STAR-BULLETIN / 1950 Lava flows from the 1950 Mauna Loa eruption reached the ocean, but then the volcano was quiet for 25 years.

Mauna Loa
rumbles awake

A flurry of earthquakes persuades
scientists the volcano could soon
begin spewing lava

"Unprecedented" activity deep under Mauna Loa this summer could be another sign that the world's largest volcano is headed toward an eruption, a scientist says.

Fast Facts: Mauna Loa » World's largest volcano and among the most active. » Rises gradually to 13,677 feet, more than 2 miles above sea level. » Has erupted 33 times since 1843. » Longest period between modern-day eruptions was 25 years, between 1950 and 1975. » Last eruption was in 1984, when lava flowed for 22 days and came to within 5.3 miles of an upland part of Hilo. SOURCE: U.S. GEOLOGICAL SURVEY

More than 350 earthquakes have been recorded since July.

Such frequent, small earthquakes have never before been measured beneath Mauna Loa's summit caldera. Data on Mauna Loa's seismic activity started to be collected in the 1930s, and Hawaiian Volcano Observatory scientist-in-charge Donald Swanson said even the instruments of that era would have detected the quakes.

"They're unprecedented," he said, "since we started studying Mauna Loa."

Despite the quakes, Swanson emphasized that an eruption should not be expected within the next few weeks or even months.

There is no good estimate on when the volcano could erupt. But before an eruption, scientists would probably see the volcano's swelling increase and detect stronger earthquakes nearer to the surface, he said.

"What we're seeing now is just a prelude to an eruption," Swanson said. "It's a clear indication that the volcano is restless, but we've known that for more than two years because it started to inflate."

In May 2002, after nearly 10 years of slight deflation, Mauna Loa's summit started to swell. Similar expansions had preceded Mauna Loa's past two eruptions, in 1984 and 1975.

Swanson said the recent quakes have not increased in frequency since July, but rather fluctuated in number from "day to day and week to week," Swanson said.

Last week, there were about 12 earthquakes recorded, he said.

The quakes occur about 25 miles beneath the volcano's summit caldera and the adjacent part of the southwest rift. Their size and depth have stayed about the same, Swanson said.

Mauna Loa, which reaches 13,677 feet high, last erupted 20 years ago. Lava from the eruption covered 16 miles in 22 days, stopping just 5.3 miles short of an upland part of Hilo.

In modern history the longest period between eruptions has been 25 years (from 1950 to 1975).

"Twenty-five years is the record," Swanson said. "We're within the length of time that ... we should be anticipating another."

Despite the unusual earthquakes, Swanson said it is still too early to step up the observatory's monitoring activities.

"We haven't seen any reason to do that. All of us are aware of what's happening. We sort of have a heightened awareness," he said, adding that the number of earthquakes recorded is "quite a large number, but it's not alarming."

Swanson also said it would be premature to start planning an eruption's path because that is still unclear, too.

"It's still quite subdued," he said, compared with what activity atop the volcano will be like when "we'll be thinking more carefully about what and where the eruption will be."

Mauna Loa is among the planet's most active volcanoes, according to the Hawaiian Volcano Observatory. Since 1843 the volcano has erupted 33 times.

One of Mauna Loa's neighbors on the Big Island, Kilauea, is also within Hawaii Volcanoes National Park and has been erupting continuously since Jan. 3, 1983.