インドネシアSemeru火山の火山性地震・微動

Abstract

Semeru volcano, an andesitic stratovolcano in East Java, Indonesia has erupted inter-mittently for more than 50 years. On February 3, 1994, the volcano erupted violently accompanied with remarkable pyroclastic flows. Seismic activity prior to the eruption was characterized by occurrence of deep volcanic earthquakes, and successive increase in shallow earthquakes and tremors continuing for 9 days until the eruption. Such a sequence of the seismic activity has been often observed at Sakurajima. Volcanic earthquakes and tremors observed at the volcano are quite similar to those recorded at Sakurajima and Suwanosejima volcanoes; those are A-type earthquakes, B-type earthquakes, explosion-earthquakes, harmonic (C-type) tremors and non-harmonic tremors. Spectral analysis for these events at the three volcanoes were done. The peak frequencies of them at Semeru and Sakurajima are almost same, but lower than those at Suwanosejima. This result suggests that Semeru has a shallow magma conduit of similar size with that of Sakurajima. It is noteworthy that harmonic tremors at Semeru have a few discrete frequency peaks of integer multiple, and the frequencies change with time, similar to those at the other two volcanoes.