Observation of the surface ruptures and displacements around the trench site
In the Miyaji area, there were two northeast–southwest striking faults (hereinafter, north and south faults) in the discontinuities of InSAR fringes (Figs. 2b, 3a, b, and 4; Ishimura et al. 2017), which we called the Miyaji faults. The Miyaji faults exhibited a dextral slip, which was consistent with the InSAR measurements (Fig. 3d–f; Fukushima and Ishimura 2020). These surface ruptures appeared on the alluvial fan surface at the foot of the Aso volcano where no traces of active faults were previously mapped. Even after the 2016 event, based on aerial photographs and high-resolution digital elevation models (DEMs), it was not possible to identify tectonic geomorphic features indicative of active faulting.
Damage was observed on the surface of paved roads and concrete walls (maximum vertical separation: 10 cm, maximum dextral slip: 5 cm) (Figs. 3 and 4; Ishimura et al. 2017). Fukushima and Ishimura (2020) created 3D deformation maps using InSAR data observed from three different directions and GNSS data (Fig. 3d, e). From these detailed vertical and fault-parallel horizontal displacement fields, it was found that the lengths of the north and south faults were approximately 1.5 km and 3.2 km, respectively. The maximum vertical and dextral displacements were 8 cm up on the south and 19 cm for the north fault, and 12 cm up on the south and 19 cm for the south fault. In addition, the vertical and fault-parallel horizontal displacement fields (Fig. 3d, e) showed left-stepping en-echelon surface ruptures in both faults. The geometry of these faults was consistent with the dextral slips on the faults. Local subsidence occurred at the trench site, forming a small graben (Fig. 3d).
At the trench site, two surface ruptures were found forming a small graben (Fig. 3c). We confirmed that the northern trace was associated with a dextral and vertical (northwest up) slip, and that the southern trace was associated with a vertical (southeast up) slip. Digital surface models were built to measure the surface displacements on paved roads based on SfM-MVS photogrammetry. Both surface ruptures showed approximately 5 cm of vertical displacement (Fig. 3c, Line A), which was consistent with the InSAR measurements (Fig. 3f, Line 1–1′).
Two inhabitants, living on the north and south faults, mentioned that the small surface ruptures appeared during the mainshock of the 2016 Kumamoto earthquake sequence, not at the foreshock and aftershocks. From this information, it was confirmed that the surface ruptures in the Miyaji area were associated with the mainshock of the 2016 Kumamoto earthquake.
Paleoseismic surveys across the small graben
Trench survey
The deposits in the trench walls were divided into 20 units (unit 00–111) based on composition, facies, and structure. Figures 5 and 6 show views of the trench and logs in the three walls and trench floor, and provide a brief description of the stratigraphic units. Table 1 shows the radiocarbon dates.
Units 00 and 01 were the cultivated soil and sediments of the post- and pre-2016 events, respectively. Units 10–80 were soil and tephra sequences, which were mostly aeolian deposits. The basement of unit 20 is a little erosive and units 30 and 31 were partially laminated. However, clear channels and large erosion features were not found. Therefore, it was interpreted that the soil and tephra sequences were continuously deposited during the formation of units 10–80. Units 90–111 were fluvial sediments, but unusual sedimentary structures and stratigraphy were recognized between units 101 and 110 on the northern wall and trench floor. Units 103 and 110 had almost similar compositions (black scoria) and sedimentary structures and unit 101 was distributed above units 103 and 110 on the western and eastern walls, however, on the northern wall, the lower part of unit 101 was interbedded between units 103 and 110. Moreover, on the northern wall, unit 101 partially intruded to unit 103, and the lower western part of unit 101 showed shear structures, indicating a fluidized deformation in unit 101. The boundary between units 101 and 110 was also traceable from the northern wall to the trench floor, and its strike was parallel to the topographic slope and was perpendicular to the faults (Fig. 6). The sedimentary structures and the direction of basal contacts of unit 101 suggest that the mobile blocks mainly composed of unit 103 on the western and northern walls were associated with liquefaction and lateral movement, and probably moved from south to north following the topographic slope.
To confirm the slip surface of the mobile blocks, a pit was excavated to a depth of 0.7 m from the trench floor across the faults (Figs. 5 and 6), and a basal gravel layer was further 35 cm below the trench floor on the uplifted side. The basal gravel layer contained horizontally bedded pebble- to cobble-sized rounded gravels (Fig. 7). On the other hand, unit 110 was deformed and thinned to the west, showing a wedge-like geometry. A slip surface was recognized inside or in the base of unit 110 (Fig. 7). From this observation, we interpreted that the mobile block slid over the gravel layer, deforming units 101, 110, and 111 on the northern wall and trench floor. In addition, unit 102 was distributed in a patchy texture on the northern and eastern walls, and their gravels were identical to the basal gravel layers in the pit (Fig. 7). Therefore, unit 102 was probably transported by liquefaction and/or involved in the lateral movement.
Tephra and radiocarbon dating were used to estimate the age of each unit. Five types of tephra were identified based on petrographic characteristics, sedimentary facies, stratigraphy, and radiocarbon ages. Unit 30 consisted of brown to green volcanic glasses whose stratigraphy and sedimentary facies are similar to those observed in the 2017 pit near our trench site (Fig. 3c; Ishimura et al. 2017). Ishimura et al. (2017) identified deposits similar to unit 30 as the N2S tephra (Miyabuchi and Watanabe 1997; Miyabuchi 2009) erupted from Nakadake volcano (Fig. 2) based on the refractive indices of volcanic glass shards. The age of N2S tephra is estimated to be 1490–1470 cal BP (68.2%) (Yamada et al. 2017) based on a high-precision age model of marine core sediments in the Beppu Bay. The radiocarbon ages below and above unit 30 are consistent with the tephra age, confirming that unit 30 is correlated with the N2S tephra. Units 50 and 51 consisted of orange scoria and volcanic ash, and radiocarbon ages above and below them are 2070 ± 20 yr BP and 2620 ± 30 yr BP, respectively. Based on the stratigraphy and radiocarbon ages in Miyabuchi and Watanabe (1997), these units correspond to the N3 volcanic activity period. Unit 70 was a characteristic layer composed of reddish scoria. Radiocarbon ages obtained above and below this unit are 2640 ± 20 yr BP and 3010 ± 20, 2990 ± 20 yr BP, respectively. Based on stratigraphy, sedimentary facies, and radiocarbon ages, unit 70 is correlated with the OjS tephra that was erupted from Ojodake volcano (Fig. 2). Although its age was estimated to be 3.6 ka (Miyabuchi, 2009), the age of OjS tephra is updated to be 3180–2790 cal BP (95.4%) based on Bayesian analysis using the OxCal program (Additional file 1: Fig. S2). Units 103 and 110 were scoria-dominated fluvial sediments, characteristically containing black scoria. The radiocarbon age obtained from unit 101 is 3070 ± 20 yr BP. Based on stratigraphy, sedimentary facies, and radiocarbon age, units 103 and 110 are correlated with the KsS tephra (4.0 ka; Miyabuchi 2009) that was erupted from Kishimadake volcano (Fig. 2). Unit 111 contained sparse white pumice (diameter: less than 1 cm). The refractive indices of volcanic glass shards of this white pumice ranged from 1.505 to 1.508 (Mode:1.505), which are similar to those of ACP1 tephra (4.1 ka; Miyabuchi 2009) reported by Furusawa and Umeda (2000). From the standpoint of refractive index and stratigraphy, the white pumice included in unit 111 is correlated with the ACP1 tephra that was erupted from Janoo volcano (Fig. 2) (Miyabuchi 2017). However, ACP1 horizon could not be clearly identified in unit 111, so we infer that unit 111 is at the same age as ACP1 or younger.
We recognized several faults cutting up to unit 01 on both walls (Figs. 5 and 6). Two faults were recognized mainly on the western wall. The northern fault cut unit 01, and two branches were identified in unit 101. The southern fault displaced the bottom of unit 40. In the eastern wall, two groups of faults were recognized. One group consisted of two faults above unit 60 and the other consisted of multiple faults between units 110 and 40. The former displaced the bottom of unit 01, and the latter cut into unit 40. These faults represented a typical flower structure, suggesting strike-slip displacement. The vertical offset of each layer is shown in Fig. 8. This shows the difference in vertical offset above and below unit 40. The vertical offset above unit 40 was about 10–15 cm and that below unit 40 was about 20–35 cm, which was twice the latest event. In addition, the vertical offset of the basal gravel layer was 25–30 cm on the eastern wall of the pit at the trench floor.
Handy Geoslicer survey
The HGS cores were obtained from the southeastern corner of the rice paddy field (Fig. 3c), about 30 m southeast of the trench. The same unit labels were used for the deposits in the HGS cores (Fig. 9). Although the HGS core deposits were roughly correlated with the trench deposits, units 20–32 were absent from the HGS coring site, probably due to the artificial modification of the rice paddy field. Units 02 and 03, which were not identified in the trench, were also related to artificial modification and cultivation.
In HGS6, a fault that cut up to unit 01 was identified, indicating that this faulting was associated with the 2016 event (Fig. 9). Based on the height difference between the layers of HGS5 and HGS6, a fault with a branch recognized in HGS6 was estimated between the cores. The vertical separation between units 03–90 was about 10 cm. Therefore, this fault moved only during the 2016 event since the deposition of unit 90. On the other hand, between the HGS2 and HGS3, a height difference was found between units 51 and 90, but the height difference was not identified in unit 01, indicating the occurrence of an event older than the 2016 event.