Quaternary Volcanoes > Active Volcanoes > Kuju

6: Hot springs and geothermal and ore deposits - 7: Current activities and observation system - 8: Characteristics of eruption activities and precautions taken in volcanic disaster prevention

6: Hot springs and geothermal and ore deposits
   Near Kuju Volcano, many hot springs occur such as Sujiyu, Hokkein, and Akagawa. In addition, many carbonated springs occur near Kurodake including the Shiramizu mineral spring. In western Kuroiwasan, in the western part of our geological map, geothermal surveys have been conducted since 1955. In 1967, Otake Geothermal Power Plant (12,500 kW) began operation; in 1977 and 1991, Hatchobaru Geothermal Power Plant units 1 (55,000 kW) and 2 (55,000 kW) began its operation, respectively. Hatchobaru Geothermal Power Plant is the largest geothermal power plant in Japan. In the northeastern part of Hosshozan, and in Iozan and Akagawa, there are active solfatara and fumaroles that release volcanic gas including hydrogen sulfide. In the Iozan area, sulfur mining since the Edo Period was discontinued in 1983.

7: Current activities and observation system
   Since the 1995–1996 eruption, volcanic tremors have occurred at Kuju Volcano in 1997; however, no significant abnormalities have been reported since then. The altitudes of fumaroles on the Iozan mountainside in northeast Hosshozan have been decreasing since 2005, and seismic activities have been calm as well. Volcanic earthquakes have occurred in shallow zones of the Hosshozan area and in Sujiyu and Otake in the western part of Kuroiwasan at depths of several kilometers (Japan Meteorological Agency, 2013). Few volcanic earthquakes have occurred in the Taisenzan and Kurodake areas. In contrast, deep and low-frequency earthquakes occurring at depths of approximately 20–25 km have occurred but have not been confirmed west of Hosshozan. Kuju Volcano is continuously observed by the Japan Meteorological Agency. The current observation system near Kujusan includes one seismometer belonging to Japan Meteorological Agency, three global navigation satellite system (GNSS) observation points, one inclinometer, one infrasound monitor, one telephoto camera, one seismometer belonging to Kyoto University, one seismometer belonging to Kyusyu University, and four seismometers from Oita Prefecture (Japan Meteorological Agency, 2013). Other wide-range observation networks include the GNSS-based control stations of the Geospatial Information Authority of Japan (GSI) and earthquake observation stations of the National Research Institute for Earth Science and Disaster Prevention (NIED) that are present within 20 km. On December 1, 2007, the Japan Meteorological Agency implemented the eruption alert level operation at Kuju Volcano. Since the operation began until July 2014, level 1, or normal, has been maintained.

8: Characteristics of eruption activities and precautions taken in volcanic disaster prevention
   Kuju Volcano has erupted magma of various compositions ranging from basaltic andesite to dacite. Its eruption style and volcanic phenomena are also diverse and include large-scale pyroclastic flows, lava flows, lava domes, block and ash flows, and debris flow. Abnormal phenomena at Kuju Volcano since its recorded history have been associated with hydrothermal activities of fumaroles; in the most recent 1,600 years, no large-scale magmatic eruption has been reported. However, in the preceding 8,000 years, active magmatic eruptions occurred near Taisenzan and Kurodake at the eastern part of Kuju Volcano. At the present (2014), no forewarnings of magmatic eruptions have been issued. However, deep, low-frequency earthquakes have been recorded in the eastern part at depths of 20–25 km (Japan Meteorological Agency, 2013), thus, future magmatic eruptions are possible.

   The 1995–1996 activity was a phreatic eruption associated with hydrothermal activities of fumaroles. Phreatic eruptions of similar scale have occurred several times during the past several thousands of years in the Iozan area of Hosshozan. These phreatic eruptions are difficult to predict, and relatively small phreatic eruptions could lead to damages from ejected volcanic blocks; this activity is particularly dangerous for hikers using a nearby hiking trail. In addition, the deposition of altered clay volcanic ash lowers the infiltration capacity of the ground surface such that even a small amount of precipitation could cause a debris flow. Therefore, extra caution is warranted.

   Ejection of andesite‒dacite magma forms thick lava flows and lava domes. The amount of falling pyroclastic materials is relatively small in such cases, although block and ash flow deposits caused by the collapse of lava domes and lava flows have been found 4 km from the summit in volcanic fan deposits. Therefore, disasters associated with these deposits should be considered. Lava flows and bombs of basaltic–andesite magma have reached areas 3–4 km from the crater, and pyroclastic fall materials such as scoria have been found in a wide extent near the foot. Damages from these pyroclastic fall materials should be considered as well.

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