Monitoring and Modeling of Ground Deformation at Three Sisters volcanic center, central Oregon Cascade Range, 1997-2009 (Invited)

Modeling of InSAR, GPS, and leveling data indicates that uplift of a broad area centered ~6 km west of the summit of South Sister volcano started in 1997 and is continuing at a declining rate. Surface displacements were measured every summer when possible since August 1992 with InSAR, annually since August 2001 using GPS and leveling surveys, and since May 2001 using continuous GPS. Our best-fit model to the deformation data is a vertical, prolate, spheroidal point-pressure source located 4.9-5.4 km below the surface. A more complicated source of this type that includes dip as a free model parameter does not improve the fit to data significantly, and other source types including tabular bodies (dike or sill) produce decidedly poorer results. The source inflation rate decreased exponentially during 2001-2006 with a 1/e decay time of 5.3 ± 1.1 years. The net increase in source volume from September 1997 to August 2006 was 36-42 x 106 m3. A swarm of ~300 small (maximum magnitude 1.9) earthquakes occurred beneath the deforming area in March 2004; no other unusual seismicity has been noted. We attribute surface deformation to intrusion of magma, perhaps at the brittle-ductile transition in hot, thermally altered crust beneath the active Three Sisters volcanic center. Elastic models like those we investigated cannot distinguish between ongoing intrusion at a declining rate and viscoelastic response of the overlying crust and hydrothermal system to an intrusion that might have ended some time ago. Repeated gravity surveys that began in 2002 might help to resolve this ambiguity; gravity results through summer 2009 will be presented separately at this meeting. Similar deformation episodes in the past probably would have gone unnoticed if, as we suspect, most are caused by small intrusions that do not culminate in eruptions.