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Huge Alaska Quake Could Offer Lessons for California

The site of last year's 7.9 temblor resembles the San Andreas fault. Scientists hope what they learn can be used in heavily populated areas.

November 09, 2003|Dan Joling | Associated Press Writer

FAIRBANKS, Alaska — On Nov. 3, 2002, the largest strike-slip earthquake in North America in 150 years ripped through central Alaska, tearing up 209 miles of the Earth's surface like a box cutter on carpet -- all in about 100 seconds.

Scientists now hope that seismic lessons learned from the 7.9-magnitude earthquake in a state with one person per square mile can be applied to places where strike-slip faults cut through heavily populated areas such as California.

Researchers have swarmed to the Denali fault because of its resemblance to the San Andreas fault, California's 800-mile crack that's a rumble away from high-density cities.

The Denali fault starts in southwest Alaska, arcs east to Canada and then curves south through Alaska's southeast panhandle for some 1,300 miles.

The 2002 quake began on the Susitna Glacier fault about 90 miles south of Fairbanks with a thrust earthquake, in which one part of the Earth's crust is pushed up over another.

The quake immediately triggered the strike-slip rupture on the Denali fault, where blocks of crust move mostly horizontally.

An energy wave ran east until it reached an intersection with the Totschunda fault, where it triggered a third earthquake on that fracture.

The power was awesome. Above the mile-wide Black Rapids Glacier in the Alaska Range, the earthquake whip lashed an unnamed mountain and tossed down house-size boulders. Rock tumbled more than a mile across the pure-white ice and part way up another mountain.

"It ruined a real good, epic ski trip," said engineer Mike Malvick, who helps oversee the integrity of the 800-mile-long trans-Alaska oil pipeline less than 10 miles away. "You used to be able to ski up the glacier, through a pass and come out at Denali National Park."

At Northway, 40 miles from the east end of the fault, the earthquake turned the ground into goo, cracking the airport runway. Unseasonably warm temperatures had left only about the top foot or so of ground frozen. That left the underlying soil susceptible to liquefaction, in which prolonged shaking transforms loose, water-saturated sediments into a slurry. Liquefaction beneath buildings can cause major damage.

The earthquake caused more than $44 million in damage in Alaska, tearing up four highways, damaging six bridges, including two that must be replaced, and knocking over dozens of village fuel tanks.

Just one person was injured -- a woman descending stairs fell and broke her arm.

And Alaskans, many of whom remember the 1964 magnitude 9.2 thrust earthquake, the second largest ever recorded, took the Denali earthquake in stride.

"Alaska just happened to have had one of the largest earthquakes in the world, so people in Alaska don't think the Denali earthquake was very large," said Donna Eberhart-Phillips, U.S. Geological Survey geophysicist. "In terms of a rupture-to-the-surface earthquake, it's the largest earthquake in North America in 150 years."

If Alaskans didn't appreciate what happened, the people who monitor the San Andreas fault for strike-slip earthquakes did.

In power and surface disruption, the quake resembled the magnitude 7.8 San Francisco earthquake of 1906, which killed at least 700, and the magnitude 7.9 Fort Tejon earthquake north of Los Angeles in 1857. It had plenty in common with a magnitude 7.8 earthquake on the southern end of the fault east of Los Angeles and San Diego. That one went off in 1685 and geologists figure that it's due for another episode.

The California earthquakes occurred before today's seismologists took their first earth science class and well before modern earthquake measuring equipment was invented. The Denali quake gave geologists the chance to see if their computer models, often based on smaller quakes, measured up to the real thing.

Almost from the minute the ground stopped shaking, federal, state and university geologists headed for the field and spent 20-hour days recording crucial measurements before fine detail disappeared.

Peter Haeussler, a USGS geologist from Anchorage, was part of a crew of 11.

"The phrase that still goes through my head, it really felt like a ticking time bomb," he said. "There was so little snow on the ground. We were concerned we were going to lose all information about the fault rupture to a snowstorm the next day."

The scientists measured cracks in the ground and cracks on ice. Some of the most precise measurements were taken on glaciers, where the earthquake had clearly offset crevasses.

Researchers found that slabs of ground had shifted as much as 29 feet past each other on the east end of the rupture. Beneath the trans-Alaska pipeline, the ground shifted sideways 18 feet.

"The reason it's important to measure these offsets is it's a direct measurement of how much energy came from different parts of the fault," Haeussler said. "Seismologists are dealing with models all the time, but they don't have these sort of direct measurements of what happened in a particular location."

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