Region’s faults under study

By Peggy Andersen

Associated Press

SEATTLE – A network of about 90 seismographs is being buried around Puget Sound – in flower beds, under decks, alongside businesses – for a three-month study of how the region responds to earthquakes large and small.

On Friday, the devices – in heavy gray plastic carrying cases – were lined up in a spare ground-level room at North Seattle Community College’s High Tech Learning Center.

Over the next several days, volunteers will plant them in a grid extending north to Everett, south to Tacoma and west to the Olympic Peninsula. The focus is the so-called Seattle Basin, an area about 17 miles wide, 50 miles long and five miles deep, said Thomas Pratt, one of the U.S. Geological Survey scientists leading the study.

The timing of the study, through early May, should not be interpreted as meaning anything is imminent.

The three-month window is simply “when we could get the money, people and instruments together,” Pratt said.

Without folks donating space in their yards and businesses, and contributing the electrical power to run the records – and the 20 volunteers working in pairs to bury the devices – “we would not able to do this study,” he said.

The devices will record shaking from the region’s small daily quakes and from larger temblors occurring around the world, where about a half-dozen magnitude 7 to 7.5 quakes occur each year.

The data will further define the basin and enable scientists to fine-tune seismic models of the region’s response to shaking.

The study – the fourth in a series called Seismic Hazards Investigation in Puget Sound, or SHIPS – is being conducted by the community college, the U.S. Geological Survey, the University of Washington and the National Science Foundation-financed Incorporated Research Institutions for Seismology, which is providing the instruments and will archive the data.

The three earlier parts of the study were:

The March 1998 Wet SHIPS, which used marine air guns towed through area waters by the UW research vessel RV Thompson, and 300 seismographs, to investigate faults, basins and the Juan de Fuca tectonic plate.

The four-day September 1999 Dry SHIPS investigation, which used explosions detonated in bore holes and more than 1,000 seismographs.

The March 2000 Kingdome SHIPS, which used demolition of the Kingdome as a source of seismic waves logged by 200 USGS recorders throughout Seattle.

The 1998 and 1999 studies showed the Seattle basin amplifies low-frequency ground shaking, but decreases the shaking at higher frequencies.

Analysis of the 1999 study data found low-frequency shaking – seismic waves that take 2 to 5 seconds between side-to-side motions – was eight to 12 times stronger in the basin than in the Olympic and Cascade mountains. Such waves could damage 20- to 50-story buildings, requiring stronger design.

“This increase in shaking in the Seattle basin is huge. It is like having the earthquake happen 10 times closer to you than it really did,” said Tom Brocher, another lead scientist involved in the studies.

The research will help scientists predict the behavior of seismic waves that get trapped in the area’s layer of shallow, weak deposits, Pratt said. “They’re bouncing around in there and … can’t get out.”

Reaction in shallow areas is “sometimes the hardest to understand,” he said. “We feel like we’ve got a pretty good hold of the deeper materials,” which are more compacted and stronger.

A film based on the Kingdome recordings showed the waves generated by the demolition “were prolonged and increased by the young riverbed deposits along the Duwamish River,” Brocher said. As a result, damage and soil liquefaction along the waterway during last year’s Nisqually Earthquake “didn’t surprise us.”

Each of the studies “has taught us something important and unexpected about the way the Puget Lowland responds to shaking from earthquakes,” Brocher said. “We’re hoping that our luck will hold for the Seattle SHIPS.”

Wet SHIPS provided “spectacular images” of the Seattle fault zone, considered the most dangerous fault for Seattle, he said.

It also helped sketch the so-called Tacoma fault zone, a large geologic structure that runs northwest from Tacoma. Evidence is still incomplete, but there are indications that this also represents an active fault zone capable of producing large quakes.

“The things that make the Seattle and Tacoma fault zones so dangerous (are), first, their proximity to major populations centers and, second, their proximity to the Seattle and Tacoma basins,” Brocher said.