Study on Eruption Process and Seismicity in Southeast Asia

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Five Indonesian volcanoes which were seismically active or erupted in 1994 were investigated from geophysical and geochemical points of view in cooperation with the Volcanological Survey of Indonesia. The main results are as follows ;1. Precursor of the big pyroclastic flows generated on Nov.22,1994 at Merapi volcano was observed with geochemical and geophysical observation, though seismic precursor was not clearly identified. Chemical composition of volcanic gases were changed about one month before. The emission rate of sulfur dioxide clearly increased and, a tiltmeter indicated inflation of the summit one week before the pyroclastic flows. These results suggest that new magma came up just before the activity, and how important comprehensive observation is for prediction of volcanic activity.2. The waveforms of volcanic earthquakes and tremors observed at Semeru are quite similar with those at Sakurajima. Before the 1994 explosive activity, A-type earthquakes occurred frequently and then B-type earthquakes increased. Finally, swarms of volcanic earthquakes and tremors were observed. Such a seismic sequence before eruptions has been frequently experienced at Sakurajima and Asama Volcanoes. This result suggests that seismic events and volcanic process may be common at andesitic volcanoes which have repeated explosive eruptions.3. Precise hypocentral determination of volcanic earthquakes was done at Guntur volcano. An aseismic zone at deeper part than 5 km beneath the summit was found. This is similar with that at Sakurajima, Merapi and Asama Volcanoes, and suggests the probable location of the magma chamber at the volcano.4.Before eruptions at Batur and Guntur, the Volcanological Survey of Indonesia recognized seismic precursors and succeeded the forecasting eruptions.