## 4 Results  
### Field Data and Sample Description  
Deformation bands developed in the borehole core and outcrop samples as mentioned below. The geological route map which we constructed incorporating the previous results [37] is shown in Figure 1b, where the lithology and strike-and-dip of the strata are indicated. The cross-section was constructed based on the geological map (Figure 1c). The basic features of the geological structure are the same as those described by the previous study [37]. An important structure revealed by both studies is the steeply dipping zone in the Shakubetsu Formation, which was interpreted as a large-scale fault between the Shitakara and Shakubetsu formations,as mentioned above [37]. However, the present study interpreted the zone without any fault gouge or breccia as a flexure (Figure 1c, represented as an overturned syncline in Figure 1b). Here, the northwestern limb trended NNE-SSW to NE-SW and dipped southeast at 75\({}^{\circ}\) to vertically (Figures 1b and 2a), while the southeastern limb trended E-W and dipped south at 6\({}^{\circ}\) (almost horizontally, Figures 1b and 2b). Further, the boreholes (B4, 5 and 6), which are located at a position between the steeply and horizontally inclined strata of the Shakubetsu Formation (Figure 1b,c), were drilled by Meiji Consultant Co. Ltd, Sapporo, Japan. to monitor the ongoing landslide. The strata below the landslide slip surface dipped SE at c. 20\({}^{\circ}\) (Figure 3a), which can be inferred from the bedding in the borehole core samples as explained below, indicating that the flexure axis is located directly below it (compare Figures 1c and 3a). Deformation bands were developed in core samples from the three boreholes (B4, 5 and 6 in Figure 3a). In the borehole core samples, very-fine- to fine-grained sandstones and mudstones alternated with many thin coal layers (Figure 3b,c), the thickness of which ranged between a millimeter to 20 cm. The deformation bands are clearly recognized as black seams in the core samples (Figure 3b,c). Deformation bands are also developed in very-fine- to medium-grained sandstones from outcrops (KT1, 2 and 3, Figure 1b) of the Shakubetsu Formation constituting the northwestern (KT1) and southeastern (KT1 and 2) limbs of the flexure, where coal layers up to 30 cm thick are intercalated (Figure 2b). Although the deformation bands are barely seen by naked eyes at the outcrops, they could be recognized in the chips of sandstone as faint seams. Deformation bands only occurred in the flexure consisting of the Shakubetsu Formation, not in other areas consisting of the uppermost unit of the Shitakara Formation (e.g., KT4, Figure 1b). In this study, microstructural and microchemical analyses were conducted for sandstones with deformation bands in not only one core sample from borehole B6 but also three samples from outcrops (KT1, 2 and 3). In the borehole core sample B4, gauges also developed other than deformation bands, the microstructures of which are described below. Further, microstructures in one outcrop sample (KT4, Figure 1b) from the uppermost unit of the Shitakara Formation were analyzed for reference, which is a fine- to medium-grained sandstone without deformation bands. All these analyzed samples are summarized in Table 2.  
Figure 2: Outcrop photographs of the Shakubetsu Formation. (**a**) Steeply dipping strata located at the northwest of flexure (outcrop KT5). The bedding plane is shown by dashed lines. (**b**) Almost horizontally dipping strata located at the southeast of flexure (outcrop KT2), where the sandstone sample for image analysis is collected. See Figure 1b for the localities. Figure 3: (**a**) Cross section of landslide and localities of boreholes (B4, 5 and 6). See Figure 1b,c for the drilled site. (**b,c**) Photographs of drilled core samples. (**b**) Borehole 4 (B4) core. (**c**) Borehole 6 (B6) core. Bedrock and landslide debris was divided by a failure surface, which was framed by white solid lines.