Lava could reach Hilo on the eastern side of the island and the Gold Coast resorts of Kona in the west, and inundate neighborhoods in the southwest rift zone above South Point - possibly without much warning, said Peter Cervelli, a research geophysicist at the U.S. Geological Service's Hawaiian Volcano Observatory.
Mauna Loa has erupted 33 times since 1843. In 1984, Mauna Loa erupted for three weeks, sending a 16-mile lava flow toward Hilo. Since then, more than $2.3 billion has been invested in new construction along Mauna Loa's slopes, according to the USGS.
"In some cases they're building on lava flows that are less than 100 years old," Cervelli said.
Scientists from Stanford University recently joined the observatory in monitoring the 13,500-foot volcano, which began to stir on May 12.
Recent data has revealed that Mauna Loa's summit caldera, the basin inside the volcano, has begun to swell and stretch at a rate of 2 to 2½ inches a year, which can be a precursor of an eruption, scientists said.
"We're at a stage where it's months to years, rather than days to weeks," before the next eruption, Cervelli said.
Scientists are working to detect an eruption as early as possible to give people a chance to evacuate.
"Earthquakes will always precede the movement of magma to the surface," Cervelli said.