Structural development of the caldera


Structural development of the caldera

Caldera structure on 9 July, 18 hours after collapse

Collapsed caldera on July 9 consists of the vertically-subsided central part without major deformation and the extended marginal zone tilting inward of the caldera. The central part shows oval shape with 600*500 m across elongated in NE-SW direction. Small explosive craters were observed at the center of the central part. Width of the marginal zone is max. 300m in NW side. Open cracks and normal faults developed on the surface of the zone parallel to the outline of the collapsed caldera.

After the appearance of the collapsed caldera, the caldera floor subsided intermittently. Photographs taken on July 9, 10, and 11 show that the depth of the caldera floor increased and the marginal zone tilted toward the inside of the caldera. No eruption occured duirng this period. （Temporal change of the depth of caldera）．
．Subsidence speed duirng this period is about 90m per day.

The collapsed caldera grew till middle of August and the horizontal across of the caldera reached 1.6 km. （Structural change ）During the caldera growth, the growth rate is nearly constant.


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