Large Earthquake Has Hawaiian Islands All Shook Up

Richard A. Lovett
for National Geographic News

October 16, 2006

A strong earthquake rattled Hawai'i at 7:07 a.m. local time on Sunday morning, damaging buildings, disrupting power and telephone service, and startling athletes preparing for this weekend's Ironman triathlon World Championships.

Nobody was killed by the magnitude 6.7 temblor, although there were numerous reports of minor injuries.

The quake was centered offshore from the Big Island's Kona Coast, 10 miles (16 kilometers) from the town of Kailua Kona (Hawai'i map).

The main quake, which was felt on all seven of the state's inhabited islands, was followed minutes later by an aftershock of magnitude 6.0.

Earthquakes aren't rare in Hawai'i, but the vast majority are smaller than the most recent quake and are associated with the underground movement of magma during volcanic activity.

Yesterday's earthquake was not only larger than average but deeper—centered 24 miles (39 kilometers) beneath the surface, according to data reported on the U.S. Geological Survey (USGS) Web site.

In addition, the quake's location on the opposite side of the island from current volcanic activity suggests that magma movements were not the cause, says Paul Okubo, a seismologist at the USGS Hawaii Volcano Observatory.

Such earthquakes are probably caused by stresses in Earth's crust created by the weight of the growing volcanoes.

"In general terms," Okubo said, "it's a lithospheric flexure process where the mass of the island has deformed the underlying earth." The earthquake resulted when Earth's crust readjusted to the pressure.

Land Rolled Like a Ship

Yesterday's temblor is by no means the largest quake ever recorded in Hawai'i. That honor goes to a quake on April 3, 1868, now estimated to have been a magnitude 7.9.

Because that earthquake predated modern instruments, its cause is not well understood.

But, Okubo says, the quake occurred in conjunction with eruptions of both the Mauna Loa and Kilauea volcanoes.

The 1868 quake must have involved a much larger fault zone along the junction of the island's "volcanic pile" and the adjacent ocean-bottom crust, he says.

What seismologists do know is that the powerful quake was centered on the Big Island's southeastern coast.

Frightened residents said at the time that the temblor caused the ground to roll like a ship at sea and created a three-mile-long (five-kilometer-long) landslide that killed 31 people along with thousands of cattle, sheep, horses, and goats.

(Yesterday's temblor created a much smaller landslide that blocked a major highway.)

The 1868 earthquake also created a tsunami with 65-foot (20-meter) waves that killed an additional 46 people.

"It was the most significant geologic event in historic time in Hawai'i," he said, "depending on how you define 'historic time.'"

The most severe recent earthquake, according to USGS historical data, was a magnitude 6.9 temblor that occurred on August 21, 1951, not far from the location of yesterday's earthquake.

The 1951 quake damaged dozens of homes and caused rocks to fall from a cliff, creating a 12-foot (3.6-meter) wave.

Hawai'i's largest earthquake threat, however, isn't from home grown temblors—it is from tsunamis created by distant quakes along the Pacific Rim in Asia or the Americas.

Since 1813 eighty-five tsunamis have been reported in the island state, the USGS says (related: "Tsunami's: Facts About Killer Waves" [January 14, 2005]).

The most devastating tsunami occurred on April 1, 1946, following a large earthquake in Alaska's Aleutian Islands.

Fifty-five-foot (16.7-meter) waves struck the city of Hilo on the Big Island, killing 173 people and doing millions of dollars in damage.

Now seismologists monitor earthquake activity throughout the Pacific Ocean, issuing warnings whenever tsunami-inducing temblors occur.

No such warnings were issued in the aftermath of yesterday's earthquake, because it did not generate significant waves.

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