諏訪之瀬島における空振観測から考える噴火ハザード

Abstract

Eruption hazard caused by volcanic bombs and volcanic ash at Suwanosejima volcano is examined using infrasound. The explosive eruption, which ejected volcanic bombs to distances around 1 km, occurred during high explosivity with infrasound larger than 200 Pa. The high explosivity accompanied only a low plume (<1500 m). Smaller (<100 Pa) infrasound pulses are successively recorded prior to the high explosivity. The frequent occurrence of infrasound pulses is classified into Type L: 1-9 days duration and minute order time interval and Type S: 1-4 hours duration and second order time interval, and it is regarded as precursor of increase in hazard due to volcanic bombs. Eruptions with plumes higher than 3000 m sometimes occurred. The amplitudes of infrasound onset pulse were much smaller than the explosive eruptions, but tremor-like vibration continued for a longer time (several minutes). Suwanosejima volcano repeats hazardous states of volcanic bombs and volcanic ash alternatively.