Analysis of GPS-measured deformation associated with the 2004-2006 dome-building eruption of Mount St. Helens, Washington

Detecting far-field deformation at Mount St. Helens since the crater-forming landslide and blast in 1980 has been difficult despite frequent volcanic activity and improved monitoring techniques. Between 1982 and 1991, the systematic extension of line lengths in a regional GPS trilateration network is consistent with recharge of a deep magma chamber during that interval. The rate of extension, however, averages only 3 mm/yr, and some of this apparent deformation may result from systematic scale error in the electronic distance measurements. Subsequent GPS surveys and data from a continuous GPS station, located 9 km north of Mount St. Helens and operating since 1997, show no significant volcanic deformation until the start of unrest on September 23, 2004. The current eruption has been accompanied by subtle but widespread inward and downward movement of GPS monitoring stations, exponentially decreasing with time and totaling as much as 30 mm. The observed 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 around 7 to 8 km and with a total cavity-volume loss of about 16–24×10⁶ m³ . The discrepancy between the estimated cavity-volume loss and the >83×10⁶-m³ volume of the erupted dome can be explained, for the most part, by exsolution of gas in the stored magma and by minor input of new magma during the eruption.