Geologists there upgraded the aviation color code for Redoubt from yellow to orange, indicating that an eruption may be imminent.
The 10,197-foot peak -- located about 50 miles west of Kenai and 100 miles southwest of Anchorage -- last erupted during a five-month period from December 1989 through April 1990.
Scientists keep a close eye on ground monitors on Redoubt to detect any movement.
"Around 1 o'clock (Sunday) morning the seismic activity really started to pick up," said AVO volcanologist Dave Schneider.
The shaking quieted about five hours later, but it still remains well above normal "background" tremor levels, Schneider said.
In the afternoon, a team from the observatory flew over Redoubt and determined that it hadn't yet erupted.
"There was steaming through pre-existing holes, but there were no new holes. ... and there was no ash on the snow cover," he said.
The crew members smelled sulphur as they flew over the volcano.
Now they'll be monitoring seismic activity at Redoubt around the clock, as well as satellite images that detect changes there in temperature, Schneider said.
Weather radar scanners near the Kenai airport, which transmit data every six minutes, will also be able to detect an ash plume should one appear, he said.
During the 1989-90 eruption, mud flows from Redoubt sped down the Drift River drainage and partially flooded the Drift River Oil Terminal facility.
The ash plume disrupted international air traffic and coated Anchorage and other nearby communities in a thin layer of ash.
The 1992 eruption of Mount Spurr, located about 70 miles due west of Anchorage, left a thicker layer of ash on Anchorage.
The volcanic activity Sunday followed by just one day a magnitude 5.7 earthquake at the mouth of Cook Inlet.
Both earthquakes and volcanoes can generally be traced to the tremendous heat that's generated far below the surface of the earth along a subduction zone where tectonic plates collide.
But that doesn't mean that the earthquake caused the volcanic activity, Schneider said. And the fact that the two were more than a hundred miles apart makes it more unlikely.
"Volcanic earthquakes are typically within five miles or so of the volcano," Schneider said.
Images of Mount Redoubt photographed by scientists who flew over the volcano on Sunday -- as well as any updates on the volcano's status -- are available online at avo.alaska.edu.
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