The 6.7-magnitude earthquake that rocked Hawai'i at 7:07 a.m. Oct. 15 was the most powerful seismic event in the United States last year, and one of largest in the world, according to U.S. Geological Survey data.

The powerful earthquake destroyed several homes on the Big Island and damaged hundreds of other residential, business and government structures across the state. Damage to government facilities alone has been estimated at over $100 million.

The shaking was felt for 60 seconds by people near the epicenter under Kiholo Bay, about 10 miles northwest of Kailua, Kona, and for 20 seconds in Volcano Village.

It was followed seven minutes later by another jolt, of 6.0 magnitude. Since then, more than 400 aftershocks have been noted, according to the USGS Hawaiian Volcano Observatory.

There were only four other earthquakes in the United States that measured between 6.0 and 6.9 magnitude in 2006, all of them in Alaska's Aleutian Islands.

Hawai'i, Alaska and California are usual suspects for seismic activity, but data from the USGS National Earthquake Information Center show that earthquakes were felt across the country last year from Maine and Illinois to Montana, Colorado and New Mexico.

In all, 2,728 U.S. earthquakes were noted and located in 2006 by the center.

Globally each year, there are an average of 134 "strong" earthquakes measuring 6.0 to 6.9 magnitude, and an average of 17 "major" quakes of 7.0 to 7.9 magnitude.

A "great" earthquake of 8.0 or higher happens on average only once a year, which was the case in 2006, when an 8.3-magnitude event occurred Nov. 15 off the Kuril Islands near Japan. That quake generated a small tsunami that caused minor flooding at a few coastal points in Hawai'i.

The largest of the two Oct. 15 Hawai'i temblors was tied for 18th on the list of largest quakes internationally last year.

Many earthquakes in the Islands are caused by underground movement of magma, which may erupt on the surface or intrude into cracks in the rock underneath the surface, according to the Center for the Study of Active Volcanoes at the University of Hawai'i-Hilo. In both cases, the island gets bigger and heavier, causing the lithosphere — the Earth's upper mantle and crust — to flex.

USGS scientists say the Oct. 15 quakes were caused by the release of long-accumulating stresses on the lithosphere.

Jim Kauahikaua, who heads the Hawaiian Volcano Observatory on the Big Island, said scientists are still wading through the data generated by the two Hawai'i earthquakes and expect to learn more about the flex-release phenomenon.

"It should tell us more about how that flexing occurs underneath the island. All of the data are fitting into a nice pattern that it's not really something breaking along a single fault zone, like the San Andreas Fault, but a flexing," he said.

"Similar earthquakes have occurred, but they don't happen that often."

The location of the Oct. 15 quakes was somewhat unusual. Since 1960, only 31 quakes larger than magnitude 4 have been recorded in that vicinity, Kauahikaua said. The largest such previous earthquake, a 4.8-magnitude event, occurred on May 14, 1982.

The 6.7-magnitude earthquake was the largest recorded on the Big Island since Nov. 16, 1983, when another 6.7 rumbled under the east flank of Mauna Loa. The only two larger earthquakes in recorded history were a 7.2-magnitude event beneath Kilauea on Nov. 29, 1975, and an estimated 7.9-magnitude quake under Mauna Loa on April 2, 1868, that destroyed more than 100 homes and killed 81 people.

A 7.9 earthquake is more than 10 times stronger than the larger of the two Oct. 15 events, according to the Center for the Study of Active Volcanoes.

Earthquakes can't be predicted, so officials say it is important to be prepared for the next big shake. That includes securing bookshelves, strengthening your home's foundation and preparing an emergency supply kit.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.