Seismic tremors and magma wagging during explosive volcanism
Seismic tremors and magma wagging during explosive volcanism
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DOI：
10.1038/nature09828
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发表时间：
2011-02-24
期刊：
影响因子：
64.8
通讯作者：
Bercovici, David
中科院分区：
文献类型：
--
作者：
Jellinek, A. Mark;Bercovici, David
火山震动是爆炸性喷发的普遍特征。这种振荡持续数分钟到数周，其特征是频率范围从0.5 Hz到7 Hz非常窄（参考文献1-4）。在大喷发之前，震颤可能会与气体流量增加和相关的地面变形（5-7）相一致。因此，火山震颤对于火山喷发预测具有特殊价值（6，8）。大多数火山震颤模型依赖于火山管道的几何形状、结构和组成以及喷发岩浆的气体含量的特定性质。由于不同火山的初始结构和岩浆管道系统的演化都不相同，因此不同火山的震颤特征如此一致令人惊讶。事实上，震颤特性的这种普遍性仍然是一个主要的谜。在这里，我们采用了当代的观点，即岩浆在管道中上升，作为一个圆柱形的塞子，周围是一个高度泡状的剪切气泡环（9，10）。我们证明，对于大多数地质相关的条件下，岩浆柱将振荡或“摇摆”对恢复“气弹簧”力的环在观察到的震颤频率。与以前的模型相比，岩浆摆动振荡对管道结构和几何形状相对不敏感，这解释了在世界各地观察到的震颤频率的窄带。此外，该模型预测，随着喷发的进行，最大频率和总信号频率带宽都将向上漂移，其性质取决于喷发的爆炸性，这是经常观察到的。
Volcanic tremor is a ubiquitous feature of explosive eruptions. This oscillation persists for minutes to weeks and is characterized by a remarkably narrow band of frequencies from about 0.5 Hz to 7 Hz (refs 1-4). Before major eruptions, tremor can occur in concert with increased gas flux and related ground deformation(5-7). Volcanic tremor is thus of particular value for eruption forecasting(6,8). Most models for volcanic tremor rely on specific properties of the geometry, structure and constitution of volcanic conduits as well as the gas content of the erupting magma. Because neither the initial structure nor the evolution of the magma-conduit system will be the same from one volcano to the next, it is surprising that tremor characteristics are so consistent among different volcanoes. Indeed, this universality of tremor properties remains a major enigma. Here we employ the contemporary view that silicic magma rises in the conduit as a columnar plug surrounded by a highly vesicular annulus of sheared bubbles(9,10). We demonstrate that, for most geologically relevant conditions, the magma column will oscillate or 'wag' against the restoring 'gas-spring' force of the annulus at observed tremor frequencies. In contrast to previous models, the magma-wagging oscillation is relatively insensitive to the conduit structure and geometry, which explains the narrow band of tremor frequencies observed around the world. Moreover, the model predicts that as an eruption proceeds there will be an upward drift in both the maximum frequency and the total signal frequency bandwidth, the nature of which depends on the explosivity of the eruption, as is often observed.