Development of prediction method of volcanic eruption using the synchronism between volcanic activities and the earth and ocean tides.

本研究旨在探讨火山活动与地球和海洋潮汐之间的同步机制，并评估利用同步预测火山爆发方法的可能性。为此，我们采取了两种方法：(1)从理论上评价火山活动与地壳应变（包括潮汐应变）同步的可能机制；(2)收集火山活动资料，分析火山活动的时间变化与潮汐力和应变变化的关系。在方法(1)中，我们发现当应变的方位和量使岩浆储层的体积变化最大时，地壳应变可以产生火山活动。模型计算在理论上支持了这一观点，对伊豆大岛火山观测到的火山地震与火山附近构造地震相关的地壳应变之间关系的分析也证明了这一观点。我们还发现，火山岩浆喷发的累积模式可以记录由地壳应变变化引发的喷发历史。方法(2)系统收集了伊豆大岛火山1300年来喷发的历史文献，发现历史上喷发的起始与两周潮汐分量同步。分析了1991年云禅火山火山喷发与潮汐应变的关系，发现在1991年5月24日至6月12日，火山活动达到最高峰，潮汐应变具有东西向或上下挤压分量，引发了大规模的火山碎屑流。综合上述结果，澄清了利用同步性预测火山喷发方法存在的问题。

This study aimed to examine mechanisms of the synchronism between volcanic activities and the earth and ocean tides, and to evaluate the possibility of a method for predicting volcanic eruptions using the synchronism. For this purpose, we made two approaches : (1) theoretical evaluation of possible mechanisms of the synchronism between volcanic activities and crustal strain including tidal strain ; (2) collection of data of volcanic activities and analysis of the relationships between temporal variations of volcanic activities and variations of tidal force and strain. As for approach (1) , we revealed that crustal strain can generate volcanic activities when the azimuth and amount of the strain give maximum volumetric change to a magma reservoir. This idea is theoretically supported by model calculation and also is demonstrated by the analysis of the relationships between the volcanic earthquakes observed in Izu Oshima Volcano and the crustal strain associated with tectonic earthquakes near the volcano. We also revealed that a cumulative pattern of magma discharge from a volcano can record a history of the eruptions triggered by changes in crustal strain. Concerning approach (2) , we systematically collected historical documents describing the eruptions occurred for the past 1300 years in Izu Oshima Volcano, and revealed that the initiation of the historical eruptions were synchronized with the fortnightly component of the earth tide. We also analyzed the relationships between the tidal strain and the 1991 pyroclastic eruptions in Unzen Volcano and revealed that the tidal strain with an E-W or up-down compressional component triggered large pyroclastic flows during May 24-June 12,1991, when the volcanic activities attained a maximum level. Synthesizing all these results, we clarified problems of the method for predicting volcanic eruptions using the synchronism.