Surprisingly little distal ground deformation associated with the 2004-2005 eruption of Mount St. Helens, WA

The 2004-2005 eruption of Mount St. Helens, Washington, began with a small earthquake swarm on September 23, 2004. Activity peaked on September 24, gradually declined throughout the day, then increased dramatically before culminating in a series of phreatic explosions on October 1-5. By October 11, seismicity had dropped to 2-3 small (0.5-1.5 magnitude) earthquakes per minute with larger events (2-3 magnitude) occurring at a rate of several per day. This decrease coincided with the onset of lava dome extrusion, which has continued without pause through early February 2005. Deformation of the volcano and its surroundings has been monitored primarily by a combination of GPS and InSAR. Only one dual-frequency and one single-frequency GPS unit were running at the onset of activity. On September 26, 2004, several campaign GPS stations on and around the volcano were reoccupied, and by mid-October a network of 10 continuous dual-frequency stations had been established (installed by both the USGS and UNAVCO Inc.- Plate Boundary Observatory). Measurements from locations outside the crater show surprisingly little deformation. A site 8 km north of the volcano abruptly moved 1 cm towards the crater between about September 23 and October 5, suggesting volume loss at depth, and has moved an additional 1 cm through early February. Unfortunately, these data cannot be corroborated since no other stations were operating at this distance during the period of most rapid displacement. A second dual-frequency site high on the SE flank of the volcano moved 4 cm SE between mid-November 2004 and January 2005, likely due to impingement of the new lava dome against the SE crater wall. A systematic deformation pattern is not readily apparent from analysis of GPS data from other sites. Similarly, InSAR results have shown no significant surface displacements before or during the eruption. Inside the crater, surface motion is monitored by a network of single-frequency GPS receivers that have been placed by helicopter onto both the 1980-1986 and 2004-2005 lava domes. Results suggest that the 1980-1986 dome has been moving north, away from the site of the current eruption, at rates of 0.5-1 cm/day. GPS units on the active dome have recorded displacement rates as high as 10 m/day. Together with time-lapse photography, these measurements demonstrate that the velocity of lava extrusion remained nearly constant between November 2004 and February 2005. Results from the mid- and far-field GPS stations do not agree with classic models of volcano deformation that predict pre-eruption inflation and co-eruption deflation; thus, monitoring of surface displacements has largely failed to indicate the course of the eruption. This suggests a need to reexamine models of volcano deformation so that future monitoring efforts at Mount St. Helens and other volcanoes are more effective at mapping subsurface magma migration.