Study of the caldera collapse process and magma plumbing system of Miyakejima volcano

2000年宫城岛火山喷发的方式与岛上居民近百年来所经历的有很大不同。由于构造原因，岩浆在宫城岛外的横向迁移是被动发生的，导致了山顶的下沉，与2000年夏天山顶的爆炸性喷发有关。2000年的喷发有两种年轻物质：6月27日喷发的玄武质玻璃安山岩和8月18日喷发的黄褐色玄武岩作为火山炸弹喷发。从2000年7月8日到8月18日，每天观测到几次长周期的地震脉冲(VLP脉冲)，与阶梯式倾斜增加同步。脉冲宽度为40到50秒，无论其幅度如何，几乎都是恒定的。8月18日峰顶喷发后，极低频脉冲的出现完全停止。这些信号的波形分析显示…此外，这些脉冲的源机制以10m的大体积膨胀为特征。已经提出了几种模型。一种是火山导管中的活塞间歇性下沉。另一种是地下蒸汽膨胀模型。对VLP脉冲前震群的分析也揭示了活塞式崩塌的前兆细节。我们在5-9月期间进行了密集的地震观测，发现地震波在穿越岛屿南部时出现了异常衰减。根据对峰顶活动脱气期间的GPS数据的分析，我们可以提出一个合理的模型，即岩浆储集层的体积随着岩浆释放的火山气组分的数量而减少。为了探测峰顶管道周围的岩浆运动和热液系统的时间变化，我们进行了电磁和连续的绝对重力观测，观测到了2001年10月至11月期间可能由管道内岩浆上升引起的系统变化。较少

The manner of the Miyakejima 2000 eruption was largely different from what islanders had experienced these hundred years. Lateral migration of magma outside Miyakejima, which occurred passively due to tectonic reasons, generated the summit subsidence, being associated with explosive eruptions from the summit in the summer of 2000. There are two kinds of juvenile materials in the 2000 eruption; aphyric glassy basaltic andesite erupted in the 27 June spatter and phyric basalt erupted as volcanic bomb of the 18 August eruption. It is likely that juvenile materials was replaced with from basaltic andesite to basalt in the middle August.Very long period seismic pulses (VLP pulses) were observed a few times a day from July 8 to Aug 18, 2000, synchronized with the step-like tilt increase. The pulse width is 40 to 50 sec and is almost constant regardless their amplitude. The occurrence of VLP pulses completely ceased after the summit eruption on Aug 18. Waveform analyzes of these signals show … More that the source mechanism of these pulses is characterized by large volume expansion of 10 m . Several models have been proposed. One is an intermittent subsidence of a piston in the volcanic conduit. Another is an underground vapor expansion model. An analysis of the seismic swarms prior to the VLP pulses also revealed the details of precursors to the piston-like collapse of the summit.We conducted a dense seismic observation during the period of May to September, and could found that the seismic waves were anomalously attenuated when crossing beneath the south part of the island.Based on analyzes of GPS data during the period of active degassing from the summit, we could propose a plausible model that the volume of the magma reservoir decreased corresponding to the amount of the volcanic gas components released from the magma in the reservoir.In order to detect the magma movements and temporal changes of hydrothermal system around the summit conduit, we have conducted electromagnetic and continuous absolute gravity observation, and observed systematic variations possibly caused by a temporal up rise of magma in the conduit in October to November 2001. Less