INSAR ANALYSIS OF SURFACE DEFORMATION AT MOUNT ST. HELENS, WASHINGTON

Since the end of dome-building eruptions at Mount St. Helens in 1986, little surface deformation has been detected at the volcano. EDM measurements in 1991 and 2000 of a network around the edifice did not detect any displacements greater than about 3 cm. GPS observations also in 1991 and 2000 of a much larger network surrounding the volcano likewise revealed no detectable changes. However, a study of earthquakes between 1987 and 1992 by Moran (1994, JGR vol. 99) suggests that the shallow magmatic system is being repressurized beneath a solid plug at 2 km depth. Additional work by Musumeci et al. (2002, JGR, vol. 107) shows that the increase in pressure continued through the late 1990s. Such a pressure increase could lead to surface uplift over an area of 5-10 km (based on the depth of the earthquakes). Limited GPS data collected on the Mount St. Helens lava dome (approximately 1 km in diameter) by the USGS Cascades Volcano Observatory show several centimeters of subsidence, most likely due to cooling and compaction of the young lavas. Initial InSAR studies have not produced useful interferograms because coherence around the volcano breaks down after 1-2 years. We will present the results of an InSAR analysis that attempts to obtain better coherence over longer time periods by stacking many short timespan interferograms constructed using ERS SAR data. It is our hope to image both subsidence of the lava dome as well as any broader deformation sources resulting from pressurization of the shallow magmatic system. The results will be important for understanding the processes that characterize magmatic systems at recently active stratovolcanoes and can be extrapolated to infer behavior elsewhere in the Cascades.