A synthetic study on the forecast of occurrence of large-scale sediment disasters and the counter plan on the suburbs of the Mt.Unzn.

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The objective of this study is an analysis of the large-scale collapse at Mt.May-uyama and the debris flows from deposit of volcanic ash and pyroclastic flow on the erupting Mt.Unzen-Fugenndake. Since the large-scale collapses are consid-cred to be the causes of unusual variations of hydrothermal water and ground water and the influence of earthquakes, we analyzed and observed as follows ; hazard area of the large-scale collapses was zoned from the geological profile and fault lines estimated by morphological analysis, radioactivity survey and electric prosoecting. Observations have been made using extensometer, strain meter, inclinometerand accelerometer at Mt.Mayu-yama.We also surveyed the sign of collapse and the pore water pressure including indoor investigations. The results were used in mapping of hazard collapse areas with calculations of critical maximum acceleration and pore water pressure. Relation between volcanism of Mt.Unzen and activities of graben zone at Unzen were also investigated with continuous observations using G.P.S.. We are obser-ving spring water temperature, groudwater level, groudwater and geological electric current which were concerned with carthquakes and unusual variations of ground water quality in order to investigate the sign of large-scale collapse. Continous analysis of condition of gully on the deposit of volcanic ash and pyroclastic flow indicates the relation between gully and scale of the debris flows. The hazard map of flooding was made using analysis of increasing mudflow and energy of flow by rhecology models of volcanic ash.The results of this study provide important data for prevention of secondary disasters caused by volcanic activities.