The mystery has yet to be solved.

"I wish I knew what was happening," said seismologist Paul Okubo of the Hawaiian Volcano Observatory, whose headquarters is perched at the rim of Kilauea's caldera.

The volcano shut down right about the time a massive earthquake off Indonesia launched a catastrophic tsunami that swept through the Indian Ocean in late December. But volcano scientists can't show any direct connection between the big quake off Sumatra and Mauna Loa's sudden peacefulness, said Stuart Koyanagi, seismologist with the Hawaiian Volcano Observatory, although he admits the timing is suspect.

"We don't really know what happened," Koyanagi said.

The mountain continues swelling, although slowly, Koyanagi said. Pick any two distant points across the top of the volcano, and they are growing farther apart, indicating that molten rock is being pumped into the region deep under the volcano.

Mauna Loa is Hawai'i's second-most active volcano after Kilauea, which has been in constant eruption for more than 20 years.

Mauna Loa has erupted 13 times in the past 100 years, but the pace has slowed in the past half-century. Its most recent eruption was in 1984.

Last summer, the volcano launched a pattern of deep quakes whose long-period seismic signatures suggested magma was churning and cracking new pathways underground. "The quakes came in waves, with periods of seven to 10 days of very high activity," Koyanagi said.

In August and twice in October, there were periods with 25 to 35 earthquakes daily, although they were unnoticeable to Big Island residents. The observatory's "Volcano Watch" newsletter for Thanksgiving 2004 said: "In the over-40-year span covered by HVO's modern earthquake catalog, no other period has included such large numbers of Mauna Loa earthquakes at the depths now being observed."

Starting last Dec. 15, the shaking reached new highs — more than a week of 40-plus temblors a day.

And then it stopped.

For the past six months, Mauna Loa has been dramatically quieter than in the previous half-year, with roughly one earthquake a day and most of them very deep. The observatory has seismic recorders all over the mountain, and by carefully comparing the arrival times of earthquake signals, scientists can establish where a given quake is centered.

As long as quakes remain small and deep — 12 to 25 miles below the surface — volcano scientists don't get anxious. When those quakes start showing up nearer the surface, meaning magma is moving up, that's another story.

"The earthquake rate has to increase big time, and we have to see them being shallow — a progression where it's shallower and shallower. It can happen any time, but it hasn't happened yet," Koyanagi said.

The current quake rate is three times the background frequency of the period before last June. But it is difficult to predict anything when there is not much going on, and even this rate is nothing near what scientists need to feel comfortable making predictions.

"I'll make a prediction when I find some shallow seismicity," said geologist Frank Trusdell, the observatory's chief Mauna Loa scientist.

"It could be another decade, or it could be next week. Until we get shallow seismicity, we're not ready to forecast the next eruption," he said.