Dynamics of seismogenic volcanic extrusion at Mount St Helens in 2004–05

Abstract

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.

Cyclic ground tilt associated with the 2004–2008 eruption of Mount St. Helens

“…As noted by Iverson et al [2006], the 2004-2008 eruption of Mount St. Helens also offers an exceptional opportunity to link shallow volcanic seismicity with extrusion dynamics. However, the bulk of such analysis is beyond the scope of this paper, and here we summarize only key observations with the caveat that some of these observations may be superseded by future work.…”

“…Hybrid earthquakes were interpreted by Harrington and Brodsky [2007] as brittle failure events involving low rupture velocities and strong path effects. Iverson et al [2006] modeled the drumbeat events as caused by repeating slip on the margin of the ascending magma plug. Waite et al [2008] however, analyzed long-period seismic events recorded during the summer of 2005, and concluded that these events were inconsistent with a shear-faulting source and suggested instead the repetitive opening and closing of a steam-or fluid-filled crack or cracks located beneath the growing lava dome.…”

“…Examples include Mount St. Helens in the 1980s, where tiltmeters were used to predict effusive eruptions [Dzurisin et al, 1983], and Soufriere Hills Volcano (Montserrat), where tiltmeters revealed remarkable cyclic eruptive activity correlated with seismicity, gas emissions, explosions, and pyroclastic flows [Voight et al, 1998;Watson et al, 2000] and helped place constraints on source processes and boundary conditions on the conduit walls [Green et al, 2006;Widiwijayanti et al, 2005;Voight et al, 1999;Lensky et al, 2008]. Cyclic or repeating behavior on a variety of timescales has also been observed with different types of instruments during other silicic eruptions including Mount St. Helens during the 2004-2008 eruption [Iverson et al, 2006], Pinatubo [Mori et al, 1996], Santiaguito [Bluth and Rose, 2004], and Unzen [Nakada et al, 1999].…”

“…Lensky et al [2008] include the effects of gas loss as well as nonlinear friction on the margins of the plug, and ascribe tilt cycles to influx of new material and short-timescale pressurization due to gas exsolution beneath a solid plug. At Mount St. Helens, Iverson et al [2006] model periodic seismicity as due to slip on a solid magma plug underlain by compressible magma.…”

Mount St. Helens: A 30‐Year Legacy of Volcanism

“…Petrologic and geodetic modeling studies suggest that the 2004-2008 eruption was fueled by a magma source about 5 kilometers deep; apparently, old thoroughly degassed magma moved to the surface in the more recent eruptions from this depth without significant recharge of fresh magma. The triggering mechanism for such an eruption remains enigmatic, but a model describing forces driving and resisting dome extrusion implies a balance between magma pressure and friction so delicate that even slight changes could have altered the eruption character [Iverson et al, 2006].…”