The present contribution investigates the effective layout of Ground Control Point (GCPs) that enables the construction of high-precision three-dimensional topographic model of steep cliff slopes using UAV-SfM survey. The surveys were conducted for the coastal cliffs (Benten island in Tomari Village and Obira, Shimamaki Village) in Hokkaido. The terrain model constructed from UAV-SfM (UAV reference model) was first compared with the terrain model derived from TLS survey. Both surveys were performed under the condition that GCPs were evenly arranged in the vertical and horizontal directions. The difference between the terrain models of UAV-SfM and the TLS was calculated to be less than 10 cm. Secondly, we compared the following three UAV models generated under different GCP arrangement with the UAV reference model; 1) “Model with GCP placed only in the lower part of the slope” (the lower model), 2) “Model with GCP placed in the lower and central part of the slope” (the lower and 3) “Model with GCP placed in the lower part and upper of the slope” (the lower and The results of this second part of the research have revealed that the lower model caused larger difference with the UAV reference model in both study sites, while the lower and upper model and lower and central model showed only small differences with the reference model. Moreover, the number of GCPs in lower and upper model was smaller than the other model. In conclusion, the present research shows that the most effective arrangement of GCPs on the studied cliffs were positioning on the lower and the upper slopes.

Three-dimensional models (digital surface models (DSMs) and ortho mosaic images) of the study area were created for multiple periods before and after snow accumulation through structure from motion and multi-view stereo processing, using only aerial images with precise positioning data (i.e. without ground control points) taken with a consumer-grade unmanned aerial vehicle (UAV) with a built-in realtime kinematic (RTK) global navigation satellite system. Snow depth distribution was estimated from the difference between DSMs during the snow-free and snow-covered periods. Accuracy of the obtained values was verified by comparing them with the actual values measured on the ground. The calculated snow depth was almost always smaller than the actual measured value, although a strong correlation between the values (r＝0.98) was observed. Moreover, the accurate estimation of snow depth distribution could be obtained using an appropriate conversion formula. A comparison was made between RTK-UAV snow depth measurement and the conventional method. Although the former method was slightly inferior in terms of equipment cost and the measurable range, it was superior in terms of spatial resolution, data accuracy, temporal resolution (measurement frequency) and measurement labor.

The 2016 Kumamoto earthquake caused surface ruptures along the Futagawa and Hinagu fault zones in the western part of Aso Caldera, triggering numerous landslides of tephra. Janoo Volcano is located on the northeast side of the along-strike area with surface ruptures and damage zones. We conducted excavation and drilling operations on the southern slope of Janoo Volcano, analyzing the chemical compositions of lava samples and radiocarbon ages of volcanic soils in order to evaluate activities at the NE edge of the fault zones. Our results revealed the stratigraphy of volcanic edifice following the formation of the Janoo Scoria Cone, and indicate that a potential occurrence of slope failure in about 4,000 years ago. The calibrated eruption ages of key stratigraphic units are as follows: Janoo Scoria: >15.4 ka; Aso Central Pumice 1 (ACP1): ~4.0 ka; Kishimadake Lava and Kishimadake Scoria:~3.7 ka; Ojodake Scoria: 3.5 ± 0.1 ka; Kometsuka Lava: 3.0 ± 0.1 ka. Both a fine sandy ash layer at depths of >8 m and the top of the Janoo Scoria had weathered to a yellowish color and been cut by small nontectonic faults parallel to the surface of the Janoo Scoria, showing normal displacement. The volcanic soil layers immediately overlying the sandy ash layer have been separated by fractures into 1-m-high and wide blocks that show evidence of imbrication and overturning. This deposit is covered by 3 m of soft volcanic soil displaced from further up the slope. These tephra layers are overlain by ACP1. Consequently, these deformation and sedimentary structures indicate that the tephra on Janoo Scoria Cone slid down just before the eruption of ACP1. This ancient slope failure seems to have been triggered by earthquakes, because its depth and transported materials are similar to those of landslides associated with the 2016 Kumamoto earthquake.