Decaying Rate of Volcanic Inflation near Three Sisters, Oregon, Measured with GPS and InSAR

Thursday, 18 December 2014
Michael Lisowski1, Robert McCaffrey2, Charles Wesley Wicks Jr3 and Daniel Dzurisin1, (1)USGS, Vancouver, WA, United States, (2)Portland State University, Portland, OR, United States, (3)USGS, Menlo Park, CA, United States
A joint analysis of GPS and InSAR measurements from 1992 to 2014 is used to track ongoing volcanic deformation in a broad area centered ~6 km west of South Sister Volcano in central Oregon. We use the tdefnode software to model the observed deformation with a linear combination of steady background tectonic deformation across the Cascades volcanic arc and time-dependent inflation of a buried magma chamber with variable depth and geometry. We find that the observed InSAR interferograms and GPS-measured uplift and radially outward displacement of the ground is consistent with that predicted by an exponentially decaying rate of inflation within a ~7-km-deep, nearly spherical magma chamber. The main pulse of deformation started sometime in 1998, and the rate of deformation has since decayed with an exponential time constant of ~7 years. The post-2001, 3-D displacements measured by the continuous and campaign-mode GPS stations only are best-fit by decaying inflation of a slightly shallower 5-km-deep prolate spheroid with an aspect ratio of ~1.4. The continuous GPS displacement time series from station HUSB, installed in May 2001, provides the clearest evidence for smoothly decaying rate of deformation. This study does not support Riddick and Schmidt’s (G3, 2011) conclusion, based on 1992-2009 InSAR data only, that the volcanic deformation in this region is best fit by a piece-wise linear rate of decay, and it conflicts with their hypothesis that the source of deformation is an inflating sill. Our results are consistent with those of Dzurisin and others (JVGR, 2009), which were based on GPS data only.