Fewer severe quakes hit Big Island

By Jan TenBruggencate
Advertiser Science Writer

Geologists are recording and locating more earthquakes than ever under the Big Island, but for the past 13 years, there have been no big ones.

"We haven't had a large, damaging earthquake since 1989," said Paul Okubo, seismologist with the Hawaiian Volcano Observatory. "Thirteen years without a magnitude 6 earthquake — that's a long window."

Okubo and his associates are trying to determine why it's been so quiet lately.

It could be because the 19-year continuous eruption of Kilauea is relieving underground stress that might otherwise cause earth movements, said Don Swanson, the observatory's scientist in charge. It's also possible the severe quake of Nov. 29, 1975, which took the lives of two campers at Halape, had an impact.

"We're not ruling that out," Okubo said. "It certainly relieved a lot of the accumulated strain energy" in the rock under the Big Island.

It might be the natural variability of earthquake activity.

"It's what makes earthquakes so interesting," he said. "They don't follow fixed patterns."

Seismologists have been tallying more quakes than ever on a global basis, as well as locally.

The National Earthquake Information Center of the U.S. Geological Survey was able to detect and determine the location for more than 23,000 quakes around the globe in 2001. That's a record high for one year.

But they found no increase in the number of big quakes, the ones that would have been detected and located even before improved technologies. The increase was among smaller temblors.

"We attribute the rising earthquake totals to an increase in the number of smaller earthquakes that we are able to study because seismological tools have rapidly increased and improved over time," said an article in the observatory's weekly "Volcano Watch" report.

In the past at Kilauea, seismologists could detect lots of small quakes, but they didn't have the staffing or the time to calculate the location for each of them.

Location is generally conveyed as a certain distance below a spot on the surface of the earth. For example, at 11:01 p.m. on May 13 of last year, there was a magnitude 2.7 quake, located 1.4 kilometers below the summit of Kilauea.

In the old days, seismologists would have to study the recordings from multiple sensors and calculate a location manually. Today, computers are programmed to do much of the work.

"Now, we can automatically trigger on selected quakes," Okubo said. "We can do so much more of our data processing automatically."

Through the 1960s, volcano scientists weren't able to do the calculations for quakes of less than magnitude 2.5. In the 1970s, the threshold dropped to 1.5. As time goes on, it drops further.

The number of accurately located small events gives scientists lots to work on, but Okubo and others are tantalized by the apparent decrease in the big ones. He cautioned that he's not entirely certain there is an overall decrease in major quake activity. It could be just a pause between events, albeit slightly longer than most.

"There seems to be a decrease in the numbers of earthquakes from the early '90s," Okubo said. "It's still pretty iffy if it really does exist, but it's my perception (that it does). We're trying to determine the reason. We're trying to still put a box around it."

"Volcano Watch" is available on the Web.