Source mechanism of a very-long-period event at Mt Ontake, central Japan: Response of a hydrothermal system to magma intrusion beneath the summit
Source mechanism of a very-long-period event at Mt Ontake, central Japan: Response of a hydrothermal system to magma intrusion beneath the summit
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DOI：
10.1016/j.jvolgeores.2009.09.006
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发表时间：
2009-11-10
影响因子：
2.9
通讯作者：
Obara, Kazushige
中科院分区：
文献类型：
--
作者：
Nakamichi, Haruhisa;Kumagai, Hiroyuki;Obara, Kazushige
2007年1月25日发生在日本中部翁塔奇山下的一次甚长周期火山事件被附近的宽带地震仪以及日本全国Hi-net和F-net地震网络记录下来。的VLP波形显示脉冲状的形状，持续时间约为1分钟。VLP事件伴随着长周期（LP）事件的突发序列。我们进行了波形反演，以确定VLP事件的震源机制和位置。我们的分析表明，体积变化的倾斜裂缝（地震矩为10（14）Nm）的VLP源，这是位于海拔0.6公里的山顶下的山Ontake和以上的堤坝从GPS数据确定。VLP事件的源-时间函数呈脉冲状三角形，表明源处裂纹先膨胀后收缩。我们对波形反演的结果解释如下。从侵入的岩浆中释放出的气体加热了翁塔克山顶下的含水层系统。系统中的热液裂缝中的不稳定过热水的破裂引起水的蒸发和相关的裂缝膨胀，随后从裂缝中排出蒸汽，导致其坍塌。这些过程导致了VLP事件，这可能是2007年3月下旬发生的小型潜水喷发的前兆。(c)2009爱思唯尔有限公司版权所有。
A very-long-period (VLP) volcanic event, which occurred on 25 January 2007 beneath Mt Ontake, central Japan, was recorded by nearby broadband seismometers as well as the Japanese nationwide Hi-net and F-net seismic networks. The VLP waveforms show a pulse-like shape with a duration of approximately 1 min. The VLP event was accompanied by a burst sequence of long-period (LP) events. We carried out waveform inversion to determine the source mechanism and location of the VLP event. Our analysis indicates volumetric changes in an inclined crack (seismic moment of 10(14) Nm) at the VLP source, which is located at 0.6 km above sea level beneath the summit of Mt Ontake and above dikes determined from GPS data. The source-time functions of the VLP event showed a pulse-like triangular shape, indicating inflation followed by deflation of the crack at the source. We interpreted the result of the waveform inversion as follows. Gases released from the intruded magma heated an aquifer system beneath the summit of Mt Ontake. A break of unstable superheated water in a hydrothermal crack in the system caused vaporization of water and associated crack inflation, which was followed by discharge of steam from the crack resulting in its collapse. These processes caused the VLP event, which may have been a precursor of a small phreatic eruption that occurred in late March 2007. (c) 2009 Elsevier B.V. All rights reserved.