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Seismic tremors and magma wagging during explosive volcanism



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 deformation5,6,7. Volcanic tremor is thus of particular value for eruption forecasting6,8. 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 bubbles9,10. 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.

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Figure 1: Variation of tremor signal with eruption intensity.
Figure 2: Sketch of the magma wagging model of volcanic tremor.
Figure 3: Wagging frequency and shear strain rate in the annulus versus annulus thickness for several conduit radii.

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A.M.J. acknowledges support from the Canadian Institute for Advanced Research and NSERC. D.B. acknowledges support from the National Science Foundation. This manuscript has benefited from discussions and comments from S. McNutt and C. Michaut. We thank S. McNutt, J. Neuberg, M. Hagerty, K. I. Konstantinou and M. Ibs-von Seht for data and preliminary figures that were incorporated into various versions of Fig. 1.

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Authors and Affiliations



A.M.J. and D.B. conceived the physical model for magma wagging together. D.B developed the mathematical model. A.M.J. collected and analysed the seismic and acoustic data, developed the model for fragmentation applied in Fig. 3 and was the lead author for the paper.

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Correspondence to A. Mark Jellinek.

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Jellinek, A., Bercovici, D. Seismic tremors and magma wagging during explosive volcanism. Nature 470, 522–525 (2011).

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