Hawaiian waters 'natural laboratory' for quake research
Thousands of small earthquakes occur every year under the Big Island without much notice, except by scientists trying to understand them better.
"There are lots of little earthquakes that don't hurt anybody," said Cecily Wolfe, geophysicist/seismologist at the University of Hawaii Institute of Geophysics and Planetology.
They provide "a wonderful natural laboratory" for seismologists worldwide to study earthquakes and fault zones and how they interact with volcanoes, she said.
Wolfe is collaborating with Paul Okubo, geophysicist/seismologist at the U.S. Geological Survey's Hawaii Volcano Observatory, on projects to learn more about the causes and structure of local earthquakes.
The 1906 San Francisco earthquake led to great advances in understanding what happened along the San Andreas Fault and how fault zones occur, Wolfe said.
STAR-BULLLETIN / 1951
A damaged house in Kealia.
1868 earthquake considered Hawaii's worst
The most destructive Hawaii earthquake occurred on April 2, 1868, causing 81 deaths, destroying more than 100 homes and generating a tsunami along Kilauea's south coast. It was estimated at 7.9 magnitude based on the effects reported at the time, said Paul Okubo, geophysicist/seismologist at Hawaii Volcano Observatory.
The largest earthquake recorded by instruments in Hawaii was in 1975 in Kalapana with a 7.2 magnitude, Okubo said. It caused damage in Hilo, as well as at the source, and generated a local tsunami.
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Seismologist Harry Leeds proposed one of the most important ideas, saying earthquakes are the result of long-term buildup of stress and strain in the earth, she said. Faults are basically areas that store the stress and they snap when they can't hold it any longer, she said.
But it took another 60 years before scientists had "the final piece of the puzzle," she said. "It was not until the plate tectonic revolution that we began to understand how these earthquakes occurred."
Plate tectonics is the theory that the Earth is divided into large plates that change over time. They grind against each other, override or subduct the other, causing earthquakes and other geologic activity.
In Hawaii, which sits in the middle of the Pacific Plate far from plate boundaries, earthquakes are related to volcanism.
"Earthquakes are occurring on faults as well, but of course everything points back to volcanoes," Okubo said.
Wolfe and Okubo are studying an unusual swarm of about 2,000 earthquakes that occurred about 25 miles beneath Mauna Loa from 2002 through 2004.
Most were due to sheer faulting, but some tiny earthquakes had unusual characteristics people think might be from little reservoirs of magma that resonate, Wolfe said.
She said the area under Mauna Loa rarely has earthquakes and "all of a sudden, we have 2,000. That is a new phenomenon. We haven't seen these big, long-period swarms in the history of seismic reporting off the Big Island."
The episode seems to be associated with the recent inflation of Mauna Loa, she said. "By studying these earthquakes, we may learn the deep magmatic processes."
"We have to try to understand different types of observations we get from volcanoes," Okubo said, "and we're always concerned that ones not currently erupting will suddenly start to" rev up.
The swarm of earthquakes could be related to an active magmatic process but the location must be more precisely calculated to understand the cause, he said.
"It's still very much a work in progress. It's a test of our ability to apply what we think we know about seismology to different types of problems."
Okubo also uses earthquake data to learn details of the volcanic structure, "like using seismic waves to generate X-rays through the volcanoes."
Seismic waves are affected by properties of Earth as they travel from the earthquake to where they're recorded, he said.
By doing computations, the seismologists hope to learn what the physical properties were along the way that control how fast or how slow the waves travel through those areas, he said.
"On a smaller scale, we've been doing those calculations for the volcanoes and the island," but they can't go very deep or far offshore with instruments, he said.
Data from a $2.5 million experiment headed by Wolfe will be combined with the HVO network data for a more integrated, larger scale picture, he said.
Wolfe is principal investigator for the National Science Foundation project called PLUME -- Plume-Lithosphere Undersea Melt Experiment.
The goal is to try and understand the so-called "hot spot" believed to build the Hawaiian Islands with rising lava as the Pacific Plate moves over it.
Special seismometers developed by collaborating institutions were placed on the ocean floor around the Hawaiian Islands in 2004. They were recovered after one year and are being redeployed to listen to earthquakes around the globe for another year, Wolfe said.
Scientists have made progress in estimating the hazards of earthquakes but aren't able to predict when the next one as big as the 1906 San Francisco earthquake might happen, Wolfe said.
"I'm not sure if we will ever have that ability," she added. "They may be inherently chaotic and unpredictable."