Changes in Geochemical Characteristics of Thermal water observed at Sumikawa and Akagawa Spas, after the 1997. 5. 11 Sumikawa Landslide, Akita Prefecture

Abstract

In May 8-11, 1997, a landslide occurred at the Sumikawa Spa, the northern flank and about 4 km northeast from the summit crater of Akita-Yakeyama Volcano, northeastern part of Akita Prefecture. The scale of the landslide was 400 m wide and 800 m long, the volume 2, 500, 000 m3, and the height of the mainscarp was 60 m. In May 11, when the main landslide mass movement occurred, the toe (front unit) of the landslide mass was transformed into the debris avalanche, 500, 000 m3 in volume. The debris avalanche rushed over the River Sumikawa, and to the Akagawa Spa, 1100 m northward and 190 m low of the Sumikawa Spa. The debris avalanche was finally transformed into the debris flow, that flowed about 800 m northward to the confluence with the River Kumazawa, one of the upper reaches of the River Yoneshiro. At the same time, when the main landslide and the debris avalanche occurred, the intermittent phreatic eruptions (hydrothermal explosions) broke out at the Sumikawa and Akagawa Spas. The smoke (fume) of these eruptions rose up to a height of>100 m at the Sumikawa Spa and up to a height of several decades of meter at the Akagawa Spa.Three times of observations of water temperatures and chemical concentrations were carried out for thermal waters and interstitial waters centrifuged from altered clay and ash samples obtained from at and around the Sumikawa and Akagawa Spas, to know the possibility of contribution of high temperature volcanic gas and deepseated hydrothermal water to the occurrence of the 1997 Sumikawa Landslide with phreatic eruptions (hydrothermal explosions).The results lead us to conclude the followings.1) The increase of ground water temperature and chemical concentrations, and the decrease of pH observed at the Sumikawa Spa were caused by the decrease of the rate of the intrusion of subsurface, meteoric and/or river water to the Sumikawa hot spring layer.2) The increase of chloride ion concentrations compared with other reference values observed at the Akagawa Spa were caused by the increase of the rate of the intrusion of interstitial water to the Akagawa hot spring layer, because of the movement of spring head, usually observed in hot spring area.These results obviously denied the possibility of contribution of high temperature volcanic gas and deepseated hydrothermal water to the occurrence of the 1997 Sumikawa Landslide with phreatic eruptions (hydrothermal explosions).