Coseismic landslides have been one of the major causes of human suffering and house damage in recent large-scale earthquakes. Estimating the occurrence of coseismic landslides in advance is important to promote rapid and efficient emergency response and disaster prevention measures in advance. Various previous studies have been conducted on landslide hazard assessment. Methods that can evaluate the occurrence site over a wide area can “relatively”evaluate the hazard and probability of occurrence, but cannot “absolutely” quantitatively evaluate the area of collapse or the amount of collapsed sediment. In addition, methods that can quantitatively evaluate the scale of collapse cannot evaluate the location of landslide occurrence over a wide area. Therefore, based on the actual results of landslides during the 2016 Kumamoto earthquake, we developed a formula for estimating the logarithmic landslide cell percentage (log(P)) for a range of topographic and geological conditions. The log(P) obtained from the estimation formula for each range is consistent with the actual results of the Kumamoto earthquake regarding the following three points: total landslide area, trend for increase of actual landslide cell percentage with increase of log(P), and correspondence of actual landslide cell percentage and estimated landslide cell percentage with the cells belonging to the log(P) class. From these results, we judged that the developed estimation formula can represent the actual results of the Kumamoto earthquake in terms of the spatial and quantitative absolute index. Then, using the 2008 Iwate-Miyagi Nairiku Earthquake as a verification example, the estimation formula was applied to an area with similar geological features. As a result, it was found to be well suited to the validation data with similar topography and geology. And the effectiveness as a quantitative absolute index was shown even in different regions.

The purpose of this paper is to report long-term stream dynamics based on photographic observations of fixed spots in Inamata Valley in Yamanashi prefecture, Japan. We focused on erosion and sedimentation caused by large discharges in 1982 and 2020. In this valley field survey was initiated in 1978 and continued almost annually from 1982 to 2003. Additional surveys were conducted in 2008 and 2020. The fixed spots for photographic record and observation were set on easily recognizable terrain features such as check dams and stream junctions. As a result, sedimentation of more than 10 m on streambed was recognized in some places in 1982. The successive photographs showed a downward trend of the streambed with repeated erosions and sedimentations in the downstream area. The streambed lowering lessened around 1990 and by 1995 it almost stopped. During the same period, however, streambed fluctuations on the order of several meters were still occurring in the upstream area. Since 1997, erosion has accelerated and the streambed had dropped to the same level as that in 1978 at downstream near Inamata-Bridge due to the construction of the Inamata third check dam (H=50 m), which was filled up by 2008. After the catastrophic landsliding and debris flows in 2020, streambed conditions of the downstream area have become similar to those in 1982 despite the new third check dam, while the upstream area became completely different than it had been before. Successive photographs of fixed spots enabled comprehensive interpretations of the streambed changes along with field surveys, based upon minute changes such as grain size, large gravel shifting and burying and exposure of falls, which are difficult to detect with LiDAR or satellite images. It is desirable to continue field observations using photographs of fixed spots in order to closely track changes over time after the 2020 sediment discharge.

A deep-seated collapse occurred in an isolated hill of Nishime, Tsuruoka City, Yamagata Prefecture, at around 1 a.m. on 31 December 2022. The height of the collapsed slope was approximately 25 m, and the volume of sediment was approximately 20,000 m³. More than a dozen houses were destroyed, and two people lost their lives as a result of the collapse. The question is why the deep collapse occurred in early winter when there was no heavy rain. One possible trigger is that total precipitation throughout December 2022 was 1.8 to 1.9 times higher than usual. Two models were used to answer this question. First, since the number of elapsed days until collapse depends on the scale of the collapse, the following equation Te = 0.0142 H^1.99, or roughly Te ∝H², was obtained from the known data on the height of the failed slope H and the number of elapsed days Te. Based on this equation, the author concluded that the number of days to collapse was 8.6 days for the slope height of 25 m in this case. This result, together with the winter rainfall situation, suggests 24 December as the date of crack initiation. Second, the effective rainfall model predicted the limit of water storage in the slope to be 303 mm on the day of the crack initiation, and the coefficient (a) of this model was indicated as a = 0.96. The hydrological model can be applied to heavy summer precipitation as well as winter precipitation by setting the maximum water inflow into the landmass as R₀.That is, R₀ was 30 mm/d before the 2019 earthquake and increased to 60 mm/d after the earthquake. The water storage reached a value close to the limit D_c on 24 December. After the subsequent process, it can be inferred that a deep-seated collapse occurred on 30-31 December.