Seismic sleuths say history may haunt Seattle

December 04, 1992|By San Francisco Chronicle

In a remarkable feat of seismic detective work, five teams of scientists have gathered compelling evidence to show that a strong, shallow earthquake about 1,000 years ago rocked the ground precisely where Seattle stands now.

A similar quake today -- well within the bounds of probability -- could churn the ground violently beneath the modern city and damage a wide and heavily populated region, according to experts assessing the new findings.

Although the Puget Sound region is not known for constant, damaging quakes, as are the infamous San Francisco and Southern California segments of the San Andreas Fault, seismologists in the Seattle area say the new findings indicate that the quake danger there may be greater than scientists had believed.

"The Northwest is big-time earthquake country," said Craig Weaver, coordinator of the U.S. Geological Survey's seismic hazards program in the region,commenting on the new findings.

The research, he said, documents a new source for damaging earthquakes in the region.

The ancient quake apparently lifted sections of Puget Sound's shores by more than 20 feet and sent a tsunami, or giant wave, dumping broad sand deposits into tideland marshes more than 35 miles away, the scientific teams discovered.

It also triggered massive avalanches of rock that dammed streams and created three lakes on the Olympic Peninsula.

And, the evidence indicates, it drowned large stands of Douglas fir trees when huge landslides tumbled directly into what is now Seattle's inland Lake Washington.

As the earth scientists compared results, they concluded that the quake must have struck at least as powerfully as California's disastrous temblor of 1989, which registered a magnitude of 7.1.

In five separate reports appearing in the journal Science, the researchers attributed the ancient event to the Seattle Fault, which runs on an east-west line about 10 miles beneath the modern city.

Their discovery adds a third source of dangerous seismic activity known to exist in the Puget Sound region.

The Seattle region has had occasional severe quakes in the very recent past, including one in 1965 with a magnitude of 6.5 that caused $12 million in damage to Seattle; and one in 1949 that measured 7.1 on the Olympic Peninsula.

Those quakes were centered in faults more than 25 miles underground, where the edge of an oceanic slab of the earth's crust called the Juan de Fuca Plate has been slowly crunching eastward beneath the North American Continental Plate for thousands of years.

Geophysicists in the Pacific Northwest also have determined from fossil evidence that many far more monstrous quakes -- with magnitudes as great as 8 or even 9 -- have shaken the region again and again over past millenniums.

The reports in Science today mark the most successful research achievement in a new field of earth science known as paleoseismology -- the quest for firm evidence of major quakes in prehistoric times.

For example, Brian Atwater of the U.S. Geological Survey in Seattle and Andrew Moore of the University of Washington detected and measured large deposits of sand that had covered a long-ago tidal flat at an inlet in Puget Sound called Cultus Bay some 25 miles north of Seattle. They found another deposit at West Point, which juts into Puget Sound less than five miles from the ancient Seattle fault.

The sand flats had obviously been dumped during a massive tsunami that sent its waves rolling through Puget Sound between 1,000 and 1,100 years ago. Scientists determined the time frame through radiocarbon techniques that date the roots of buried arrow grass plants that had grown in the quake-triggered deposits.

The tsunami, in turn, must have been caused when the quake abruptly lifted the land about 21 feet along the shore at Restoration Point, a cliff-like terrace that still exists north of the long, invisible Seattle Fault.

Along the shore south of the fault, there also is evidence that the land subsided with equal suddenness when the ancient quake struck, the scientists have found.

The rising shoreline caused tidal flats to change abruptly into freshwater swamps and meadows.

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