桜島火山におけるハーモニック微動の時空間特性について

Abstract

桜島火山において発生するハーモニック微動について、群発性B型地震から推移して発生するもの（HTB）と爆発的噴火の直後に発生するもの（HTE）に分類した。両者のスペクトルは基本周波数とその整数倍にピークを持つ特徴がある。1990年7月20日に発生したHTBは基本周波数が1.4-1.7 Hzの範囲で一定であり，9つの高次モードが見られた。2002年10月11日11時15分の爆発的噴火の3分後に始まったHTEは，微動の初期の基本周波数は0.8 Hzであったが，時間が経つにつれ高周波側にシフトし，3.7 Hzまで増加した。HTEの高次モードも基本周波数の整数倍を保ちつつ高周波側にシフトした。両者の振動軌跡はRayleigh波が卓越しており，その回転方向は地震計を設置している深度が最も深いHAR観測点でprograde，他の観測点ではretrogradeであった。火口の南もしくは西の観測点では波形にLove波が見られた。HTB，HTEともに表面波の振動パターンは類似していた。

Eruptive activity with Vulcanian type has been continued at the summit crater of Sakurajima since 1955. Harmonic tremors have been observed several hours after B-type earthquake swarms or immediately after explosive eruptions. Here, we call the first one HTB (Harmonic Tremor following B-type) and second one HTE (Harmonic Tremor after an Eruption). Particle motions and spectra of HTB and HTE were analyzed. Spectra both of HTB and HTE have peaks at fundamental frequencies and higher frequencies of multiple integers of them. Peak frequencies of HTB were kept in a certain range. In contrast, those of HTE gradually increased. Spectra of HTB, which occurred on July 20, 1990, have fundamental frequencies ranging 1.4 Hz - 1.7 Hz stably and at least 9 peaks of higher modes. While the fundamental frequencies of HTE, which occurred 3 minutes after an explosive eruption at 11h15m on October 11, 2002, gradually increased from 0.8 Hz to 3.7 Hz. The peak frequencies of higher modes of HTE were also gradually increased according to shift of the fundamental mode toward higher frequency. Particle motion analysis showed that Rayleigh waves are dominant from prograde elliptical motion at the deepest borehole station HAR and retrograde motions at the other stations. Love waves were sometimes mixed at the stations south and west of the craters. The distribution patterns of Rayleigh and Love waves of HTB are similar to those of HTE.