Detection of buried snowpack in landslide sediments using ground penetrating radar
2015 年 36 巻 2 号 p. 25-33
詳細
2015 年 36 巻 2 号 p. 25-33
When landslides occur in snow-covered areas, the presence of snow may cause a secondary hazard. In the Tatsunokuchi area of Tsunan-machi, Niigata Prefecture, Japan, a large landslide was induced by the seismic motion resulting from the North Nagano Prefecture earthquake (magnitude of 6.7) on March 12, 2011, which caused a snow avalanche at the same time. In this study, a ground penetrating radar (GPR) survey was applied to find areas of buried snow and ice underground in the landslide area. We first conducted experimental field surveys of artificially buried snow and ice to verify the effectiveness of the GPR survey in terms of investigating ice and snow within collapsed land. The results of the experiments successfully detected strong radar reflections from areas of ice and snow. When the ice and snow began to melt, the survey data showed both multiple reflections and the attenuation of reflections by meltwater. Then, we investigated the presence of buried snow in the landslide area of Tatsunokuchi using GPR surveys. A GPR survey conducted in August 2011, five months after the landslide event, showed a clear response indicating the presence of a snowpack underlying the sediment. A response related to an underground snowpack was also observed in November 2012, about one year after the first GPR study. An excavation was conducted and the underground snowpack was ascertained. The snowpack was found to be compressed into an ice-like state that may be appropriate for long-term storage underground. The survey results of the buried snowpack in the landslide area at Tatsunokuchi were consistent with the results of the experimental surveys. Snowmelt water may cause the ground to loosen, which can increase the risk of secondary hazards such as subsidence and sediment discharge by heavy rains. The results of our study indicate that GPR surveys are useful for examining buried snowpacks in landslide areas, as well identifying the potential areas where secondary hazards can be caused by the melting of snow and ice within the landslide area.
