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Scientists weigh possible causes of small Oahu quake

"Seesaw" shifts within the island chain might explain the temblor, which was felt on Maui

By Rob Shikina

POSTED:
LAST UPDATED: 05:52 p.m. HST, Feb 25, 2011

The cause of a small earthquake off East Oahu yesterday was not immediately known, but scientists suspect it might have been caused by fault lines or the weight of the islands on the earth's crust.

The U.S. Geological Survey reported the 3.6-magnitude quake hit at 2:12 p.m. in the Kaiwi Channel. An earlier report, later revised by the USGS, described the quake as at 3.3 magnitude and centered just south of Lanai.

Victor Sardina, a geophysicist with the Pacific Tsunami Warning Center, said the inaccuracy of the initial report was likely because of the scarcity of seismic recording instruments on Oahu. There are only five seismic meters on Oahu compared with about 50 on Hawaii island.

While the Kaiwi Channel quake was too small to cause a tsunami, it shook residents around Oahu. The warning center reported receiving calls from people in Hawaii Kai, Kailua and Honolulu, and residents from East Oahu to Manoa reported their homes rattling for several seconds.

People reported feeling the shaking 40 miles away at Schofield Barracks and 161 miles away in Kula, Maui, according to the USGS.

Kanoa Koyanagi, a Pacific Tsunami Warning Center geophysicist, said earthquakes occur about once a year in the channel and are not something to be concerned about.

"People shouldn't get shook up," he said. "We don't expect a magnitude 6 to occur here."

Koyanagi said the fault lines around Oahu are typically not long enough to produce damaging quakes, such as those known to occur along long fault lines such as California¹s San Andreas.

On the Big Island, earthquakes of a magnitude 3 occur roughly every week, many related to volcanic activity, he said.

He said the location of the quake is still uncertain because of the lack of readings means the epicenter could be off by 10 to 20 miles.

Floyd McCoy, a professor of geology, geophysics and oceanography at Windward Community College, said a sea floor crack called the Molokai fracture zone, which is filled with fault lines, extends from California to Molokai and could have triggered yesterday's quake. An earthquake near Lanai of about 7 magnitude in the late 1800s is believed to have been within that fracture zone.

McCoy said another possible source of the quake is the shifting islands.

As the weight of Hawaii island pushes down on the earth's crust, the mantle beneath pushes up elsewhere, slightly lifting Molokai and Maui. Meanwhile, Oahu appears to be subsiding, he said.

"My guess is it's the differential movement in this upward movement between Molokai and Oahu," he said. "They're responding to the Big Island. ... That huge mass is pushing down on the earth's crust, and the response again is a seesaw."

"It's just another wonderful signal that the earth is alive," he said.






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