AKITA-YAKEYAMA, Northeast Honshu

(1.366 m, 39o57'40"N, 140o45"38"E)

(8/25/93) According to the JMA's "Volcanic Observation Report No.10" on 25 Aug.,
8/22/97 N of volcanic EQ's= 14, N of tremors=0
8/23/97 N of volcanic EQ's= 18, N of tremors=0
8/24/97 N of volcanic EQ's= 14, N of tremors=0
8/25/97 N of volcanic EQ's= 10, N of tremors=0 (by 15:00)
Access to the crater area is limited.

No visible sign showing increasing activity had been observed on the ground since 17 Aug. The JMA's "Volcanic Observation Report No.7" on 22 Aug. showed,
8/16/97 N of volcanic EQ's= 62, N of tremors=2
8/17/97 N of volcanic EQ's= 81, N of tremors=1
8/18/97 N of volcanic EQ's= 71, N of tremors=0
8/19/97 N of volcanic EQ's=448, N of tremors=1
8/20/97 N of volcanic EQ's=226, N of tremors=0
8/21/97 N of volcanic EQ's= 27, N of tremors=0
8/22/97 N of volcanic EQ's= 14, N of tremors=0 (by 15:00)

Total N of volcanic EQ's=292, and total N of tremors=4 since 8/16.

JMA estimated that volcanic earthquakes had occurred just below the summit. They installed new seismometers near the summit and northern slope of the volcano, a microphone in the western foot, and cameras (color, high resolution monochromatic, and infrared) on the eastern foot on 20 and 21 Aug.; all telemetered to Sendai and Akita. JMA had continued helicopter survey assisted by GSDF. Geophysical monitoring of this activity is also kept by Tohoku University, and field survey of the eruption products has been done by geologists of Geological Survey of Japan, Tohoku University, and Akita University.

Shintaro Hayashi, a geologist of Akita University, who did the field inspection on 18 Aug., estimated the volume of fallout from the "new" crater to be about 1000 m^3 (report on 22 Aug). He pointed out generation of mud flow just before the phreatic explosion of the 16 Aug. noon, and told that the mud which looks puddle, was issued from small depressions just below the "new" crater, all of which locate in the SW rim of the Karanuma crater (main crater). He continued that the mud was deposited around the depressions, partly flowed down to the floor of the main crater, and the total volume was estimated as much as about 20,000 m^3.

Tatsunori Soya, Geological Survey of Japan, introduced an old document written by the late Prof. H. Tsuya, on 22 August. The document, which appeared in the 10th Anniversary Report of Tamagawa Hot Spring Study Group (1954), describes explosions in 1949, 1950, and 1951 at Akita-Yakeyama (the last two were not documented officially). According to that, large explosion caters (C1-C4) had been already formed before the 1949 eruption, one of which is the Karanuma crater (C1 crater). Eruptions in 1949 occurred in the eastern margin of the Karanuma crater, resulting in the formation of small craters, a1, a2, and a3 (15, 10, and 5 m across respectively). Nobody in the hot spring area 3 km east of the summit, heard sounds of explosion and felt the quakes. The eruption products were about 1 m thick in the rim of the a1 crater and contained old lava blocks up to 1 m across. Another explosion occurred at a1 crater in February 1951; as a result, it widened as much as 50 m across. Products of the 1951 explosion were smaller in volume than those of the 1949 eruption.

Shintaro Hayashi proposed referring to the introduction by T. Soya that the present explosion occurred in a2-a1, and mud spouted out from a3 crater .

Information contact: Noritake Nishide, Sendai District Meteorological Observatory (JMA); nnishide@redc-sn.eqvol.kishou.go.jp; Shintaro Hayashi, Faculty of Education, Akita University, hayashi@ipc.akita-u.ac.jp; Tatsunori Soya, Geological Survey of Japan, soya@gsjrstn.gsj.go.jp; Volcano Research Center, ERI, Univ. of Tokyo: nakada@eri.u-tokyo.ac.jp.

Link to photos of Akita-Yakeyama phreatic explosion by Tatsuro Chiba (in Japanese): photo #1-2="new" crater, #3-4="mud" flow, #9-11=main crater (top shows one year before eruption)

According to "Volcanic Advisory" issued from JMA in the evening of 16 Aug. 1997, a tourist reported a small-scale phreatic explosion at the Karanuma (meaning of empty pond) Crater (about 200 m across) near the summit (1,366 m) of Akita-Yakeyama Volcano around the 16 Aug. noon. Seismometers of JMA installed at the volcano recorded volcanic tremors during one hour, 10:53-12:04 (JST). The JMA's "Volcanic Observation Report" on 17 Aug. reported the finding of a new hole (crater) of 20 m across in the southeastern rim of the Karanuma Crater. Eruption products including cinders as large as 20 cm across were found around the new crater, and pastes of volcanic ash were sprayed about 300 m south of it. No eruption cloud remained in the new crater on 17 Aug. Approach to the crater area was limited.

16 Aug. 97
N of volcanic EQ's=62, N of tremors=2
17 Aug. 97
N of volcanic EQ's=81, N of tremors=1
18 Aug. 97 (by 15:00)
N of volcanic EQ's=53, N of tremors=0
(JMA Volcano Observation Reports on Akita-Yakeyama, No. 3)

T. Oba and T. Hasenaka, geologists of Tohoku University, who did the field inspection on 17 Aug. reported that the "new" crater was quiet, and the depth of it reaches about 30 m. The cinders up to 30 cm across were thrown about 25 m away from the crater, and no juvenile materials were included. Ash deposits on the ridge 20-30 m south of the "new" crater were as thin as 4-5 mm. They suggested that the present eruption occurred using one of old craters, which probably had appeared in 1949, because the volume of eruption products is too small to fill up the space of the "new" crater.

Akita-Yakeyama is a stratocone (about 7 km across) mainly of andesite. The historic eruption occurred for 3 days in1949. A small-scale mud flow as short as 200 m long occurred during the eruption activity.
The summit of this volcano is about 3 km southwest of Sumikawa-Onsen (hot spring resort) where a phreatic explosion was triggered by landslide in May 1997 (see below).

Information contact: Noritake Nishide, Sendai District Meteorological Observatory (JMA); nnishide@redc-sn.eqvol.kishou.go.jp, Tsukasa Ohba and Toshiaki Hasenaka, Tohoku University; ohbatu@mail.cc.tohoku.ac.jp, hasenaka@mail.cc.tohoku.ac.jp, and Volcano Research Center, ERI, Univ. of Tokyo: nakada@eri.u-tokyo.ac.jp.

A series of landslide, steam explosion, mud-flow and debris-flow occurred in a hot spring resort at the foot of Akita-Yakeyama on 11 May, 1997. The Sumikawa Hot Spring is about 4 km northeast of the summit of the Akita-Yakeyama.

The followings are based on the report to the National Committee of Volcanic Eruption Prediction by Dr. Shintaro Hayashi of Faculty of Education, Akita University. Although the precursory phenomena of landslide began a few days before, the movement of sliding was accelerated 20 min. before the explosion. A group of the city mayor witnessed the fast-moving landslide near there and took quick refuge before 8 a.m. (JST), May 11. The explosion was witnessed at 8 a.m. by a pilot flying over this area, who reported water-steam column rising like geyser; approximately 10 s later, followed by projection of black smokes. The sound of explosion was heard 1.4 km away from the explosion site. Mud-flow generated together with the explosion, and descended along the Akagawa River, developing into debris-flow in the down-stream. The field inspection showed mud-flow deposits were covered by thin ash. Cinders which were estimated to be issued from the explosion site, were observed, also covered with ash. Dr. Hayashi presents a model that the explosion was triggered by sudden depressurization of a hot water reservoir under the hot spring due to removal of the overlying mass of debris (landslide). The depressurization introduced sudden boiling of hot water, generated overpressure high enough to explode. The volume of erupted materials was roughly estimated as 1,000-10,000 cubic meters.

According to Faculty of Science, Tohoku University, the precursory vibration and tremor of the events were clearly recorded by the short-period seismometer of Tohoku University, which was installed 1 km NNE of the hots pring. In the morning, May 10, a landslide as large as 500 m wide, 150 m high, and 500 m deep started, being associated by vibration signals whose amplitudes increased with time and declined by the midnight. The first tremor was recorded at 20:00, May 10. Tremors and vibrations associated with landslide were activated again at 4 a.m., May 11, the tremor activity reached the maximum at 07:32, successively followed by a hiatus of tremors and vibration (07:53-07:57). And then, short-period event and the following long-period event took place at 07:57 and 07:58. No signals at 8:00 due to electric halt.

Akita-yakeyama is a small andesite stratovolcano of 7 km-diameter and 700 m-height with the summit crater of 600m across. Fumarole activity is strong around the summit so that sulfurous alteration of rocks is heavy, and there are plenty of hot springs on the slopes and at the foots. Geothermal wells were drilled in the volcanic field including Akita-Yakeyama. The recent eruptions occurred in 1890, 1929,1948, and 1949. All were small phreatic eruptions. The volcanic area is covered thickely with snow in winter season.
Faculty of Science, Tohoku University, keeps monitoring the activity at Akita-Yakeyama.

Information contact: Dr. Shintaro Hayashi, Faculty of Education, Akita University, e-mail: hayashi@ipc.akita-u.ac.jp, Prof. Hiroyuki Hamaguchi, Faculty of Science, Tokoku University, e-mail: hama@aob.geophys.tohoku.ac.jp, and Volcano Research Center, ERI, U-Tokyo, e-mail: nakada@eri.u-tokyo.ac.jp.

Link to photos in "The land slide and steam explosion at Sumikawa-onsen" by Tatsuro Chiba.

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