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Spokane, Washington  Est. May 19, 1883

Geologists Say Seattle Fault Is Bigger Than Expected Exploration Data Collected By Oil Companies, And Kept Secret For 20 Years, Sheds Light On Puget Sound Earthquake Faults

Associated Press

The Seattle Fault, a major geological fault that triggered a massive, land-heaving earthquake more than 1,000 years ago, also passes directly under Tacoma and Federal Way, geologists have discovered.

Previously, researchers have said the fault runs under downtown Seattle near the Kingdome and west across Puget Sound.

A new study indicates the fault also dips southward, diving beneath Tacoma and underlaying a large block of the Puget Sound basin, said geophysicist Thomas Pratt of the U.S. Geological Survey.

When the fault slips, that entire block and all the cities on it will feel the jolt, he said Tuesday.

“Tacoma is going to be a part of that - the big one that destroys Seattle when the Seattle Fault goes,” he said.

Using exploration data collected by oil companies in the 1970s and kept secret for years, Pratt and his co-workers conducted the most detailed study yet of the Seattle Fault.

Their latest work, which will be published soon, also revealed several other small faults in the Puget Sound basin.

Three months ago, the same research team reported the discovery of a major fault along Whidbey Island.

“We now have a model that shows the geometry of the Seattle Fault and links it to these other faults,” Pratt said. “There’s a whole series of them up and down the coast … like cracks in a window.”

The epicenter of a magnitude 5.0 quake that jolted the region Saturday night was near the edge of the Seattle Fault, at the eastern tip of Maury Island, where a series of lesser faults crisscross the Tacoma area.

Initial evidence indicates the quake, the largest to hit the region in 30 years, did not occur on the Seattle Fault itself, University of Washington scientists said at a news conference Tuesday.

Scientists reported in 1992 that the Seattle Fault is believed responsible for a magnitude 7 earthquake 1,100 years ago that jacked up land masses, spawned a huge tidal wave and sent forests sliding to the bottom of Lake Washington.

They don’t know whether those disasters reached as far south as Tacoma, Pratt said Tuesday. Nor do they know how many smaller, but potentially damaging, earthquakes have occurred without leaving such dramatic tracks, USGS geologist Brian Atwater said.

To better understand the fault and more accurately predict the size and frequency of earthquakes it could trigger, geologists hope to study it more intensively over the next few years, said Craig Weaver, head of the Survey’s National Earthquake Hazard Reduction Program.

They would like to collect more data like that the oil companies accumulated when they were scouring the region for profitable oil deposits. The corporations found nothing worth drilling for, which is why they were willing to share their work with Pratt and his colleagues.