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

Abstract

[1] The long-term behavior of the 2004-2008 effusive eruption of Mount St. Helens was characterized by a gradual decline in the rates of seismicity, dome growth, and broad-scale ground deformation, but shallow near-periodic "drumbeat" earthquakes over timescales of minutes indicated episodic short-term behavior. In part to better characterize this behavior and any associated ground deformation, a network of tiltmeters was installed and recorded thousands of cyclic tilt events within the crater. The duration of these events, from minutes to hours, was too long for them to be directly caused by the drumbeat seismicity. Tilt events were characterized by a recoverable, asymmetric pattern of rapid tilt away from the vent followed by a more gradual reversal, were highly correlated between different tiltmeters in the crater, sometimes occurred in association with volcanic seismicity, and ceased at the end of the eruption. Tilt vectors converged on a point just south of the center of the preexisting 1980s lava dome, and the absence of detectable tilt outside the crater suggests a shallow source (<1 km). We examine several models, including cycles of conduit pressurization and plug slip or gas loss and stick-slip behavior on the interface between the ascending plug and the 1980s lava dome. The small number of stations within the crater prevents a unique determination of source type or geometry, but results are consistent with a mechanism involving extrusion of the semisolid dacite plug and/or cycles of conduit pressurization.

Conditions for detection of ground deformation induced by conduit flow and evolution

“…The blue boxes indicate the tilt amplitude recorded in two andesitic volcanoes: boxes 1 and 2 at Soufrière Hills (Montserrat) in May-August 1997 and December 1996, respectively [Voight et al, 1998], and box 3 at Mt. St. Helens between July 2006 and January 2007 [Anderson et al, 2010]. Parameters used for the calculation are the same as in Figure 4.…”

“…St. Helens, variations of the tilt amplitude were also measured during the active period between the years 2004 and 2008 by Anderson et al [2010]. Tiltmeters, located within the crater and close the dome (around 150-250 m from the vent), recorded many cyclic tilt events characterized by a duration from minutes to hours and a mean amplitude close to 1 mrad.…”

“…Results were compared with the solution given by the following analytic expression [Love, 1987]: U r = a 2 c P c 2rG , where r is the radial distance. Secondly, the tilt induced, at the surface, by a constant shear stress applied along the conduit wall, was numerically estimated and compared to the analytic expression provided by Anderson et al [2010], which corresponds to an approximate solution derived from the integration of the Green's functions of a vertical point force. The comparison between numerical results and analytic solutions as well as the estimated error are shown in Figure B1.…”

“…However, deformation of the conduit wall due to the magma flow was not taken into consideration and most of studies considered only one component of the stress field acting on the wall rocks, either shear stress [Beauducel et al, 2000;Green et al, 2006] or normal stress [Hautmann et al, 2009]. Only few studies consider both components [Chadwick et al, 1988;Anderson et al, 2010]. However, in these studies, the two effects are evaluated independently despite their obvious link throughflow motion equations [Nishimura, 2009].…”

Tilt offset associated with local seismicity: the Mt. Etna January 9, 2001 seismic swarm.

“…These permanent offsets are assumed to represent the change produced by the fault dislocation in a tectonic context (e.g. [5,6]) while in an active volcanic setting, the tensile component (e.g. a dike) can dominate the deformation.…”