From 1986 to 1987, the USGS surveyed the Three Sisters vicinity with tilt-leveling networks and electro-optical distance meters, but South Sister was not the subject of close geodetic analysis for the next two decades. The volcano was found to be potentially active in 2000, when satellite imagery showed a deforming tectonic uplift west of the mountain. The ground began to bulge in late 1997, as magma started to pool about underground. Scientists became concerned that the volcano was awakening, but examination of interferograms, or diagrams of patterns formed by wave interference, revealed that only small amounts of deformation occurred. A 2002 study inferred that the composition of intruding magma was basaltic or rhyolitic. A map at the Lava Lands Visitor Center of the Newberry National Volcanic Monument south of Bend shows the extent of the uplift, which reaches a maximum of . In 2004, an earthquake swarm occurred with an epicenter in the area of uplift; the hundreds of small earthquakes subsided after several days. According to a 2011 study, this swarm may have initiated a significant decrease in the peak uplift rate, which dropped by 80 percent after 2004. By 2007 the uplift had slowed, though the area was still considered potentially active.