Fundamental Research on Application of Magma Exploration Method

了解火山喷发机制，提高火山预测预报的可靠性，离不开对地下结构的了解。我们研制了一套体积小、重量轻、性能高的数据测井系统，并应用该系统对万代火山的地下结构进行了探测，由10所国立大学和来自上述机构的JMA 76名成员组成的万代火山地震勘探课题组参加了本次实验。我们在距离火山顶峰15公里的范围内部署了292个临时无人值守地震台站，并观测到从1997年10月2日102时至2时12分每隔10分钟发出8次爆炸的地震波。利用约2300个初至时间资料反演了三维P波速度结构，主要结果如下：(1)万代火山及其邻近地区的地下速度具有较大的水平速度差。在海拔0.5公里处，努马诺泰拉火山口以下的P波速度为5.3公里/S，而在巴纳代火山南侧与伊纳瓦夏湖之间的地区，P波速度下降到2-2.4公里/S。(2)在Numano-taira斜面正下方探测到垂直延伸的高速窄体。该岩体具有较强的局部化特征，类似于侵入岩墙状，水平范围小于2公里。(3)该侵入体被马蹄形火山口墙的凸起所包围，这些凸起与过去的巨大喷发有关。这表明，来自深处的类似堤坝的入侵是过去破坏性爆炸的来源。

The knowledge of subsurface structure is indispensable to understand an eruptive mechanism of volcano and to increase reliability of prediction of volcanic prediction We have developed the data logging system with small-size, lightweight and high performance and, in the present study, we applied this system to explore the subsurface structure at Bandai volcano, The research group for seismic exploration of Bandai volcano consisted of 10 national universities and JMA 76 members from the above institutions participated in the present experiment. We deployed 292 temporary unattended seismic stations in and around the volcano within 15 km from the summit and observed seismic waves loin 8 explosions that were fired at 10 minutes interval from 102 till 2 : 12 on October 2,1997. We determined three-dimensional P-wave velocity structure inverted from about 2300 first arrival time data The present results are summarized as follows ; (1) the subsurface velocity beneath Bandai volcano and its neighborhood has a large horizontal velocity contrast. At the elevation of 0.5 km above the sea level, P wave velocity is 5.3 km/s beneath the Numano-taira crater, while it decreases to 2-2.4 km/s in the region between the southern flank of Banadai volcano and lake Inawashire. (2) Narrow high velocity body extending vertically is detected just under the Numano-taira miter. This body is much localized feature, similar to intrusive dyke-like and has horizontal extent less than 2 km. (3) This intrusive body is encircled by the convexities of horseshoe shaped caldera wall associated with the past gigantic eruptions. This suggests that the dyke-like intrusion from depths had worked as source of the past destructive explosions.