NEW RESULTS FROM A PROPOSED PBO CASCADE VOLCANO CLUSTER III: POST-ERUPTIVE DEFORMATION OF MOUNT ST. HELENS, WASHINGTON, FROM EDM AND GPS

The major eruption of Mount St. Helens on May 18, 1980, was followed by five smaller explosive eruptions during the next five months and 16 dome building eruptions through October 1986. In 1982, the USGS established a 14-station Geodolite EDM network centered near Mount St. Helens and extending 30-km across and 90-km along the volcanic arc. All 26 line lengths were remeasured in 1991, and 10 line lengths between 7 central stations were recovered by GPS in 2000. Strain accumulation between 1982 and 1991 was dominated by areal dilatation (0.15 ± 0.06 µstrain/yr) in the central part of the network around Mount St. Helens. No significant strain accumulation was observed in the same area between 1991 and 2000. The continuous GPS station JRO1, located 9 km north of the dome and installed in 1997, has been moving steadily north-northeast with a velocity of 8 ± 1 mm/yr relative to interior North America. This velocity and those of other regional GPS stations along with the lack of strain accumulation near the volcano are roughly consistent with rigid rotation of coastal Oregon as determined from modeling of broadscale GPS-measured deformation. 2003 measurement of a dense 40-station GPS network around Mount St. Helens (established in 2000) will reveal whether there has been localized deformation near the edifice or across the right-lateral St. Helens seismic zone.