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An experimental study of volcanic tremor driven by magma wagging Dehghanniri, Vahid


Pre-eruptive seismic tremor with similar spectral properties is observed at active volcanoes with widely ranging conduit geometries and structures. Accordingly, the ``magma wagging'' model introduced by Jellinek & Bercovici[21] and extended by Bercovici et al.[6] hypothesizes an underlying mechanism that is only weakly-sensitive to volcano architecture: Within active volcanic conduits, the flow of gas through a permeable foamy annulus of gas bubbles excites and maintains an oscillation of a central magma column through a well-known Bernoulli effect. In this thesis, we carry out a critical experimental test of this underlying mechanism for excitation. We explore the response of analog columns with prescribed elastic and linear damping properties to forced annular airflows. From high-speed video measurements of linear and orbital displacements and time series of accelerometer measurements we characterize and understand the excitation, evolution, and steady-state oscillating behaviors of analog magma columns over a broad range of conditions. We identify three distinct classes of wagging: i. rotational modes which confirms predictions for whirling modes by Liao et al.[26]; as well as newly-identified ii. mixed-mode; and iii. chaotic modes. We find that rotational modes are favored for symmetric, and high intensity forcing. Mixed-mode responses are favored for a symmetric and intermediate intensity forcing. Chaotic modes occur in asymmetric or low intensity forcing. To confirm and better understand our laboratory results, and also extend them to conditions beyond what is possible in the laboratory, we carry out complementary two-dimensional simulations of our analog experiments. Our combined experimental and numerical results can be applied to make qualitative predictions for natural testable in future studies of pre- and syn-eruptive volcano seismicity. Long before an eruptive phase, the total gas flux is low and we expect magma wagging in a chaotic mode, independent of the spatial distribution of the gas flux. At a pre-eruptive state signaled by gas flux increasing, if the distribution of gas flux is approximately symmetric, we expect a transition to mixed and possibly rotational wagging modes. During an eruption, fragmentation and explosions within the foamy annulus can cause spatial heterogeneity in permeability resulting in non-uniform gas flux that favors chaotic wagging behavior.

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