Volcanic tremors and magma wagging: gas flux interactions and forcing mechanism

Abstract

Volcanic tremor is an important precursor to explosive eruptions and is ubiquitous across most silicic volcanic systems. Oscillations can persist for days and occur in a remarkably narrow frequency band (i.e. 0.5-7 Hz). The recently proposed magma-wagging model of Jellinek & Bercovici provides a basic explanation for the emergence and frequency evolution of tremor that is consistent with observations of many active silicic and andesitic volcanic systems. This model builds on work suggesting that the magma column rising in the volcanic conduit is surrounded by a permeable vesicular annulus of sheared bubbles. The magma-wagging model stipulates that the magma column rattles within the spring like foam of the annulus, and predicts oscillations at the range of observed tremor frequencies for a wide variety of volcanic environments. However, the viscous resistance of the magma column attenuates the oscillations and thus a forcing mechanism is required. Here we provide further development of the magmawagging model and demonstrate that it implicitly has the requisite forcing to excite wagging behaviour. In particular, the extended model allows for gas flux through the annulus, which interacts with the wagging displacements and induces a Bernoulli effect that amplifies the oscillations. This effect leads to an instability involving growing oscillations at the lower end of the tremor frequency spectrum, and that drives the system against viscous damping of the wagging magma column. The fully non-linear model displays tremor oscillations associated with pulses in gas flux, analogous to observations of audible 'chugging'. These oscillations also occur in clusters or envelopes that are consistent with observations of sporadic tremor envelopes. The wagging model further accurately predicts that seismic signals on opposite sides of a volcano are out of phase by approximately half a wagging or tremor period. Finally, peaks in gas flux occur at the end of the growing instability several tens of seconds after the largest tremors, which is consistent with observations of a 30-to 50-s lag between major tremor activity and maximum gas release. The extended magma-wagging model, thus, predicts tremor frequency and its evolution before and during an eruption, as well as a driving mechanism to keep the tremor excited for long periods.

Origin of Shallow Volcanic Tremor: The Dynamics of Gas Pockets Trapped Beneath Thin Permeable Media

“…Previous studies aimed to explain shallow volcanic tremor through frictional faulting (Dmitrieva et al, ; Hotovec et al, ) and fluid‐elastic processes (e.g., nonlinear oscillations of volcanic conduits and cracks during magma ascent (Balmforth et al, ; Julian, ; Lipovsky & Dunham, ), forced coalescence of bubbles (Ripepe & Gordeev, ), gas‐liquid two‐phase flow (Fujita et al, ; Lane et al, ), acoustic resonance of fluid‐filled crack/cavities (Chouet, ; Chouet, ), acoustic resonance of magmatic foams attached to the conduit walls (Bercovici et al, ; Jellinek & Bercovici, ), and the formation of turbulent eddies behind obstacles (Hellweg, )). Although we do not reject the possibility that different mechanisms may act simultaneously, our results suggest that the accumulation of gases beneath permeable caps generates shallow volcanic tremor; this is consistent with a localized seismic source and supports the idea of cap‐controlled tremor invoked by Hellweg (), Johnson and Lees (), and Valade et al ().…”

“…Several mechanisms have been proposed to explain volcanic tremor, including fluid‐elastic resonance (Balmforth et al, ; Bercovici et al, ; Chouet, ; Chouet, ; Fujita et al, ; Hellweg, ; Jellinek & Bercovici, ; Johnson & Lees, ; Julian, ; Lane et al, ; Lipovsky & Dunham, ; Ripepe & Gordeev, ) and frictional processes (Dmitrieva et al, ; Hotovec et al, ). These mechanisms are able to explain some tremor features.…”

Coupling Between Magmatic Degassing and Volcanic Tremor in Basaltic Volcanism

“…This behavior, which has also been observed on long-time scales by Leonardi et al, 2000b, has been interpreted as due to increasing pressurization of the volcano's shallow feeder system, which simultaneously increases tremor but with a lag time before gas release at the onset of eruptive activity (e.g., Young et al, 2003;Nadeau et al, 2011). A further process might rely on the turbulent magma-flow rate in the upper conduit triggered by gas-slug dynamics during ongoing eruptive events (e.g., Parfitt, 2004) coupled with instability of magma column (Bercovici et al, 2013).…”

An approach to source characterization of tremor signals associated with eruptions and lahars

“…Harmonic tremor signals can be generated by repetitive triggering of a fluid-filled cavity (e.g., Aki et al 1977;Chouet 1986), self-excited oscillations associated with a fluid flow in a volcanic conduit (Julian 1994), or magma column oscillations (Jellinek and Bercovici 2011;Bercovici et al 2013). Our results may be explained if we consider that the inharmonic tremor signal is directly related to mass transport associated with magma eruptions and that the harmonic tremor is triggered by a gas flow without tephra fallout.…”