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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
© The Author(s) 2017
Received: 31 July 2016
Accepted: 14 January 2017
Published: 2 February 2017
In the recent years, InSAR analysis enables us to reveal the detailed and spatially comprehensive ground deformation such as faulting and tilting associated with an earthquake for an area with a few thousands square kilometers. It also often displays small displacements on the preexisting as well as unknown faults located up to ~100 km away from a source fault [e.g., 2010 El Mayor–Cucapah, California, earthquake (Wei et al. 2011)]. Nishimura et al. (2008) reported an episodic slip event on an existing shallow reverse fault that uplifted a hill by 10 cm, ~10 km away from the source fault of the 2007 Chuetsu-oki, Japan, earthquake of M w 6.7 by analysis of InSAR data. Fujiwara et al. (2016) also found numerous small slips as interferogram fringe offsets occurred even 10 km far from the main Futagawa fault trace associated with the 2016 Kumamoto earthquake.
Here, we report our tectonic geomorphic investigations of active faults and surface ruptures which might be triggered by the 2016 Kumamoto earthquake in and around the downtown of Kumamoto City based on the interpretation of the topographical data and field survey. This study focused on the small surface ruptures along the preexisting fault that cannot be explained by the coseismic slip of a strike-slip fault as a source fault.
We also interviewed local people whether they have noticed any deformational features along the fault trace, such as surface ruptures or offsets and tilts of artificial features immediately after the large shocks on 16 April.
Geomorphic interpretation and field survey
The Aso-4 and M surfaces can be subdivided into several surfaces, because the abandoned channels and small terrace rises are clearly observed on these surfaces. The M surfaces are distributed adjacent to the Aso-4 surface and are considered to be strath terraces of the Aso-4 surface. The L1 and L2 are well-developed surfaces in the area, while the L3 is subdivided from L2 or alluvial plain and is locally distributed. In addition, the longitudinal profiles of the L2 surfaces are steeper than those of the L1 surfaces. Thus, the development of the L1 and L2 surfaces may be closely related to the global climate change. The widespread AT tephra erupted from the Aira caldera in south Kyusyu at about 28–29 ka (Murayama et al. 1993) during MIS 3 is contained in the middle part of aeolian deposit overlying L2 gravels (Watanabe et al. 1995). Therefore, the L1 and L2 terraces were likely formed between MIS 5a and MIS 3.
Suizenji fault zone
We found the NW-trending flexure scarps down to the southwest that cut across the M, L1, and L2 terraces in the downtown of Kumamoto City based on the interpretation of the anaglyph images (Fig. 3). Three discernible subparallel fault strands (F1–F3) are called collectively as the Suizenji fault zone in this paper.
F2 is located ~800 m west of F1 and parallel to the northern part of F1. The NNW-trending flexure scarps of F2 cut across the L2 and L3 surfaces formed by the Shirakawa River flowing to the southwest. The amounts of vertical offset on the L2 and L3 surfaces are 3.5–4.3 and 2.7 m, respectively (Fig. 4).
The L2 surfaces are incised by the meandering Shirakawa River near the downtown of Kumamoto, suggesting an uplift of the area. At the south bank of this river, the gently sloping L2 surface is offset by the NW-trending steep scarp of F3. On the east side of the southern extension of this scarp, the L3 terraces are exceptionally well developed. The L3 terraces are separated from the alluvial plain with the NW-trending straight scarps of F3. Along the possible southern extension of F3, the L1 surfaces are also juxtaposed against the alluvial plain and wetland along the Kasegawa River and Lake Ezu gushing spring water out of the underground, which may suggest the existence of a buried fault at the southern extension of F3.
Subsurface structures derived from shallow borehole data and Bouguer anomalies indicate that the flexure scarps were formed by the buried active fault. A geological cross section derived from borehole data across F1 (Kumamoto City Waterworks Bureau 1980) shows a 10-m vertical offset of the Aso-4 pyroclastic flow deposits retrieved from all the boreholes on the both sides of the scarp (Fig. 4b). The upward convex deformation of the Aso-4 unit on the downthrown side is similar to the F1 scarp profiles of K4 and K5 in Fig. 4a, suggesting a rollover anticline associated with the SW-dipping normal fault. The amount of the vertical offset of the Aso-4 deposits, more than 10 m, is evidently larger than that of the M1 surfaces, which may indicate cumulative fault slip as a result of repeating movements. The long-term vertical slip rate of the F1 is estimated to be around 0.1 mm/year, based on the amount of the vertical offset of the Aso-4 deposits and its age.
In addition to the geomorphic and surface stratigraphic analyses, one of the distinctive subsurface structural boundaries estimated from Bouguer anomalies (Matsumoto et al. 2016) is consistent with the Suizenji fault zone and the Akitsugawa flexure zone.
Akitsugawaa flexure zone
The Kiyama fault is believed to be a buried active fault on the eastern extension of the Akitsugawa flexure zone, based on the subsurface distribution of the Tokawa lava (Watanabe et al. 1979). The Akitsugawa flexure zone may be connected to the Kiyama fault at seismogenic depth and form a single active fault system.
Surface ruptures of the 2016 earthquake
Along the northern part of F3, continuous surface deformation including right-stepping en echelon open cracks and 0.5-cm left-lateral offset of the concrete block wall were observed (Fig. 7g–j) . Houses and concrete buildings standing between the visible deformation features like open cracks on asphalt roads were heavily damaged, implying that the structural basements might be also deformed by faulting. According to the local residence, these features were found in the morning of 16 April after the main shock of the 2016 Kumamoto earthquake.
Small deformation like continuous cracks and buckling was also observed near the base of flexure scarp of the Akitsugawa flexure zone at several places (Fig. 7k–q). From the west of downtown Mashiki, where surface fault ruptures evidently emerged (Kumahara et al. 2016) to Hirosaki, Mashiki town, the continuous cracks and buckling of asphalt were intermittently extended (Fig. 3). A five-storied reinforced concrete building just on the continuous cracks was tilted at Shoryo in Mashiki town (Fig. 7k).
Our interviews with the local residents enable us to conclude that the series of minor ruptures on the Suizenji fault zone and the Akitsugawa flexure zone were the products of the 2016 Kumamoto main shock. However, regarding the relation between the 2016 ruptures and the preexisting fault zone, one could raise critical questions about whether these slips were indeed triggered by the coseismic deformation, and/or whether they occurred only on the shallow part of the fault zones, and/or whether the Suizenji fault still keeps stress unreleased at seismogenic depth.
To confirm these shallow slip and aftershocks were indeed triggered by the 2016 Kumamoto main shock, we calculated static Coulomb stress change (ΔCFF, e.g., King et al. 1994) in an elastic half-space of Okada (1992) with Poisson’s ratio of 0.25 and Young’s modulus of 80 GPa. We used a source fault model of GSI (2016b) and then resolved ΔCFF on NW-trending normal faults and NW-trending reverse faults (Fig. 8c, d). We found that the normal faults around the Suizenji fault zone were favorably oriented for failure due to the 2016 Kumamoto earthquake, and the NW–SE stretched stressed lobe is totally consistent with the trend of the minor ruptures on the Suizenji fault zones (Fig. 8c). In contrast, the thrust faults in the Suizenji area are calculated to have moved farther away from failure (Fig. 8d), which may deny a possibility that the Suizenji fault zone is an east-dipping reverse fault.
Remotely triggered small amounts of slip other than our study area are also found by the InSAR analysis (Fujiwara et al. 2016). In the northwest of the outer rim of Aso caldera, in particular, slip on numerous short EW-trending lineaments, some of which are previously mapped as active faults, contributed to develop low valleys by their sawtooth normal faulting displacements. These peripheral small faults, responded to the movement of the major fault movement, may provide us a clue to understand low slip rate and short active faults ubiquitously exist. No aftershocks beneath the northwest of the outer rim of Aso caldera also support the idea that these faults are always passively moved for adjusting local stress disturbed by major earthquakes nearby.
On the other hand, we suspect the Suizenji fault zone has a potential to generate its own earthquake. Adopting the empirical relation of fault length and maximum displacement by Matsuda (1975), ~6-km-long Suizenji fault zone could have brought ~0.5-m vertical displacement. However, maximum vertical slip measured was ~0.1 m that might be a tiny fraction of its potential slip. Thus, we do not believe that these triggered slips completely released stress that the Suizenji fault has stored. Instead, they are now brought closer to failure (Fig. 8c) if stress at the seismogenic depth is indeed accumulated. Furthermore, the elongated shallow seismic cluster implies that the subsurface fault structure might be further extended to the northwest as inferred from the Bouguer anomalies (Matsumoto et al. 2016), which may have a capability to produce an earthquake as large as M ~ 6.5. It may be a part of the post-Kumamoto earthquake hazard as well as the widespread off-fault aftershocks beneath the entire Kumamoto Plain.
Several NW-trending and SW-facing flexure scarps (the Suizenji fault zone) were newly identified for a length of 5.4 km departed from the main coseismic Futagawa surface fault trace in the downtown of Kumamoto City based on the interpretation of topographical stereo images (anaglyph) produced from 5-m-mesh DEM of GSI, which was generated from LiDAR data. Up to 10-cm vertical offset of surface deformation such as cracks, tilts, and buckling associated with the main shock of the 2016 Kumamoto earthquake were observed along the Suizenji fault zone. Our field observations are consistent with the interpretation of the Interferometric SAR image (GSI 2016) processed using ALOS-2/PALSAR-2 data spanning the main shock of the Kumamoto earthquake (Fujiwara et al. 2016). The ENE-trending and SE-facing long-wavelength flexure scarps (Akitsugawa flexure zone) were also identified on the anaglyph images in the area between the apparent surface ruptures of the 2016 Kumamoto earthquake and the Suizenji fault zone. The surface deformation along this zone seems to have been caused by the buried fault beneath the Akitsugawa flexure zone. Because the amount of offset (~0.1 m) observed in the field and on the Interferometric SAR image is considerably smaller than the slip (0.5 m) empirically calculated from the fault length, it is likely that the 2016 Kumamoto earthquake only caused small amounts of triggered slip on this fault zone. The risk of another large earthquake from these structures still remains high in the downtown of Kumamoto, although the city is located a few km away from the Futagawa-Hinagu fault zone (FHFZ).
HG planed and conducted all the fieldwork and drew the figures. HG wrote the text with HT and ST. HT and YK discussed the interpretation of tectonic landform with HG and did the fieldwork. ST calculated stress change in Fig. 8. ST and YK discussed the interpretation of InSAR image and distribution of aftershock with HG. All authors read and approved the final manuscript.
This work was partly supported by MEXT KAKENHI Grant Numbers 16H06298 and 16K01221. We thank staffs of Kumamoto Technical High School, who allowed us to conduct our fieldwork inside the school property. We are grateful to Dr. Hisao Kondo and Dr. Jian-Cheng Lee for providing valuable comments and suggestions, which helped us in progressing our study and improving the manuscript.
The authors declare that they have no competing interests.
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