Structual development of the collapsed caldera

Structural character of the early stage of the collapsed caldera

Structure of the collapsed caldera at noon of 9 July, at about 18 h after the first summit eruption was reconstructed from the aerial images taken by Asia Air Survey

Left: Structure of the collapsed caldera on 9 July, 2000 (after Geshi et al. 2002. Bull Volcanology)
Right: Vertical view of the collapsed caldera on 9 July, 2000. Taken by Asia Air Survey

The collapsed caldera consisted of caldera floor surrounded by steep caldera wall. The caldera floor can be subdivided into an undeformed central zone and an extended marginal zone bordered by concentric faults. The central zone consisted of a coherently subsided block, which was oval in shape, about 300~500 m across, and elongated in a SW--NE direction. The original ground surface, which was covered by a small amount of the ejecta from 8 July, remained undeformed in its central zone. At the center of the block, two or three craters, 50--100 m in diameter, were lined up in WNW--ESE direction and eruptive materials were scattered around these craters. No steam emission was observed from these craters during the morning of 9 July. The marginal zone consisted of blocks that tilted inward towards the caldera. Some extension cracks and normal faults subparallel to the outline of the caldera developed in this zone. The marginal zone was wide in the northwestern area (250 m) and absent in the southeastern area. A circumferential slope, 50 m high, divided the central and marginal zones. There was little talus deposit at the foot of the slope. The caldera wall surrounding the floor was 50--100 m high. Talus developed from the caldera wall on the marginal extended zone of the caldera floor. Horizontal displacements of the subsided caldera floor were estimated by using landmarks on aerial photos. The central zone subsided almost vertically with a small horizontal displacement, whereas the marginal zone moved a maximum of 100 m towards the center of the caldera with extensional deformation.

After the summit eruption of 8 July, the collapsed caldera enlarged continuously. Oblique aerial images taken on 10 and 11 July show continuous subsidence of the caldera floor without eruption. Depth of the caldera floor exceeded 300 m on 11 July: 3 days after the beginning of the caldera collapse. The central zone subsided vertically with only minor deformation during this period. The northeastern part of the marginal zone also subsided with an increase of inward tilting and extension. Talus deposit from the caldera wall covered the original ground surface of the subsided caldera floor.
Structure of the caldera did not show remarkable change in the daytime of 9th July and the depth of the caldera became deeper more than 100m in the next day. During the midnight between 9 and 10 of July, a step-like tilt change was observed. This strongly suggests that the caldera floor subsided intermittently at the step-like tilt change.

Change of the depth of the collapsed caldera in the first 3 days (from 9 to 11, July.)

Continuous subsidence of the caldera floor: left:9 July, middle: 10 July, and right: 11 July, 2000. Taken by Mainichi Newspaper and Chunichi Newspaper .

Collapse of the caldera wall enlarged the diameter of the caldera from July to August, 2000. The caldera enlarged mainly towards the northeast and southwest in July, and towards the southeast in August. By the middle of August, the outline of the collapsed caldera had reached the rim of Hatchodaira Caldera, which was formed 2500 years B.P. (Fig. 9A). The center of the final outline of the caldera is located at the point where the craters were first observed on 9 July. The southern rim of the caldera floor, which was nested by craters and fumaroles, corresponds to the caldera rim on 9 July.

The diameter, area, and volume of the collapsed caldera increased linearly from July to the middle of August . The rates of increase in diameter, area, and volume were 20 m/day, 3.5~10--2 km2/day, and 1.4~107 m3/day, respectively from 9 July until the middle of August. The depth increased rapidly in the middle of July and remained at almost the same level (~450 m).

Growth of caldera from July to September, 2000. (after Geshi et al. 2002. Bull Volcanology)

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