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

这项研究旨在研究火山活动与地球和海洋潮汐之间同步的机制，并评估利用这种同步来预测火山喷发的方法的可能性。为此，我们采取了两种方法：(1)对火山活动与包括潮汐应变在内的地壳应变之间可能的同步机制进行了理论评估；(2)收集了火山活动的数据，分析了火山活动的时间变化与潮汐和应变变化的关系。对于方法(1)，我们揭示了当应变的方位和大小使岩浆储集层的体积变化最大时，地壳应变可以产生火山活动。这一观点得到了模型计算的理论支持，并通过分析伊豆大岛火山观测到的火山地震与火山附近构造地震的地壳应变之间的关系得到了证实。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。关于方法(2)，我们系统地收集了描述过去1300年来伊豆大岛火山喷发的历史文献，发现历史喷发的开始与潮汐的双周分量是同步的。我们还分析了潮汐应变与1991年温泉火山火山喷发的关系，发现具有东西向或上下挤压分量的潮汐应变在1991年5月24日至6月12日期间触发了大量的火山碎屑流，当时火山活动达到最大水平。综合所有这些结果，我们澄清了利用同步性预测火山喷发的方法存在的问题。

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 Concerning approach (2) , we systematically collected historical documents describing the eruptions occurred for the past 1300 years in Izu Oshima Volcano, and revealed that the initiation of the historical eruptions were synchronized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.