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

活火山附近的地震波场在0.01 ~ 1.0Hz的频率范围内包含了丰富的火山活动信息，而常规的火山地球物理监测在这一范围内，除极少数情况外，一直没有引起人们的重视。在火山中运作的物理学是极其复杂的;在不同的频带中，不同的物理过程支配波场，在阿索部署了密集的宽带地震仪网络，揭示了火山口下方1至1.5公里处的热液储层，该储层持续共振，15秒，即使在火山口没有明显的表面活动。当观察到潜水喷发时，宽频带地震记录生动地阐明了整个喷发过程：在泄流开始前的100- 200 s期间，水库逐渐增压和长周期（约20 s）脉动，随后随着泄流，水库逐渐放气。所观察到的信号幅度的空间变化表明，15秒长周期震颤（LPT）的来源包括一个各向同性的膨胀（收缩）和膨胀（放气）的倾斜拉伸裂纹与罢工几乎平行的陨石坑链。埋藏的裂缝面的延伸与火山口链相遇，包括表面的活动火山孔，这表明裂缝在将蒸汽（气体）和/或熔岩从下面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，以限制活火山的地下结构。

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.