University of Utah Professor Robert Smith said the rise is likely due to a huge area of molten rock forming 6 miles beneath the caldera.
"There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That's the bottom line," said Smith, lead author of the study. "A lot of calderas (volcanic craters) worldwide go up and down over decades without erupting."
Smith, researcher Wu-Lung Chang and colleagues said the upward movement -- nearly 3 inches annually for the past three years -- is more than three times greater than ever observed since such measurements began in 1923.
"Our best evidence is the crustal magma chamber is filling with molten rock," said Smith. "But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again."
The study, which included doctoral students Jamie Farrell and Christine Puskas, with geophysicist Charles Wicks of the U.S. Geological Survey, appeared in the Nov. 9 issue of Science.