2000年新島・神津島近海地震とその後の降雨による崩壊発生の地質・地形的要因について : 流紋岩質溶岩と火砕物からなる斜面の崩壊の例

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The Niijima and Kozushima earthquake struck Kozushima Island to the south of Tokyo on 1 July 2000, with an intensity of 6-low on the Japanese Meterological Agency scale. The earthquake was followed by heavy rain (cumulative storm precipitation 291mm in 24 hour and maximum hourly precipitation 52mm) on July 7th to 8th and another earthquake occurred on July 9th. These cumulative events triggered many landslides, which we identified on air photographs taken before, during, and after the events, and then mapped on GIS-based topographic and geologic maps to assess the cumulative effects of consecutive earthquake and rainfall on slope failures on a rhyolitic volcano. The successive July 1st earthquake, rainfall, and smaller July 9th earthquake triggered 618, 123, and 102 slope failures, respectively, on rhyolitic lava and pyroclastics that dominate the island. Most of the failures occurred on slopes steeper than 30 degrees near the "erosion front". On jointed rhyolite lava slopes, many earthquake-initiated slope failures occurred because of the high inertial energy within the lava mass during the earthquake. Conversely, few landslides were triggered by rainfall because of the high permeability of the lava. Areas of coarse pyroclastics and crumble breccia suffered from many landslides during the first earthquake but not during the subsequent rainstorm, probably due to the low cohesion and large permeability of these materials. Alternatively, silt or clay-rich pyroclastics experienced more landslides during rainfall compared to the earthquake because of the greater cohesion and lower permeability of these materials. However, rainfall on July 7th to 8th alone appears not to be of sufficient intensity to trigger widespread landslide; previous rainfall events that initiated widespread slope failures on the Kozushima Island had maximum 1-hour intensities >100mm. The earthquake on July 1st could have loosened the surface material, which failed during the subsequent moderate rainfall.
2005-07-25

2000年新島・神津島近海地震とその後の降雨による崩壊発生の地質・地形的要因について : 流紋岩質溶岩と火砕物からなる斜面の崩壊の例

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