STUDY ON SOURCE PROCESS OF VERY LONG PERIOD SEISMIC EVENTS

我研究了最近在许多活火山上观测到的长周期地震事件的源过程。我在日本东北部岩手山和万代附近部署了四到六个宽带地震仪和一个低频麦克风。这两座火山最近出现了明显的地壳变形和/或微地震活动，因此我成功地记录了两座火山非常长周期地震事件的高质量数据。考察了观测到的波形特征，确定了激发约10s长周期信号的震源矩张量的位置和时空函数。结果表明：(1)岩手山事件源位于Vp/Vs比值较高的岩浆流体存在区；(2)两个火山观测到的极长周期事件的源机制显示出相互的膨胀和收缩运动。这种震源机制在其他活火山如阿苏山的长周期地震事件中也有报道。因此，我得出结论，很长周期事件的源机制包括显著的体积变化。为了解释观测到的源机制和振荡的主要周期，我提出了一个源模型，其中不规则岩脉或火山管中的可压缩流体运动用一维流体动力学的基本方程表示。用有限差分法计算的运动表明，由狭窄通道连接的两个腔室可以作为膨胀和收缩震源。通过缩小通道，可以很容易地激发出10s的长周期波，而没有不切实际的大岩浆房和/或声速极低的岩浆流体。根据我对观测到的波形的分析和理论考虑，我得出结论，长周期地震事件是由不规则形状的火山岩脉或管道中的岩浆流体的迁移引起的。少

I studied source process of very long period seismic events that have recently been observed at many active volcanoes. I deployed four to six broad band seismometers and a low-frequency microphone around Mounts Iwate and Bandai, northeastern Japan. The two volcanoes have recently shown significant crustal deformations and/or micro earthquake activities, so I successfully recorded high quality data of very long period seismic events of the two volcanoes. Examining the observed waveform characteristics, I determined the locations and space-time functions of moment tensors of the seismic source that excites long period signals of about 10 s. The results show that : (1) the sources for Mount Iwate events are located in a region having a high Vp/Vs ratio that indicates an existence of magmatic fluid ; (2) the source mechanisms of very long period events observed at both of the two volcanoes indicate mutual inflation and deflation motions. Such source mechanisms are also reported for the ver … More y long period seismic events at other active volcanoes such as Mount Aso. I, therefore, conclude that the source mechanisms of very long period events includes significant volumetric changes. To explain the observed source mechanisms and the dominant periods of the oscillations, I presented a source model in which compressible fluid motions in an irregular dike or volcanic pipe are expressed by the basic equations for a one-dimensional fluid dynamics. Calculations of the motions using a finite difference method indicate that two chambers connected by a narrow channel can act as an inflation and deflation seismic source. The long period waves of 10 s are easily excited by narrowing the channel without unrealistic large magma chambers and/or magmatic fluid having extremely low acoustic velocity. Based on my analyses of the observed waveforms and theoretical considerations, I conclude that the very long period seismic events are excited by a migration of magmatic fluid in volcanic dikes or pipes with an irregular shape. Less