[cacher]
The following is based on a report by Shintaro Hayashi, published in the Smithsonian's Monthly Reports:
"Although the landslide began moving a few days before 11 May, the sliding accelerated 20 minutes before the explosion. A field party saw the fast-moving landslide and took refuge prior to 0800 on 11 May. The explosion was witnessed at 0800 by a pilot flying over the area; he saw a water-and-steam column rising like a geyser, followed within seconds by black smoke emissions.

The explosion, heard as far as 1.4 km away, triggered a mudflow along the Akagawa River and eventually developed into a debris flow downstream. The field party noticed a thin coat of ash covering the mudflow deposits; they concluded that the tephra had issued from the explosion site.

Hayashi suggested that the explosion was triggered by sudden depressurization of a hot water reservoir under the hot spring due to removal of the overlying debris. The depressurization led to sudden boiling, generating sufficient steam pressure to explode. The volume of erupted material was estimated to be between 1,000 and 10,000 m3.

According to Hiroyuri Hamaguchi the precursory vibration and tremor were recorded by a short-period seismometer 1 km NNE of the hot spring. The landslide was as large as 500 m wide, 150 m long, and 500 m deep. After 2000 on 10 May, tremors of increasing amplitude built up. They declined by midnight and then returned at 0400 on 11 May. A maximum amplitude was reached at 0732, followed by a hiatus during 0753-0757. Short- and long-period events took place at 0757 and 0758, respectively.

Hayakawa reported that two hotels at the foot of Akita Yakeyama were completely destroyed by the landslide and lahar; however, there were no casualties because the staff and guests had evacuated. Air photos taken on 12 May by Asia Air Survey Co. can be seen on the internet."