Evaluation of the Recent Activity at Guntur Volcano, West Java, Indonesia

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  • インドネシア・グントール火山における最近の火山活動の評価
  • インドネシア グントール カザン ニ オケル サイキン ノ カザン カツドウ ノ ヒョウカ

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Abstract

Guntur is a volcano-complex, West Java, Indonesia and is located 35km SE of Bandung. Explosive eruptions and lava flows were repeated at Guntur Crater since 17th century, however, no eruption has occurred for 150 years after the 1843 eruption. Volcanological Survey of Indonesia began to monitor the volcano by a seismometer at a station 0.8 km east of the summit crater. In 1994, 3 seismic stations were added to determine hypocenters in collaboration with DPRI, Kyoto University. Volcanic earthquakes are located in the summit area (aligned from SE to NW along the volcano line from Guntur to G. Masigit) and Kamojang area west of the summit. In Kamojang area, earthquakes are distributed from Gandapura (NE) to Darajat caldera (SW) at depth of 5-10 kim, with the strike slip mechanism, similarly to tectonic earthquakes in inland area. In contrast, volcanic earthquakes beneath the summit are shallower (<5km) with normal or reverse fault mechanism. The magnitudes are smaller (<1) than those in Kamojang (M<3). In recent 8 years, the number of volcanic earthquakes increased in May-July 1992, May-August 1993 and May-November 1997 and the seismicity increased in both summit and Kamojang area. The seismic energy release in these periods is comparable to each other. The volcanic earthquakes since May 1997 were concentrated at depth of 2-4 km beneath the summit.Many earthquakes occurred on 29 May and 7-8 October successively. The focal mechanisms of them are reverse fault-type. The seismicity increase coincided with upward tilt to the direction of the summit crater.The precise leveling survey in August and November 1997 shows the ground near the summit was elevated.Tilt and leveling data suggest the ground around summit area was inflated due to minor pressure increase at shallow part beneath the crater. The increase in seismicity at the summit area may be caused by shear fracture influenced by the pressure increase. Reverse fault-type earthquakes in swarms suggest uplift of the ground above the pressure source. The increase in seismic energy released in 1997 is comparable to the previous seismic activity. From the estimation of seismic energy, we may conclude that the 1997 seismicity increase may be one of the phenomena repeated in the time interval from one to a few years.

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