Abstract
To determine the magnitude of explosion earthquakes and B-type earthquakes at Sakurajima Volcano, where the long-period component often dominates, we installed a long-period seismograph at Kagoshima Local Meteorological Observatory Station E. We derived the magnitude formula for the vertical component of the long-period seismograph, MELV=logAV+1.37logΔ+0.69, from data on techtonic earthquakes whose focal parameters were determined by the Fukuoka District Meteorological Observatory (FDMO). This formula has no systematic discrepancy from the magnitude determined by FDMO, and is applicable to volcanic earthquakes up to M4.0. Applying this formula, we found that MELV of 171 explosion earthquakes in 1996 were 1.3-3.1 and those of 120 B-type earthquakes in January 1996 were 0.7-1.8. We similarly derived a magnitude formula for the vertical component of a short-period seismograph at Station E and calculated explosion earthquake and B-type earthquake magnitude, and compared them with MELV. We found that explosion earthquake MELV averages about 0.21 grater than the short-period magnitude, and about 0.14 grater for B-type earthquakes. This is because the short-period seismograph cannot record original explosion earthquake and B-type earthquake amplitudes where the long-period component dominates.
