3.2. Site 2 in Laue Crater
At Site 2, we measured boulder trails and small craters (D> 15 m) and their distributions along the entire crater wall and on the floor of Laue crater (Figure 6a). Boulder trails occur only on the southeast and northwest walls, and fewer boulder trails are superimposed by small craters (yellow lines in Figure 6a) at Site 2 than at Site 1 (see Figure 2a). Our measured locations of boulder trails and boulder sources are mostly consistent with those of Kumar et al. (2019), but we identified fewer boulder trails than Kumar et al. (2019) did. Comparison of the distributions of slope angles, small craters, and starting points of boulder falls indicate that at Site 2 there are fewer small craters in areas where slope angles are larger than about 30° than in areas with smaller slope angles (Figure 6b).
We selected one area with boulder trails (red rectangle in Figure 6a) and compared the spatial distribution of maximum acceleration due to impacts with the starting points of boulder falls within that area (Figure 7). Although the averaged maximum acceleration in 500 m× 500 m grids (Figure S6) was weakly correlated with the number of starting points (Figure 8a), their correlation was weaker than that at Site 1 (Figure 4a).
The model age estimated by using all measured small craters with diameters of 15−500 m at Site 2 was about 20 Ma (Figure S5a), but when only small craters on the eastern and western walls were used, the estimated model ages were about 13 and 14 Ma, respectively, and the estimated model age of the crater floor was about 70 Ma (Figure S5b). These results show that the Laue crater walls are relatively younger than its floor. We averaged slope angles and estimated the density of small craters in 800 m × 900 m grids at Site 2 (Figure S3b), and found that, similar to the trend at Site 1 (Figure 4b), the density of small craters at Site 2 decreases with increasing slope angle (Figure 8b).
OMAT values are high mainly in the southeast and northwest upslope areas at Site 2 (Figure 5b), and similar to Site 1 (Figure 5a), high values are correlated with boulder sources. OMAT values decrease toward the crater floor (Figure 5b), indicating more mature soils. We also compared OMAT values with small crater density (Figure 8c) and slope angle (Figure 8d) at Site 2. Although we found no clear correlation between OMAT and the density of small craters, OMAT values tend to increase with slope angle at Site 2, similar to the tendency at Site 1 (Figure 4d).
Kumar et al. (2019) inferred that boulder falls at Site 2 were triggered by a shallow Mw 4.1 moonquake that occurred on 3 January 1975. The epicenter, as originally determined by Nakamura et al. (1979) and relocated by Kumar et al. (2019), was on the largest lobate scarp segment. We investigated boulder falls in other craters (D> 7 km) around Site 2 within an epicentral distance of about 200 km from the moonquake (Figure 9a) and found boulder falls in some craters far from the epicenter. We then estimated the number of craters per unit area whose centers were within a radius of 70, 140, or 210 km from the moonquake epicenter (Figure 9b), but we did not find any dependence of the number of craters either with or without boulder falls on epicentral distance. This result is consistent with the findings of Bickel et al. (2021), who statistically analyzed the global distributions of boulder falls and moonquake epicenters.