1. US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct. #100, Vancouver, Washington 98683, USA
2. Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA
3. US Geological Survey, Denver Federal Center, Box 25046, Lakewood, Colorado 80225, USA
4. Deceased.

Correspondence to: Richard M. Iverson¹Daniel Dzurisin¹Cynthia A. Gardner¹ Correspondence and requests for materials should be addressed to R.M.I. (Email: riverson@usgs.gov), D.Z. (Email: dzurisin@usgs.gov) or C.A.G. (Email: cgardner@usgs.gov).

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.

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