4. Volcanic Activities in Tohoku

Unrest of Iwate Volcano

Iwate volcano lies about 20 km northwest from Morioka city, the capital of Iwate prefecture, in the northeastern Honshu, Japan. This volcano is composed of six major cones standing along the E-W direction and is subdivided in two groups; namely the western Iwate volcano group (Mitsuishi-yama, O'matukura-yama, Inukura-yama, Ubakura-yama and Kurokura-yama) and the eastern Iwate volcano one (Yakushi-dake). Eruptive rock is mainly composed of basalt and basalt-andesite. Recent eruptions occurred in 1686, 1732, 1919 and 1939, in which the largest eruption occurred at the summit (latitude 39.850 oN, longitude 140.004 oE, elevation 2,038 m) of the eastern Iwate volcano in 1686 and emitted pyroclastic ejecta (volume 8.5 x 10**7 m**3).

The premonitory deep volcanic tremors were observed at the depth of 8-10 km beneath the eastern Iwate volcano on September 15, 1995. Since then, activities of deep tremors and shallow volcanic earthquakes have been relatively at high level. Shallow weak swarm activity took place in January 1998 in the area of western Iwate volcano. Thereafter, the activities of HF events and some LF events gradually increased in the period from February to April. Distinctly high number of shallow HF events were observed in April 29 and the swarm activity was kept at very high levels till the end of August 1998 (Fig. 15). During this period, more that 30 felt shocks were registered at the nearby station. The most significant feature of seismicity is that the epicentral area of the shallow events was elongated successively about 10 km toward the west in the period of February to August and that the deep LF events at the depth of Moho (ca. 30 km in depth) increased distinctly in number beneath the volcano (Fig. 16). The tectonic earthquake of magnitude 6.1 occurred on September 3rd, 1998 and was located at the adjacent area close to the volcanic seismic swarm region. The reverse faulting associated with this shocks was clearly traced about eight hundred meters along the northern edge of existing active fault. Total number of registered volcanic earthquakes in 1998 was about 8,900 and the maximum magnitude of these events was 3.4.

The remarkable crustal deformation has been monitored by not only the borehole tilt meters and strain meters at three stations at the flank of the volcano but also the temporally installed GPS network. Horizontal displacements in the NE-SE direction were predominant at the western part of seismic swarm area, suggesting that magma or magmatic fluid was intruded into the zone beneath the elongated seismic area (Fig. 17). The seismicity and crustal deformation were declined after the occurrence of the earthquake of M6.1. At the beginnings of 1999, the unrest at Iwate volcano is still persisting though the current seismicity is relatively low level.

There are several fumarolic activities in Iwate volcano. The most active one is O-Jigoku fumarolic area where is close to Kurokura-yama in the western Iwate volcano. The fumarolic activity around the summit (Yakushi-dake) in the eastern Iwate volcano is not so high and the temperature is about 90 deg. C and gas pressure is very weak. Volcanic gases were directly collected from two fumaroles (No.1 and No.2) at O-Jigoku area during the period from June to September in 1998. The temperature of No.1 and No.2 is about 110 deg. C and 135 deg. C, respectively, and the temperature was not changed in the period.

Their general chemical features can be summarized as follows: (1) water vapor is about 98 % of the total volcanic gases. (2) hydrogen chloride, sulfur dioxide, hydrogen sulfide and carbon dioxide in dry gas range from 1 to 7 %, from 1 to 3.6 %, from 18 to 26.5 % and from 64 to 76 % , respectively. (3) residual gas, that were not absorbed in alkaline solution, is mainly composed of nitrogen with small proportion of helium, hydrogen, methane and argon.

The SO2/H2S ratios in gases from O-Jigoku are shown in Fig. 18. The ratio is gradually increased with time. The ratio changed from 0.05 in June to 0.15 in September. The concentration of HCl and total sulfur gas in No.2 fumarolic gas were increased in the period. From the isotope composition of oxygen and hydrogen of gas condensate, water vapor in fumarolic gases almost derived from magma. And the proportion of magmatic water was increased with time. The evidence of changes in chemical and isotopic composition of volcanic gases means that the volcanic activity was increased gradually from June to September in 1998.

(Hiroyuki Hamaguchi and Jun-ichi Hirabayashi)

Volcanic Gas Disaster

In 1997, three volcanic gas disasters occurred around volcanoes in Japan. Two gas accidents happened in Tohoku district. Two members of a party of the Ground Self Defense Force (Japanese army) fell in the depression (18m×12m, depth 8 m) at the northern foot of Hakkoda volcano on the evening of 12 July, and 14 members of GSDF go to the rescue of them. Three of them lost their lives.

High concentration of CO2 gas stay in the depression, the concentration of CO2 in air at the floor ranged from 15 to 20 % on 13 July. Delta 13-C values are -5.7 per mil for CO2 in the gas collected from the depression floor on 13 July, and -6.0 per mil for that in spring water from near the depression. The latter gas sample was collected on 14 July. The CO2 gas in the air at the depression and in spring water is volcanic origin. Recently, emission of CO2-rich gas from several points of the depression wall is detected, the concentration of CO2 is about 16 %. The present data suggests the volcanic origin of the CO2-rich gas that killed three members of GSDF.

On 15 September, Four hikers died by H2S gas in the Numanotaira crater at the western flank of Adatara volcano, Fukushima Prefecture. The volcanic activity at the crater is increased from September 1996. The flux of volcanic gas was increased at the crater floor. Low temperature and high concentration of H2S gas oozed out at the south edge of crater floor. The day of gas accident, it was no wind and deep foggy. Hikers get lost theirs way to the summit of Mt. Adatara. And they passed in the high concentration H2S area. About one month after the accident, more than 400 ppm of H2S was detected in the air above 1 m of ground surface at the scene.

(Jun-ichi Hirabayashi)

References

*Koshiya, S. et al. (1999) Earthquake fault associated with Iwate-ken Nairiku-hokubu earthquake of September 3, 1998. Active Fault Res., 17, 9-20.

**Kusatsu-Shirane Volcano Observatory, Tokyo Inst. Tech. (1999) Observation of volcanic gases at Mt. Iwate. Rep. Coord. Commit. Predict. Volc. Eruption, 72, 22-24.

**Sato, M. and Hamaguchi, H. (1999) Analysis of crustal deformation at Iwate volcano in February-August, 1998. Gekkan-Chikyu, 21, 312-317.

**Tanaka, S., Nakamichi, H., Hamaguchi, H. and Ueki, S. (1999) Seismic activity at Iwate volcano in 1998. Gekkan-Chikyu, 21, 273-279.

**Tohoku University (1999) Seismic activity and crustal deformation at Iwate volcano in the period from June to September, 1998. Rep. Coord. Committ. Predict. Volc. Eruption, 72, 2-21.

**Ueki, S., Miura, S., Sato, T., Tachibana, K. and Hamaguchi, H. (1999) Crustal deformation at Iwate volcano observed by a dense GPS network. Gekkan-Chikyu, 21, 296-301.

(*In Japanese with English abstract. ** In Japanese)