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The Honolulu Advertiser
Posted on: Thursday, October 19, 2006

The Big One? That probably was Sunday

By Mike Gordon
Advertiser Staff Writer

Even as scores of aftershocks continue to rattle nerves and dinner plates on the Big Island, seismologists say the tremors are not a prelude to The Big One. In fact, scientists are pretty sure that already happened Sunday morning, when seismographs scratched out the now-famous 6.7 temblor.

"What do you mean, the big one? 6.7 isn't big enough for you? This was not a small earthquake. This was a big one," said Peter Cervelli, a research geophysicist with the Alaska Volcano Observatory.

On the Big Island, where Cervelli previously worked at the Hawaiian Volcano Observatory, "this is about as big as they get," he said.

The scientific community that studies the Earth's shifting geological features has pored over the data arriving steadily after the first of two strong earthquakes shook the Big Island beginning at 7:07 a.m. Sunday. By yesterday evening, there had been 116 aftershocks, including many from the 6.0 earthquake that occurred a few minutes after the initial temblor.

What they're seeing is a relatively small amount of aftershocks that are decreasing in size and frequency, Cervelli said. The strongest aftershock, a magnitude 4.4, occurred at 10:35 a.m. Sunday.

"Nobody can predict earthquakes, but from a statistical point of view, I would be surprised if this triggered something bigger," he said.

U.S. Geological Survey seismologist David Oppenheimer, who has viewed some of the data from his office in Menlo, Calif., said the number of aftershocks "is extremely low."

He compared them to the numbers felt by Californians in the months after the 6.9 magnitude Loma Prieta earthquake of 1989. In the first 24 hours after the Loma Prieta quake, there were 998 aftershocks.

After the first 24 hours of the initial Sunday temblor, there had been 81 aftershocks, he said. There were 17 on Monday and an even dozen on Tuesday.

"It is not a very energetic aftershock sequence," Oppenheimer said. "I would expect most of the aftershocks to die out in a month, at this rate."

A month is nothing when compared with the aftershocks Californians experienced after the Loma Prieta quake. Their jittery journey ended 627 days and 7,189 aftershocks later.

Right after an earthquake there is a 5 percent to 10 percent chance that it is foreshock, but with each passing day that diminishes, Oppenheimer said.

"There's a 95 percent probability that that was the biggest earthquake in the sequence," he said.

One of the reasons for this is that the initial earthquake occurred deep below the Earth's crust — about 24 miles below the ocean floor. It occurred west of the Big Island on a previously unknown Pacific Plate fault zone.

Because the Big Island is in an active volcanic zone, scientists believe the main earthquake Sunday was caused by surging lava and rock that built up massive weight over many years. The increased weight pushes down on the sea floor and Earth's crust, causing movement farther below.

Cecily Wolfe, a seismologist at the University of Hawai'i-Manoa, said scientists do not have enough information about previous deep earthquakes in Hawai'i. The lack of that information makes it difficult to predict the behavior of the aftershocks now being felt, she said.

An aftershock larger than the original quake is unlikely, she said. But a magnitude 5 aftershock can't be ruled out, she said.

"I don't want to panic people," she said. "It does get better with time. But for this large earthquake, there could be something moderate within the next few months."

Reach Mike Gordon at mgordon@honoluluadvertiser.com.