爆発的噴火に伴う地震動・空気振動の比較研究-桜島火山と諏訪之瀬島火山の比較-

Abstract

Some characteristics of earthquakes and air-shocks are compared among three styles ofexplosive eruption, those are, explosion and successive minor eruption at Sakurajima, and ex-plosion at Suwanosejima. The following results are obtained.1) The maximum amplitudes of vertical component are 3-30 mkine in explosion earthquakesat Sakurajima, less than 13 mkine in eruption earthquakes at Suwanosejima and less than 7mkine in BL-type earthquakes at Sakurajima. The epicentral distances are around 3 km. Theamplitudes of compressional phase of air-shocks are 0.2-5 mb in explosions at Sakurajima, lessthan 2 mb in explosions at Suwanosejima and less than 0.1 mb in successive minor eruptions atSakurajima. The infrasonic microphones are 6 km and 3.3 km apart from the craters at Sakura-jima and Suwanosejima, respectively.2) Comparing the three styles of eruptions in the same amplitude-range of eruption earthquakes(5-7 mkine), the amplitudes of air-shock by explosions at Suwanosejima are larger than those bysuccessive minor eruptions at Sakurajima, but are smaller than those by explosions at Sakuraji-ma. This result suggests that the ratio of seismic energy to infrasonic one changes with theseeruption styles.3) The intensity of the explosions in the active period during 1988-1989 at Mt. Tokachi wasas large as that of the strongest explosions at Suwanosejima in 1989. However, the volume ofpyroclastic materials, the time interval of eruption and the duration of compressional phase ofair-shock at Mt. Tokachi are closer to those at Sakurajima.