STUDY ON SOURCE PROCESS OF VERY LONG PERIOD SEISMIC EVENTS

我研究了最近在许多活火山观测到的非常长周期的地震事件的震源过程。我在日本东北部的岩手山和万代山周围部署了四到六个宽带地震仪和一个低频麦克风。这两座火山最近显示出显著的地壳变形和/或微地震活动，因此我成功地记录了这两座火山的长周期地震事件的高质量数据。通过对观测波形特征的分析，确定了激发10 s左右长周期信号的震源矩张量的位置和时空函数。结果表明：（1）岩手山事件的震源位于一个具有高Vp/Vs比值的区域，表明存在岩浆流体;（2）在两座火山观测到的甚长周期事件的震源机制表明，两座火山都有相互的膨胀和收缩运动。这种源机制也报告为ver ...更多信息 在其他活火山如阿索山的长期地震事件。因此，我的结论是，非常长的周期事件的源机制包括显着的体积变化。为了解释观测到的震源机制和振荡的主导周期，我提出了一个震源模型，其中不规则岩脉或火山管道中的可压缩流体运动由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，由一个狭窄的通道连接的两个腔室可以作为一个膨胀和收缩震源。10 s的长周期波很容易通过缩小通道而激发，而没有不切实际的大岩浆房和/或具有极低声速的岩浆流体。根据对观测波形的分析和理论考虑，我认为超长周期地震事件是由岩浆流体在不规则形状的火山岩脉或管道中的迁移激发的。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