GPS-MEASURED DEFORMATION ASSOCIATED WITH THE 2004–2006 DOME-BUILDING ERUPTION OF MOUNT ST. HELENS, WASHINGTON

A prolonged period of dome growth at Mount St. Helens starting in September–October 2004 provides an opportunity to study how the volcano deforms before, during, and eventually after an eruption, using modern instruments and techniques such as GPS and interferometric synthetic aperture radar (InSAR). No precursory ground deformation was detected by differencing campaign GPS measurements made in 2000 and 2003, nor by a continuous GPS station (JRO1) located ~9 km to the north-northwest and operating since May 1997. Concurrent with the start of a shallow earthquake swarm on September 23, 2004, station JRO1 abruptly began moving downward and southward, toward a source beneath the volcano. Station JRO1 initially moved at a rate of ~0.5 mm/d until early October 2004, when its movement began to slow. By mid-October 2004 a network of fifteen continuous GPS stations had been installed around Mount St. Helens by the Plate Boundary Observatory (PBO) and by the U.S. Geological Survey to track ongoing deformation. All GPS stations showed a slowing radially-inward and downward ground displacement, with the movements varying with distance from the vent but coherent in time. Likewise, InSAR observations spanning the first year of the eruption suggest broad subsidence centered near the vent. During the ongoing eruption, the growth rate of a dacite dome decreased from a high of ~6 m³/s observed in the first few weeks of extrusion to less than 1 m³/s by late 2005 and throughout 2006. The observed surface deformation is consistent with the predictions of an elastic half-space model of a vertically elongate magma chamber with its center at a depth of ~7 km and with a total volume loss of about 15–25×10⁶ m³. This estimated volume loss is a fraction of the >83×10⁶m³ volume of the erupted dome, a discrepancy that could be explained by a combination of compressibility of stored magma and relatively minor input of new magma to the base of the chamber during the eruption.