Volcanic Activity in Japan Monitored by the Japan Meteorological Agency Using Detected Precursory Phenomena

Abstract

There are 76 potentially active volcanoes in Japan. The systematic volcanological work by the Japan Meteorological Agency (JMA) has been carried out since early 1960. The activity at 17 active volcanoes has been routinely monitored, and other volcanoes have been temporarily observed by mobile observation teams. During the period 1961–1985, 28 volcanoes erupted, and 17 volcanoes showed volcanic unrest such as earthquake swarms, fuming increase, etc. During these 25 years, therefore, there were an average of 5.5 eruptions a year, and 7.7 cases of volcanic unrest a year. According to the JMA observations up to now, the most clearly defined precursory event is the increase of seismicity before a volcanic eruption, but this pattern does not always occur. The next clearest precursor is the increase of intensity of fuming that has been observed at several volcanoes. It is difficult to predict smaller eruptions, as the clear precursor activity is lacking. In the cases of volcanoes which have been dormant for long periods, prior accumulation of relevant observational data is insufficient.