UNZEN VOLCANO, Western Kyushu
(June 1, 1996)
Slow deformation of lava dome after the halt of magma supply was monitored successfully with GPS and EDM analyses by the Shimabara Earthquake and Volcano Observatory (SEVO), Kyushu University. The deformation shows the tendency that endogenous dome is flattening: the maximum subsidence and lateral movement during this one year are about 1.5 m and 1 m, respectively; the averages are 0.5 and 0.4 m, respectively. The temperature measurement of fumarolic gas at the top of the dome by SEVO showed that gas had cooled from 500-700 deg. C to 300-500 deg. C during this one year.
Several small block-and-ash flows occurred on 10, 12 and 13 February 1996, leaving one-year quiescence, though magma supply had stopped since February 1995. According to the Shimabara Earthquake and Volcano Observatory (SEVO), Kyushu University, these flows descended to SE, traveling about 1.0 km from the source; surge-like (ash-dominated) deposits were associated with the flows. These deposits were colored in reddish brown, and the temperature of lava blocks in them was measured >60 oC using the infrared camera of the Public Work Research Instiute. These flows were generated by partial collapse of old lava (lobe 7 or 8). The witness told that the ash-cloud accompanied by these flows was much similar to those of pyroclastic (block-and-ash) flows that took place frequently during 1991-94. Simple rockfalls (without ash-cloud) also occurred simultaneously and reached about 1.5 km from the source, beyond the front of block-and-ash flows!
No volcanic earthquakes occurred before and after these events. Tiltmeter close to the dome did not show any change during these events. These partial collapses may be due either to cooling shrinkage of the dome or seasonal temperature change.
Generation of pyroclastic flows and major deformation of lava dome had stopped in February 1995. Refer to Decade Page for the 1990-1995 eruption history.
(right photo) Pyroclastic flows, Unzen Volcano, on Jun. 24, 1993 (taken by S. Nakada).
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