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

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.