Rumblings under the world’s largest active volcano prompted scientists Thursday to raise the alert level for Mauna Loa.

The volcano sent lava toward Hilo when it last erupted in 1984.

Raising the alert from normal to advisory does not mean that an eruption is certain, the scientists emphasized, but it indicates that the giant might be awakening after a 31-year slumber.

Scientists with the U.S. Geological Survey, which runs the Hawaiian Volcano Observatory, said a recent flurry of small, shallow earthquakes, the largest at magnitude 3.5, and deformation of the mountain’s flanks could mean that Mauna Loa’s magma chamber is filling up once again, signs that preceded eruptions in the past.

“It’s possible that the increased level of activity at Mauna Loa could continue for many months, or years, without leading to an eruption,” Tina Neal, the observatory’s scientist-in-charge, said in an emailed statement. “It is also possible that the current unrest could be a precursor to the next eruption of Mauna Loa. But at this early stage, we cannot determine precisely which possibility is more likely.”

The next alert levels would be a watch, meaning an eruption is likely, and a warning, meaning an eruption is imminent. There would have to be much more activity than is happening now to justify a watch, said Janet Babb, a scientist at the observatory, by telephone.

The alert levels correspond to a separate warning code for aircraft that relates to the possibility of volcanic ash in the atmosphere. That was raised from green to yellow Thursday. The next ones would be orange and red.

The observatory’s seismic stations have recorded elevated rates of shallow, small-magnitude earthquakes beneath the summit, the upper Southwest Rift Zone and the west flank for at least the past year, Babb said. Over the same period, instruments have measured ground swelling or inflation that is consistent with recharge of the volcano’s magma chamber, which is separate from that of Kilauea.

A magnitude-3.8 earthquake south of Fern Acres on Thursday was related to Kilauea, not Mauna Loa, Babb said.

The last time the observatory changed the status for Mauna Loa to advisory was in 2004-05, after another period of rapid inflation, but no eruption resulted.

The most recent eruption of Mauna Loa began March 25, 1984, and lasted just over three weeks. It was foreshadowed by up to three years of increased earthquake activity, which was more intense than what is now observed, scientists said.

The eruption began at the summit, which is typical, according to Babb, then moved to the northeast flank, sending lava toward Hilo and Kulani Prison. Both were spared.

What is certain is that Mauna Loa will erupt again someday, “so we should be prepared,” said Frank Trusdell, another observatory geologist.

Michael Garcia, a professor of volcanology at the University of Hawaii at Manoa, said Mauna Loa goes through episodic quake activity and deformation, so it is difficult to predict which episode will lead to an eruption.

“In late 2004 there was a big hike in earthquakes, and nothing happened,” he said. “The same thing happened early this year. There was an increase in quakes and an increase in deformation.”

The advisory alert is merely a formal acknowledgment that the mountain is stirring again, he said.

“They are going to put more attention on it than they have in the past,” he said. “It goes through these cycles, and we just have to keep watching and be alert.”

If an eruption happens, it will most likely begin at the summit, he added.

Mauna Loa rises to 13,681 feet above sea level. It’s part of a more massive edifice that starts at the seafloor, which in turn is depressed by the mountain’s mass. That means the summit is 56,000 feet above its base.

Since 1843, the first well-documented event, the volcano has erupted 33 times. The intervals between eruptions have been a short as months to as long as decades.

The eruptions tend to be large, producing big, fast-moving flows, according to the USGS.

Since the mid-19th century Hilo has been threatened by seven Mauna Loa flows. And flows reached the ocean to the south and west in 1859, 1868, 1887, 1926, 1919 and three times in 1950. In July 1975 the volcano erupted for less than a day.