Three Sisters encompasses  South Sister and the North Sister Field.
In 1985 and 1986 the U.S Geological Survey established tilt-leveling and EDM at Three Sisters, making baseline measurements that showed no deformation (Yamashita and Doukas, 1987). InSAR measurements made between 1992 – 2001 revealed uplift of an area ~ 10 km x 20 km at a rate of 3 – 5 cm/yr (Wicks et al. 2002). In response to this, the tilt-leveling and EDM networks were resurveyed using GPS, and measurements showed uplift at rates consistent with InSAR (Dzurisin et al. 2006). In 2002 – 2003, new leveling lines and 30 new campaign GPS marks were added (http://earthquake.usgs.gov/monitoring/gps/Sisters/). Three continuous GPS stations were also installed between 2001 and 2004 (http://earthquake.usgs.gov/monitoring/gps/Pacific_Northwest/). Continuous GPS measurements, and additional InSAR measurements between 2004 and 2006, showed that the deformation rate had decreased to ~1.4 cm/yr (Dzurisin et al. 2007). InSAR timeseries show that this decreased rate of uplift continued until at least 2010 (Riddick and Schmidt, 2011). Geochemical anomalies that coincide with the area of uplift suggest the existence of magma in the crust, and uplift is interpreted as inflation caused by magma intrusion (Wicks et al. 2002). In total ~0.05 – 0.07 km³ of magma was intruded as of Autunmn 2010 (Riddick and Schmidt, 2011), with models for all datasets suggesting a source depth of 5 – 7 km.