An experimental study of volcanic tremor driven by magma wagging
An experimental study of volcanic tremor driven by magma wagging
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岩浆摆动驱动的火山震颤实验研究
DOI：
10.1093/gji/ggab404
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发表时间：
2021
影响因子：
2.8
通讯作者：
Jellinek, A. Mark
中科院分区：
文献类型：
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作者：
Dehghanniri, Vahid;Jellinek, A. Mark
0.5-7赫兹的喷发前火山震颤（PVT）的持续发作在活成层火山中很常见。与岩浆运动过程相关的可靠联系，从而使PVT的属性作为诊断喷发前兆成为可能。PVT的一个具有挑战性的特点是，信号的一般频谱和振幅特性类似地演变，独立于大多数物理模型所依赖的广泛变化的火山结构和管道几何形状。Jellinek & Bercovici（2011）提出并由Bercoviciet al.（2013），Liao et al. Liao & Bercovici（2018）取得了进展，因为它依赖于对火山结构只有弱敏感性的岩浆动力学：气体通过气泡的可渗透泡沫环的流动激发，调节和维持了中央岩浆柱在喷发管道中上升的摇摆振荡。“岩浆摇摆”和由此产生的PVT是通过与环空气流速度的方位角变化相关的“伯努利模式”的能量传递来驱动的。与观测结果一致，PVT的光谱和振幅特性的预测，以发展喷发前的环的宽度随着气体流量的增加而减少。为了确认这一关键的伯努利摇摆能量转移，我们使用广泛的实验和限制的数值模拟粘弹性列环形空气流激发摇摆振荡。我们还探索了摇摆的空间和时间特征对不对称环形气流的敏感性，这些气流在现有的岩浆摇摆模型中是难以解决的，并且预计会在自然界中随着环空渗透率的空间变化而发生。从高分辨率的时间序列的线性和轨道位移的模拟列的顶部和时间序列的轴向偏转和加速度的列中心线，我们的特点的激励，演变，和稳态振荡前所未有的细节在广泛的条件。我们表明，伯努利模式对应的时间尺度的轴向弹性弯曲应力的建立，在响应于压力变化有关的空气流量的高度列。我们确定了三种不同的摇摆模式：（i）旋转（参见。Liao et al.混合模式和混沌模式。旋转模式有利于对称的，高强度的强迫和最大限度地提供机械能的基本岩浆摇摆模式。混合模式振荡制度有利于对称，中等强度的强迫。混沌模式，涉及最低效率的能量输送到基本模式，发生不对称的强迫和施加的气流强度低。数值模拟还表明，在强迫频率与自由振荡的自然模式相当的情况下，由外围气流传递的功率通常集中在最低频率的基本模式处，并且在空气压力和柱弹性力相当的较高频率的自然模式之间传播。我们结合实验和数值计算的结果作出定性预测的性质的火山震颤及其在地震或次声阵列在自然事件的PVT和同喷发火山震颤的实地研究是可测试的演化。
Protracted episodes of 0.5–7 Hz pre-eruptive volcanic tremor (PVT) are common at active stratovolcanoes. Reliable links to processes related to magma movement consequently enable a potential to use properties of PVT as diagnostic eruptive precursors. A challenging feature of PVT is that generic spectral and amplitude properties of the signal evolve similarly, independent of widely varying volcano structures and conduit geometries on which most physical models rely. The ‘magma wagging’ model introduced in Jellinek & Bercovici (2011) and extended by Bercoviciet al. (2013), Liaoet al. and Liao & Bercovici (2018) makes progress because it depends on magma dynamics that are only weakly sensitive to volcano architecture: The flow of gas through a permeable foamy annulus of gas bubbles excites, modulates and maintains a wagging oscillation of a central magma column rising in an erupting conduit. ‘Magma wagging’ and resulting PVT are driven through an energy transfer from a ‘Bernoulli mode’ related to azimuthal variations in annular gas flow speeds. Consistent with observations, spectral and amplitude properties of PVT are predicted to evolve before an eruption as the width of the annulus decreases with increased gas fluxes. To confirm this critical Bernoulli-to-wagging energy transfer we use extensive experiments and restricted numerical simulations on wagging oscillations excited on analogue viscoelastic columns by annular air flows. We also explore sensitivities of the spatial and temporal characters of wagging to asymmetric annular air flows that are intractable in the existing magma wagging model and expected to occur in nature with spatial variations in annulus permeability. From high-resolution time-series of linear and orbital displacements of analogue column tops and time-series of axial deflections and accelerations of the column centre line, we characterize the excitation, evolution, and steady-state oscillations in unprecedented detail over a broad range of conditions. We show that the Bernoulli mode corresponds to the timescale for the buildup of axial elastic bending stresses in response to pressure variations related to air flows over the heights of columns. We identify three distinct wagging modes: (i) rotational (cf. Liaoet al. ); (ii) mixed-mode and (iii) chaotic. Rotational modes are favoured for symmetric, high intensity forcing and a maximal delivery of mechanical energy to the fundamental magma wagging mode. Mixed-mode oscillations regimes are favoured for a symmetric, intermediate intensity forcing. Chaotic modes, involving the least efficient delivery of energy to the fundamental mode, occur for asymmetric forcing and where the intensity of imposed airflow is low. Numerical simulations also show that where forcing frequencies are comparable to a natural mode of free oscillation, power delivered by peripheral air flows is concentrated at the lowest frequency fundamental mode generally and spread among higher frequency natural modes where air pressure and column elastic forces are comparable. Our combined experimental and numerical results make qualitative predictions for the evolution of the character of volcanic tremor and its expression in seismic or infrasound arrays during natural events that is testable in field-based studies of PVT and syn-eruptive volcanic tremor.