Hunting A Quake's Clues
When a major earthquake ripped through Black Rapids Glacier in central Alaska, most people dove under tables or ran out their doors. Geologist Peter Haeussler headed for a helicopter.
The seismographs told Haeussler and other geologists from the U.S. Geological Survey that the largest strike-slip earthquake in North America had just torn through the tundra like a box cutter on carpet. It was a once-in-a-lifetime event for a geologist but a blanket of snow could have covered up priceless data at any moment.
"The phrase that still goes through my head: It really felt like a ticking time bomb," said Haeussler. "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 snow held off. Haeussler and other scientists spent 10 days in the field, carefully measuring and recording the earthquake's effects. They're now hoping that seismic lessons learned in a state with less than one person per square mile can be applied where a similar event could kill thousands.
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.
Like the San Andreas, the Denali Fault is a strike-slip fault, in which blocks of the Earth's crust move mostly horizontally past each other. 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 November 2002 Alaska quake lasted about 100 seconds and its power was awesome. Above the mile-wide Black Rapids Glacier in the Alaska Range, the magnitude 7.9 earthquake whiplashed an unnamed mountain and tossed down boulders the size of houses. Rock tumbled more than a mile across the pure-white ice and part-way up another mountain.
At Northway, 40 miles from the eastern end of the fault, the earthquake turned the ground into goo. Unseasonably warm temperatures had left only about the top foot of soil frozen. The wet underlying soil was shaken into a liquid slurry from the prolonged shaking.
In power and surface disruption, the 2002 quake resembled the magnitude 7.8 San Francisco Earthquake of 1906, which killed at least 700, and a 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 quakes occurred well before modern earthquake measuring equipment was invented. The Denali Fault quake gave geologists the chance to see if their computer models, mostly based on smaller events, measured up to the real thing.
Haeussler and other 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 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."
Most people still think of an earthquake in terms of a pebble dropped into a pond, with waves emanating in a circle from an epicenter.
Readings at a seismograph at a trans-Alaska pipeline pump station less than two miles from the fault confirmed a phenomenon known as directivity, in which released energy is far more intense in a narrow band along the fault in the direction the earthquake moves.
Ground just a few miles off the Denali Fault underwent far less shaking than ground in front of it, or as California earthquake expert Lloyd Cluff puts it, at the end of the barrel of the gun. Directivity is the reason the 2002 quake sloshed water 3,400 miles away in Louisiana and also in Seattle 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 ground liquefaction was widespread throughout eastern-central Alaska, and its pattern suggested that the directivity of the rupture amplified motions as it moved east.
Cracking of ice on lakes confirmed the effect. Along the west end of the tear, ice cracked on lakes within 18 to 25 miles of the fault. On the east end, cracked lake ice extended as far as 62 miles from the fault.
The findings could change how engineers design buildings that lie in the path of a wave from a strike-slip quake. Cluff, 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.
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."
By Dan Joling