Posted on: Tuesday, August 17, 2004

New pattern of minor quakes shakes Mauna Loa

A lava flow inches along a beach at the foot of the eastern Banana delta. The glow in the background comes from where some of the lava plunges into the sea. The photograph was taken by a scientist with the U.S. Geological Survey. U.S. Geological Survey

By Kevin Dayton
Advertiser Big Island Bureau

HILO, Hawai'i — Scientists are monitoring a series of earthquakes that has been rattling Mauna Loa in a pattern unlike any detected before at the world's largest volcano.

The summit of Mauna Loa has been inflating since 2002 in what scientists believe is a swelling of the underground magma reservoir and a signal of a coming eruption.

Earthquake activity picked up before the past two eruptions in 1975 and 1984, but it is unlikely the recent series of temblors indicates an imminent eruption, said Paul Okubo, research seismologist at the Hawaiian Volcano Observatory.

"We're of the mindset here that Mauna Loa will erupt, but in terms of the lead time, I think we're still quite a ways off," he said.

The recent earthquakes are originating 25 to 30 miles below Mauna Loa's caldera, with the largest in the magnitude 2 range. It's unlikely anyone on the surface would feel temblors that small and deep, but scientists have catalogued about 50 similar earthquakes in the past three weeks, and the activity appears to be continuing, Okubo said.

"We've not seen these before," he said. "It's a new pattern."

Most earthquakes at Mauna Loa are more shallow, he said. There have been a significant number of deep earthquakes below Mauna Loa before, but never so many in such a short period of time as scientists have recorded since July, he said.

At Kilauea volcano, scientists have gathered evidence that links similar but shallower earthquakes to the movement of magma below the surface. But Okubo said scientists don't know what is causing the recent quakes under Mauna Loa.

"We can locate them, we can determine their magnitudes, we can look at their behaviors through time, but in terms of really associating them with a physical process, we don't have that kind of handle on it," he said.

Larger and shallower earthquakes have been associated with Mauna Loa's past two eruptions.

In the summer of 1974, scientists noted a series of unusual, shallow earthquakes five to six miles below the surface, and recorded a magnitude 5.4 earthquake on Nov. 30, 1974. Mauna Loa erupted on July 5, 1975.

A similar pattern of shallow earthquakes was observed by scientists in the fall of 1983, with a larger magnitude 6.6 earthquake on Nov. 16, 1983. The following March 26, Mauna Loa erupted again.

Both of the stronger, shallower earthquakes before the two eruptions were beneath the southeastern flank of Mauna Loa, Okubo said.

The recent deep earthquakes "are possibly among the very, very early indications of changes that will happen and begin to move more systematically shallower in the system," Okubo said.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com or (808) 935-3916.