Massive tectonic forces
cited in fatal tsunamis

Hawaii scientists say a rare
"megathrust" quake led
to disaster

Instruments at the Hawaiian Volcano Observatory showed waves of vibrations pulsing through the earth for six hours following Dec. 26's massive earthquake off the coast of Indonesia.

"The earth was ringing like a bell," said observatory head Jim Kauahikaua.

image: chart Geologists are using a relatively new term for the phenomenon that caused such massive vibrations. They call it a "megathrust earthquake."

It is a new name for an old danger that has caused death and destruction in Hawaii. A magnitude-9.5 megathrust earthquake off the coast of Chile in 1960, the largest quake ever recorded, created a tsunami that killed 61 people on the Big Island and caused $23 million in damage.

The quake resulted from part of the ocean floor scraping under another piece of the earth's surface where South America sits.

Where one area of land crunches beneath another, geologist call the site a thrust fault. "These happen all the time," Kauahikaua said.

But when the crunch involves two of the giant plates making up the earth's surface, the size of the site earns it a new name, a megathrust.

In last week's megathrust quake and aftershocks near Indonesia, the Burma plate slipped under the Sunda plate in an area about 750 miles long and 60 miles wide, the U.S. Geological Survey said.

Other such over-under slippages that have affected Hawaii are the magnitude-9 quake in Kamchatka, Russia, in 1952, the 9.1 quake in the Andreanoff Islands, Alaska, in 1957 and the 9.2 quake in Prince William Sound, Alaska, in 1964, the U.S. Geological Survey said.

All of those caused tsunamis that did some damage in Hawaii, and the 1957 caused one death.

In comparison to these magnitude-9 quakes, the 1975 Halape earthquake on the Big island had a magnitude of 7.2. It, too, involved one block of land thrusting under another, but since Hawaii is not at the edge of a tectonic plate, it was not a megathrust.

One of the Big Island's worst disasters, the 1946 tsunami created by an earthquake in the Aleutian Islands of Alaska, does not quite fit the megathrust pattern.

The quake originated in a megathrust area, but some scientists feel it should not have created a tsunami because the magnitude of the quake, variously calculated at magnitude 7.1 to 7.8, was too small.

Geologist Emile Okal of Northwestern University says the strength should be recalculated at magnitude 8.5.

Mayor Harry Kim said he did not need the Indonesia quake to remind him of the danger.

"I went through that living in Hilo in 1960, looking for friends and never finding them," he said.

More than a month ago Kim had a "feeling," he said, which prompted him to schedule a meeting with Big Island officials to review Civil Defense response procedures for tsunamis.

The meeting is scheduled for Thursday morning. Kim, as the county's former Civil Defense director for 24 years, will conduct the meeting himself.

East-West Center Tsunami Relief
American Red Cross Hawaii
Pacific Tsunami Warning Center

U.S. Pacific Command Tsunami Relief

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