Survey of the Liquefied Layer and Sand Boil due to Tottoriken-seibu Earthquake by Geoslicer

Tottoriken-seibu earthquake occurred in 6 October 2000, Southwest Japan. Many sand boilings occurred around Lake Nakaumi near the source region of this earthquake. Using 1.5 m wide and 4.0 m long steel box, we obtained extensive vertical slices beneath surface sand boils at Daikon-jima Island. Nineteen sand boils appeared after the earthquake in the studied area, and the ground surface subsided around sand boils. Slices contain dredged sand with faintly bedded structure which is capped decomposed granite and soil up to 1 m thick. The basement in this area consists of Pleistocene basalt and thin organic mud layer covers it which deposited on the bottom of the Lake Nakaumi. This land was formed by dredge and reclaim in about 1975. Slices show several signs not only to reveal mechanism of liquefaction, but also to utilize the common features as index of past great earthquakes. It has revealed that dredged sands liquefied and erupted on the surface through the capped layer. Detailed observation and description of liquefied layers are the clue to know the liquefaction mechanism. Some parts of liquefied layer preserved as plugs into the primary depositional structure, and the other parts deform convexly toward the injected path. Upper part of the liquefied layer indicates the finer grain size, which may have been elutriated from intra-layer. Sands are passively erupted along the fissures in the capped layer formed by shaking during earthquake.