Fig. 2

a Photograph of the outcrop exposure looking south at the Katagai site. The beds of the Uonuma Formation are steeply dipping. b Photograph of the slickenlines on the slip plane. Inset shows the slip direction of the hanging wall. c Sketch of the outcrop shown in a. Red lines show slip planes, flexural-slip faults, and reverse faults. The average spacing of bedding-plane slip surfaces at this site is ~ 4 m. Circles and diamonds indicate sampling sites of fault gouge generated by bedding-plane slip and mudstone, respectively, collected for measurements of the friction coefficient. d Lower hemisphere, equal-angle stereographic projections of the typical bedding-plane slip at the Katagai site. Triangles indicate the trend of the fold axis of the Matto Anticline