Three Sisters Bulge, central Oregon Cascade Range

Rob McCaffrey, Mike Lisowski, Chuck Wicks, Dan Dzurisin

A joint analysis of GPS, leveling and InSAR measurements from 1992 to 2020 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 TDEFNODE3D 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 a roughly exponentially decaying rate of inflation within a ~7-km-deep, nearly spherical magma chamber.

The main pulse of deformation started sometime in 1998 or 1999, 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 ~1.2. 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.



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