsu and Furuya 2016) and field survey (Goto et al 2017). The field investigations imply the complex surface phenomena and tectonic settings in this region.…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…Toda et al (2016) examine surface ruptures aligned parallel to each other near the Mashiki town and Nishihara village and present a model of slip partitioning. Goto et al (2017) examine subtle geomorphic features of surface ruptures using a digital elevation model in the city of Kumamoto and discuss their possible relationship to the earthquake source faults.…”