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Elucidation of the Pressure Source Beneath the Active Crater of Aso Volcano

阿苏火山活火山口下方压力源的解析

基本信息

批准号：
09304043
负责人：
KAWAKATSU Hitoshi
金额：
$ 22.08万
依托单位：
University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-09304043/
关键词：
Aso volcano volcanic tremor volcanic eruption hydrothermal reaction broadband seismometer 火山噴火

项目摘要

Seismic wavefields observed near active volcanoes contain plenty of information about volcanic activities in the frequency range between 0.01 Hz and 1.0 Hz, where, until recently, conventional geophysical monitoring of volcanoes had riot paid much attention except for a few rare cases. The physics operating in a volcano is extremely complex; in different frequency bands, different physical processes dominate the wavefield, which makes broadband seismic observations essential to unravel the physical processes of volcanic activity using seismic data.Deployment of a dense network of broadband seismometers in Aso revealed a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15s, even when there is no apparent surface activity at the crater. When phreatic eruptions are observed, broadband seismograms elucidate vividly the whole eruption process: gradual pressurization and long-period (about 20s) pulsations of the reservoir during the 100-200s before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The observed spatial variation of the signal amplitudes reveals that the source of 15s long period tremors (LPTs) consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The extension of the buried crack plane meets the crater chain, including the active fumarole at the surface, suggesting that the crack has played an important role in transporting steam (gasses) and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

活火山附近观测到的地震波场包含大量 0.01 Hz 至 1.0 Hz 频率范围内火山活动的信息，直到最近，除了少数罕见情况外，传统的火山地球物理监测一直受到广泛关注。火山中的物理原理极其复杂。在不同的频段，不同的物理过程在波场中占主导地位，这使得宽带地震观测对于利用地震数据揭示火山活动的物理过程至关重要。在阿苏部署的宽带地震仪密集网络揭示了火山口下方1至1.5公里处的一个热液储层，即使火山口没有明显的表面活动，该热液储层也会以长达15秒的周期持续共振。当观测到潜水喷发时，宽带地震图生动地阐明了整个喷发过程：在喷发开始前的100-200秒内，储层逐渐增压和长周期（约20秒）脉动，随后与喷发流同时发生的储层逐渐收缩。观察到的信号幅度的空间变化表明，15秒长周期震颤（LPT）的来源由倾斜拉伸裂纹的各向同性膨胀（收缩）和膨胀（紧缩）组成，其走向几乎平行于陨石坑链。埋藏裂缝平面的延伸与火山口链相交，包括表面的活跃喷气孔，表明裂缝在将蒸汽（气体）和/或熔岩从下方输送到火山口方面发挥了重要作用。这项工作还展示了宽带地震仪作为大地测量仪器的强大用途，以约束活火山的地下结构。