Chokai Volcano Eruptions

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Not many signs were noted announcing new activity. Small earthquakes beginning in December 1973 were later detected on routine recordings. A climber reported to have seen "an unusual phenomenon at the summit of the volcano", probably increased fumarolic activity.
In late February, inhabitants living on the eastern foot of the volcano, 10 km from the summit, heard sounds like gun shooting or thunder in the direction of the volcano.
A pilot first observed ash emissions ("black smoke") producing small ash clouds and ash falls on the snow on the eastern slope of Chokai. On 3 March, a new 50 m wide crater where all snow had melted was discovered at the foot of the 1801 lava dome (Shinzan). Activity decreased rapidly in following days and the last ash emissions were observed on 5 March. On 6 March, a small mud flow, 2 km long, was observed flowing slowly on the snow-covered northern floor of the caldera.
The second phase of the eruption began in April. A swarm of volcanic earthquakes was recorded on April 6, and a new fumarolic activity started on April 7. More than 10 fumaroles, arranged in an E-W direction, were formed along a fissure from the summit of 1801 Shinzan dome westward to the lower rim of the Kojindake crater.
A series of ash explosion started on 24 April from Kojindake crater and were observed from a plane. The explosions produced a small ash plume of a few 100 m height, and ash fall until the eastern base of the volcano.
During the strongest explosions, ejection of cock's tail-jets of ash led to and rapid melting of large amounts of snow resulting in a mud flow, that started at the summit crater and flowed down the W side reaching 3 km length and 2 m thickness.
Phreatic explosions continued intermittently until the end of April, and steaming until October.
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Sources:
- Smithsonian GVP monthly reports
- Nakamura and Shibahashi (1977) "1974 activity of chokai volcano, Japan", Bull. Volc., v 40 (4), pp 231-238