STUDY ON SOURCE PROCESS OF VERY LONG PERIOD SEISMIC EVENTS

我研究了最近在许多活火山上观察到的非常长周期地震事件的震源过程。我在日本东北部的岩手山和万代部署了四到六个宽带地震仪和一个低频麦克风。这两座火山最近表现出明显的地壳形变和/或微震活动，因此我成功地记录了这两座火山很长一段时间的地震事件的高质量数据。通过对观测波形特征的研究，确定了激发出约10 S长周期信号的震源的矩张量的位置和时空函数。结果表明：(1)岩手山事件的震源位于指示岩浆流体存在的高波速比区域；(2)两座火山观测到的极长周期事件的震源机制都表现为相互膨胀和收缩运动。对于VER…，这种源机制也有报道其他活火山，如阿苏山，发生了更多长周期地震事件。因此，我的结论是，极长周期事件的震源机制包括显著的体积变化。为了解释观测到的震源机制和振荡的主要周期，我提出了一个震源模型，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。用有限差分法进行的运动计算表明，由窄通道连接的两个洞室可以作为充气和放气震源。10 S的长周期波很容易通过缩小通道而激发出来，没有不切实际的大岩浆室和/或声速极低的岩浆流体。根据我对观测波形的分析和理论考虑，我得出结论：很长周期的地震事件是由不规则形状的火山岩墙或管道中的岩浆流体迁移所激发的。较少

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