Abstract
Early Pleistocene Okiura caldera, 15 by 10 km in diameter, located in the Hakkoda geothermal area, Northeast Japan, is one of the examples to address caldera structure and a caldera forming process. Volcanic sequence of the caldera filling deposits, a bout 50 km3 in remnant volume, is divided into, at least, eighteen units of major and minor pyroclastic-flow deposits, debris-flow deposit, and post caldera dacite lava. In the caldera-fill deposits, several abut unconformity relations occur between upper and lower pyroclastic-flow units are observed. Those abut planes are likely considered to be fault scarps, simultaneously created by pyroclastic flow eruptions. Estimation of paleo-current directions for the subsequent major pyroclastic-flow units reveals that these eruptions took place at inner caldera fault either in the central or southern part. Gravimetric and bore hole data, and presence of the syn-eruptive inner caldera faults suggest that coherent caldera floor was separated into several large pieces. The overall caldera structure was built up by the sequence of pyroclastic flow eruptions from inner caldera vents.
