Alaskans not rattled by 2002 shake, but researchers were jolted into some serious study

Alaska quake offers lessons for elsewhere

Posted: Sunday, November 02, 2003

FAIRBANKS 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-rise buildings and high-density cities.

The 2002 quake began early on a Sunday afternoon 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 and 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 down the fault 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 whiplashed an un-named 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 eastern end of the fault, the earthquake turned the ground into goo, cracking the community's 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 liquid slurry. Liquefaction beneath buildings can cause major damage.

The earthquake did 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 fuel tanks in villages.

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 Fault 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 U.S. Geological Survey geophysicist Donna Eberhart-Phillips. ''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 it's due for another episode.

Those 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,'' Haeussler 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 snow storm 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.

The researchers found slabs of ground shifted as much as 29 feet past each other on the east end of the rupture. Beneath the trans-Alaska pipeline, the ground split and 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.''

Bill Ellsworth of Menlo Park, Calif., the chief scientist of the USGS earthquake hazards team, said seismologists want to know how strong the big earthquakes are close to faults.

''Globally, we have very little information about this,'' he said.

Most people still think of earthquake shaking in terms of a pebble dropped into a pond, with waves emanating in a circle from an epicenter.

Readings at a seismograph at a pipeline pump station less than two miles from the fault confirmed a phenomenon known as directivity, in which energy released is far more intense in a narrow band along the fault in the direction the earthquake moves.

Ground just a few miles off the fault underwent far less shaking than ground in front of the it, or as California earthquake expert Lloyd Cluff puts it, at the end of the barrel of the gun. Directivity is the reason the November quake sloshed water in Louisiana and Washington but created less-than-expected shaking in Fairbanks, which was pointed away from the direction of the fault.

A report by the Earthquake Engineering Research Institute of Oakland, Calif., noted that liquefaction was widespread throughout eastern-central Alaska and its pattern suggested that the directivity of rupture amplified motions toward the east.

Cracking of ice on lakes confirmed the effect. Along the west end of the tear, ice cracked on lakes within 18-25 miles of the fault. On the east end, cracked lake ice extended as far as 62 miles from the fault.

The findings someday could change how engineers design buildings that lie in the path of a wave from a strike-slip quake. Cluff, the manager of the geosciences department for Pacific Gas and Electric, the largest privately owned utility in the country, persuaded his employer to help pay for research on the topic in Alaska.

Damage to structures often depends on the acceleration of an earthquake, or the change from one speed to another.

''In this earthquake, the accelerations were possibly slower than expected but the velocities were among the highest ever observed,'' Haeussler said. ''In the end, you might well want to design something differently for high velocity and low acceleration rather than the other way around.''

Landslides caused by the quake also were surprising. They were concentrated in a narrow band about 19 miles wide along the surface rupture. A 7.9 magnitude quake should have triggered landslides up to 155 miles away. The moderate concentration of landslides suggest a deficiency of high-frequency shaking, again indicating low acceleration where the slip was largest.

The extensive liquefaction and the absence of landslides in the east indicated high velocity was present.

Haeussler spent part of the summer looking for clues as to how much offset occurred in the last big Denali Fault earthquake, and when it happened. In his office, waiting to be carbon-dated, are slices of trees exposed in a quake fissure that Haeussler suspects toppled into the earth during the last big quake 500 or so years ago.

''The basic understanding of how strike-slip faults work in repeated earthquakes, we don't know,'' Haeussler said. ''So that's something that we're really hoping we can learn a lot from on the Denali Fault.''

Other geologists are looking at how faults work together and how lessons could be applied to California, where thrust faults and strike-slip faults intersect.

They believe the quake added stress to the east and west ends of the Denali Fault. The latter crosses Alaska Railroad lines and the Parks Highway, the main route between Fairbanks and Anchorage.

The answers from Alaska will have applications elsewhere.

''This will be used by engineers around the world who will try to design for a large earthquake,'' said Eberhart-Phillips.

On the Net:

USGS fact sheet on the Denali Fault earthquake: http://geopubs.wr.usgs.gov/fact-sheet/fs014-03/

Earthquake Engineering Research Institute report: http://www.eeri.org/lfe/pdf/usadenalieerireport.pdf



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