5. Eruptions of Hokkaido-Komagatake and Meakandake, Hokkaido

Phreatic Eruptions of Hokkaido Komagatake in 1996 and 1998

Hokkaido Komagatake located in the southwestern part of Hokkaido erupted twice, both in small scale in 1996 and 1998, after 54 years dormancy. Mt. Komagatake was known as a sleeping giant in recent decades considering the repeated 4 major historical explosive eruptions (VEI=4-5) since 1640.

March 1996 eruption was a phreatic explosion discharging ash approximately 120,000 ton (Ui et al., 1997a, b). All of ash, lapilli and block discharged during the eruption were derived from pre-existing andesitic lavas and pyroclastics of the volcano. No juvenile material was identified. Glassy particles found in the ash sample were derived from hydrothermally altered part of the volcanic edifice. The 1996 main crater opened at the southern floor of the 1929 main crater, and a radial fissure crater ca. 200 m long opened on the southern summit (Fig. 19).

Despite of its half century-long sleep, the volcano had been continuously monitored by Mori Weather Station of Japan Meteorological Agency (JMA) since 1967, and by Usu Volcano Observatory (UVO) of Hokkaido University since 1982. Volcano seismicity had been very low, even the lowest among the monitored active volcanoes by JMA, except seldom short burst of minor quakes and tremors.

The eruption was preceded by (1) the 3 days-long seismicity below regular detection level, (2) several years-long expansion (ca. 10 cm or 10**-5) of the1929 crater area detected by EDM measurements (Fig. 20), and (3) possible uplift reversal by leveling survey (Usu Volcano Observatory, 1997, Mori et al., 1997). Despite of invisible eruption due to snow fall and darkness, the eruption dynamics was well registered by a single pulse micro-deformation corresponding to 6 minutes-long eruption tremors and following minor seismicity (Fig. 21). Those were luckily detected by 500m deep borehole station (KMB), which were operated by UVO since 1994 and is located ca. 2.5 km south of the 1929 crater.

October 1998 eruption was also phreatic explosion discharging ash at least ca. 3,000 ton. The eruption site was the 1996 main crater, and the south fissure crater did not respond this time. Origin of ejecta was the same as the case of the previous eruption.

No short-term (days to hours) precursors to the 1998 eruption were recognized. But various observational changes had been found during the gradual subsidence and contraction of the summit since 1996 eruption in the preceding months; new geothermal anomalies and possible increase of CO2 (personal communication of Drs. P. Hernandez and K. Notsu) and micro-gravity (by P. Jousset). Comparison of observed records suggest similar eruption dynamics to the 1996 eruption.

Early cock's tail eruption of wet ash cloud, and subsequent fallback produced minor ash-flow cells (Fig. 22) were luckily photographed by a climber or a local officer. During the 1942 eruption, mysterious wet ash cloud descended the eastern slope and was well photographed (Ishikawa and Hashimoto, 1943). Similar mechanism may be responsible for generating those slope-descending ash clouds.

Despite of small size eruption, both eruptions in 1996 and 1998 produced an extensive rock fall on the northern summit, and large blocks measured 30 cm to 1 m size scattered over the trail. Luckily, first eruption occurred during winter snow season after dark and no climbers. And for the second case, local office's eager safe management allowed successfully pretending climbers accident scarcely in time; gate-open time control and crater off-limit.

Accelerating new erosion of the ash-covered summit, finally filled full two old crater depressions, and over flow from those craters caused repeated small scale lahar to the eastern and northeastern flanks during rain fall in 1996 summer, causing minor damages to roads and the settlements (Usu Volcano Observatory, 1997).

The 1929 eruption was well known in the world from the following reasons. (1) Its large explosive eruption in short time (essentially continued less than several hours) producing pumice flow type pyroclastic flows, (2) no clear immediate precursors, except the minor phreatic eruption in the preceding 9 hours, and (3) post eruption subsidence as used for classical inflation-deflation model proposed by K. Mogi.

Whether recent renewal eruptions are the possible early indication of the coming major explosive one or not, is the major concern among volcanologists and also disaster officials and people. Two important viewpoints are followings. (1) More than several minor phreatic eruptions took place during 1919-1924, prior to the 1929 major eruption (Fig. 23), that may indicates a possible intermediate-term precursor, and (2) similar summit views before the 1929 eruption and today; steaming not only from the main craters, but from the fissure craters. Renewal heat supply to shallow vent may be started, and formed recent unstable hydrothermal vent system.

Phreatic Eruptions of Meakandake in 1996-1998

Meakan-dake located in the eastern part of Hokkaido also erupted in small scale twice in 1996 and 1998. November 1996 eruption (Fig. 24) was a minor phreatic explosion discharging ash approximately 12,000 ton. All of the ejecta discharged during the eruption were derived from pre-existing andesite of the volcano. No juvenile material was identified.

November 1998 eruption was much smaller eruption discharging ash at least 650 ton. Almost all of the ash was derived from pre-existing volcano, but fresh and glassy basaltic andesite is rarely included within ash ejecta. It is supposed to be derived from new magmatic body or recently intruded mass.

The 1996 eruption was not predicted in terms of short notice (days to hours), but the eruption has noticeable precursors in weeks to months (Okada et al., 1997, Suzuki and Okada, 1998). Prior to the 1996 eruption, volcano seismicity became very low for several months, while crater temperature climbed up steadily over 500 degree C highest ever. Then seismicity resumed in late August forming three episodes, and correspondingly crater temperature declined slightly. The eruption occurred after the 2 weeks-long minor steady seismicity of the third episode.

A single large earthquake initiated the 1996 eruption, but it was not accompanied by an explosion type airwave. Seismicity only intensified dramatically shortly after the eruption, but sustained its high level only 10 hours. The 1998 eruption has also no short-time detectable precursor. Only single precursory evidence we found, is a slight recovery of minor steady seismicity from the low level in the preceding 19 days, but no events even took place in the preceding 23 hours.

At Meakan-dake, small eruptions took place from the first historical eruption in 1955 trough 1966, and resumed in 1988. All historical eruptions were minor phreatic nature. But this only implies the lack of historical documents in the area. In recent years, volcano sabo engineering group had worked efficiently with volcanologists, and reconstructed the volcano history over 12,000 years based on new drillings and trench works for tephra studies including dating. Four major magmatic eruptions including large pyroclastic flow are identified. Such knowledge is valuable basis for long-term evaluation of volcanic activity. Neighboring communities are preparing volcano hazard map in 1999.

(Hiromu Okada and Tadahide Ui)

References

**Ishikawa, T. and Hashimoto, S. (1943) Explosion of Komagatake on November 16th, 1942. J. Japan. Assoc. Mineral. Petrol. Econ. Geol., 29, 65-80 and 100-112.

*Mori, H. Suzuki, A. Maekawa, T. and Okada, H. (1997) Ground deformations before and after the March 5, 1996 phreatic eruption of Mt. Hokkaido-Komagatake. Geophys. Bull. Hokkaido Univ., 60, 121-130.

*Okada, H. Suzuki, A. Maekawa, T. Mori, H. and Nishimura, Y. (1997) Preliminary report on the November, 1996 eruption at Mt. Meakan-dake, Hokkaido. Geophys. Bull Hokkaido Univ., 60, 131-144.

Suzuki, A. and Okada, H. (1998) Activity of long-coda earthquakes - from 8 years activity at volcano Meakan-dake, Hokkaido. Geophys. Bull. Hokkaido Univ., 61, 263-274.

*Ui, T., Yoshimoto, M., Furukawa, R., Ishizuka, Y., Yoshida, M., Miyaji, N., Katsui, Y., Kito, N., Ganzawa, Y. and Nogami, K. (1997a) March 1996 eruption of Hokkaido-Komagatake, northern Japan. Bull. Volcanol. Soc. Japan, 42, 141-151.

*Ui, T. Yoshimoto, M. Sato, J. Hashimoto, I. and Miyamura, J. (1997b) March 1996 eruption of Hokkaido-Komagatake: reappraisal on eruption volume. Bull. Volcanol. Soc. Japan, 42, 429-431.

*Usu Volcano Observatory (1997) The March, 1996 eruption of Mt. Komagatake, Hokkaido. Rep. Coord. Com. Pred. Volcan. Erup., 67, 1-17.

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