Hodge theorized that it had collapsed to form a caldera just as Mount Mazama collapsed to form Crater Lake. In the 1940s, Howel Williams completed an analysis of the Three Sisters vicinity and concluded that Multnomah had never existed, instead demonstrating that each volcano in the area possessed its own individual eruptive history. Williams' 1944 paper defined the basic outline of the Three Sisters vicinity, though he lacked access to chemical techniques and radiometric dating. Over the past 70 years, scientists have published several reconnaissance maps and petrographic studies of the Three Sisters, including a detailed geological map published in 2012. Neither North nor Middle Sister is likely to resume volcanic activity. An eruption from South Sister would pose a threat to nearby life, as the proximal danger zone extends from the volcano's summits. During an eruption, tephra could accumulate to in the city of Bend, and mudflows and pyroclastic flows could run down the sides of the mountain, threatening any life in their paths. Eruptions from South Sister could be either explosive or effusive, though an eruption of ash with local volcanic rock accumulation and slow lava flows is considered most likely. The Three Sisters area does not have fumarole activity, although there are hot springs west of South Sister. 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