Many previous studies reported that the several geomorphological and geological features were often found at the area close to scars of catastrophic landslide based on detailed field survey. However, most of these studies focused on only areas close to catastrophic landslide scars, and very few studies paid attention to the areas where the catastrophic landslide did not occur. In this study, we quantitatively analyzed the positional relationship between catastrophic landslides and geomorphological and geological features, such as mass rock creep, deep-seated landslide (rotational), arc-shaped crack, gentle ridge top, active fault and geological fault at Mt. Wanitsuka in Miyazaki Prefecture. We confirmed that these geomorphological and geological features, which has been often found at areas close to catastrophic landslide scars. And, some of these features, including geological fault, deep rotational landslide, gentle ridgetop, were appeared not only at areas close to catastrophic landslide scars but also at areas far from it.
In September 2005 a prolonged period of intensive rainfall caused landslides in the 27.48 km2 combined catchment area of the Byutano, Kataino and Sakaino rivers of Wanitsuka Mountain, Miyazaki Prefecture, southern Kyushu, Japan. The landslides began as deep-seated debris slides and developed into debris avalanches and debris flows that impacted upon downstream areas. Based on the potential for such events to occur again in this area, evaluations of the spatial and temporal distribution of landslides and slope geomorphological processes are crucial for assessments of hazard levels. Evaluations were based on inventory maps for the years 1948, 1974 and 2005. Interpretation of landslide patterns indicated that large landslides in the study area were preceded by creep deformation of weathered rocks, which controlled chronic landsliding. Approximately 34% of the catchment areas retain the potential to produce landslides. Within creep deformation areas in one catchment, the sites of most potential for triggering in the near future were identified based on geomorphological signatures. Past landslide triggering scenarios were also identified. Previous landslide occurrences in the three locations were established using 14C dating, with estimated ages ranging 200-3300 year BP.
Although forests are expected to reduce sediment discharge during heavy storms by preventing slope failure, current sabo planning uses the ratio of slope failure area to the total watershed area (i.e., slope-failure area ratio) already set for diverse geology type and does not take into account the effect of vegetation conditions. This study analyzed the effects of surface vegetation conditions, forest conditions, geology, and slope gradient on the slope-failure area ratio observed in the past three sediment-related disasters caused by heavy storms. Together with the summary of the previous studies, we discuss how the effect of vegetation and forest conditions can be reflected in the sabo planning. The slope-failure area ratio analyzed in this study was 1.2 to 7.6 times higher for the grassland and new planting areas than for the mature and old forest areas, supporting the result of previous studies that mature and old forests effectively reduced slope failure. On the other hand, the results of this study and previous studies demonstrated that taking into account a distinction between coniferous and broad-leaved forests, tree height/age, and tree density is not effective for increasing the prediction accuracy of slope-failure area ratio. Hence it is concluded that setting the different standard value of the slope-failure area ratio for each geology type and surface condition (grassland/new planting area and mature/old forest) is desirable.
A pyroclastic flow accompanies a pyroclastic surge. A pyroclastic surge killed many people who were taking photographs on a far-off hilltop when a large pyroclastic flow occurred at Mount Unzen in 1991. Volcano hazard maps must include information on pyroclastic surges as well as debris flow, lava flow, and pyroclastic flow. Currently, in studying volcanic hazard maps the area one kilometer around an area prone to pyroclastic flow is designated as the area where a pyroclastic surge may reach. This seems too large. In this paper, data on the pyroclastic surge that occurred at Mount Unzen were analyzed. It was found that the distance the pyroclastic surge travels is proportional to the velocity of the pyroclastic flow. An empirical equation was obtained. This equation will be applied when making a volcano hazard map. It was also found that a pyroclastic surge leaves the pyroclastic flow's main body and flows separately when the pyroclastic flow bends at an angle larger than 20 degrees. These results may be applied in analyzing pyroclastic surges associated with the lava dome collapse type pyroclastic flow. More research is needed on other types of pyroclastic flow.
This study used two conceptual models to examine the effects of variable, heavy rainfall conditions on shallow landslides : 1) the Soil Water Index based on a tank model, and 2) the process-based model. The process-based model used a digital terrain model with 10-m resolution to calculate the regional potential for shallow landslides, based on the distribution of shallow infiltration water, Darcy's law, and a safety factor estimated by an infinite slope stability analysis. We used this process-based model as a conceptual model, rather than as a physically based model, and defined the model output value of the total area of each cell with a safety factor less than one as the Potential Landslide Area Index. The two models were applied at the Funyu Experimental Forest of Utsunomiya University in Tochigi Prefecture, Japan. At the end of August 1998, a heavy rainfall event caused many shallow landslides in the study area, whereas other heavy rainfall events from 1979 to 2008 did not cause severe landslides. A response analysis of data collected hourly during heavy rainfall events with the Soil Water Index from 1979 to 2008 revealed a maximum value in the heavy rainfall events at the end of August 1998. In addition, the relative difference ratio of the Soil Water Index value of the second largest heavy rainfall event, on 11 July 2002, was 8%. Although the response analysis with the Potential Landslide Area Index also shows a maximum value with a heavy rainfall event at the end of August 1998, the relative difference ratio to the second largest heavy rainfall event on 11 July 2002 was 30%. This result suggests that Potential Landslide Area indices obtained from the modeling are useful for discriminating between rainstorms, with and without sediment-related disasters, similar to the Soil Water Index.