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THE "SHIP"ping NEWS — USGS Scientists Launch Earth Shaking Study in the Puget Lowlands
Released: 1/25/2002

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Pat Jorgenson 1-click interview
Phone: 650-329-4011

For more information visit the UGGS and UW SHIPS web sites at: http://spike.geophys.washington.edu/SEIS/SHIPS/ http://geohazards.cr.usgs.gov/pacnw/ships/

In an effort to understand how future earthquakes will affect the central Puget Sound lowlands, U. S. Geological Survey scientists, working from North Seattle Community College, on January 25 will begin installing 90 seismographs to measure ground shaking throughout the region.

The goal of the study, the fourth phase of a project called Seismic Hazards Investigation in Puget Sound, or SHIPS, is to determine how the Seattle sedimentary basin influences shaking during large earthquakes, such as the Feb. 28, 2001, Nisqually earthquake. From now through early May, the recorders will measure the tiny vibrations caused by the small earthquakes that occur almost daily in the Puget Sound area, and from distant large earthquakes. The small amounts of shaking recorded during these earthquakes will give clues to how the area will shake during much larger earthquakes.

"Studies conducted elsewhere have shown that areas that show amplified shaking during small and distant earthquakes also amplify the strong shaking produced by large, local events," said Thomas Pratt, one of the USGS scientists leading the study. "During the Nisqually earthquake, for example, many areas that produced the strongest shaking had already been identified from previous studies of weak shaking during small earthquakes."

The 90 seismographs will be placed on the property of residents and businesses throughout the Puget Sound region. Thirty of them will be deployed along the same line as in the 1999 Dry SHIPS study in an effort to confirm shaking patterns seen then. The remaining 60 recorders will be spread out in a grid from Everett to Tacoma. USGS personnel and volunteers will install the seismographs on sites that property owners have donated sites that have electrical power for the recorders. No underground detonations will be used. "We have had a tremendous response to our call for volunteers to let us put the seismographs in their flower beds, beneath their decks, and along side their houses and businesses. Without the donation of these sites and the willingness of 20 community volunteers to give up their weekend to install the seismometers we would not be able to do this study," Pratt said.

Organizations participating in the study are the USGS, the University of Washington, North Seattle Community College and the NSF funded Incorporated Research Institutions for Seismology (IRIS) that provided the instruments and will archive the data produced. North Seattle Community College is the newest addition to the SHIPS partnership and is looking forward to future collaborations. "The faculty and students of North’s Science and Math Division look forward to having the SHIPS project on our campus," said Tom Griffith, division dean. "Classes can visit the project and in the future we anticipate that some of our students may become interns working with scientists from the U.S. Geological Survey."

Previous phases of SHIPS have provided scientists with a much greater understanding of how the region will react to strong shaking. The first phase, called Wet SHIPS, used marine air guns towed by UW’s RV Thompson oceanographic research vessel and over 300 seismographs in March 1998 to investigate crustal faults and sedimentary basins throughout the Puget Sound Lowland. Dry SHIPS, September 1999, investigated the Seattle basin using explosions detonated in boreholes and over 1,000 seismographs. The third phase in March 2000, Kingdome SHIPS, used the demolition of the Kingdome as a source of seismic waves that were recorded by 200 recorders deployed by the USGS throughout Seattle. "Each SHIPS study has taught us something important and unexpected about the way the Puget Lowland responds to shaking from earthquakes. We’re hoping that our luck will hold up for Seattle SHIPS," said Tom Brocher, the other USGS scientist leading the study and a lead scientist on the previous SHIPS studies.

The experiments in 1998 and 1999 showed that the Seattle basin amplifies low-frequency ground shaking but decreases the shaking at higher frequencies. The Seattle basin is a 50- by 17-mile depression, extending from Edmonds south to Interstate 90, filled with a five-mile-thick sequence of rocks comprised of sand, silt and mud. The basin lies under the Puget Sound Lowland from the Cascade Range to the Olympic Mountains.

The Seattle basin has been the target of several previous studies by USGS and University of Washington scientists because similar sedimentary basins have been known to amplify shaking during earthquakes. "The Wet SHIPS study set the table for us", Brocher said. "Wet SHIPS defined the structure of the Seattle and Tacoma basins, as well as provided new insight into the geometry of the Seattle fault zone, considered to be the most dangerous fault for Seattle. Wet SHIPS provided spectacular images of the Seattle fault zone and how it reaches maximum strength in Seattle."

Wet SHIPS also revealed a large geologic structure trending northwest from Tacoma. Although their evidence remains sketchy at this point, Brocher said there is evidence that this large geologic structure represents an active fault zone capable of producing large magnitude earthquakes. The mapping of this structure, named the Tacoma fault zone by Brocher and his collaborators, represents the most unexpected and dramatic result of the Wet SHIPS experiment. "The things that make the Seattle and Tacoma fault zones so dangerous, is first, their closeness to Seattle and Tacoma, and second, their closenss to the Seattle and Tacoma basins," Brocher said. "Increased shaking caused by seismic waves in sedimentary basins is believed to have caused intense damage in Mexico City during a 1985 earthquake and in some areas of Los Angeles during the 1994 Northridge earthquake," he added.

Scientists happened to have 30 seismographs deployed across the Seattle basin for the Dry SHIPS experiment in 1999 when a magnitude-7.6 earthquake struck the island of Taiwan. The seismographs recorded the waves from that earthquake and from magnitude 2.1 and 2.8 earthquakes beneath the Cascade Mountains, even though the shaking was too small to be felt by residents. The scientists also analyzed shaking from the series of underground explosions they detonated during the four-day Dry SHIPS study.

Analysis of the seismic waves from the Taiwan earthquake, local earthquakes and Dry SHIPS explosions showed that low frequency shaking was eight to 12 times stronger in the Seattle basin than on bedrock in the Olympic Mountains and the Cascade Range. The strongest increase in shaking was for waves that take two to five seconds between the side-to-side motions. Such low-frequency waves could be damaging to 20- to 50-story buildings and other large structures, so engineers must design them to withstand the greater shaking, Brocher said. "This increase in shaking in the Seattle basin for weak shaking levels is huge. It is like having the earthquake happen 10 times closer to you than it really did," he said. "We don’t expect to see such large increases during the next ’Big One,’ but we do expect to see some increase." In contrast, he said, higher-frequency waves that are more damaging to smaller buildings, such as houses, showed less of an increase, or even a decrease, in the shaking in the Seattle basin.

"Results from the 1999 Dry SHIPS study gave us a great first look at the pattern of shaking over the entire Seattle basin, but the small number of seismographs left us without information from a lot of areas," Pratt said. "The Seattle SHIPS study will have an extensive grid of seismographs, allowing us to map the ground shaking over the entire basin rather than just along a single strip."

Kingdome SHIPS provided an even more detailed look at the variation in ground shaking, made possible by knowing in advance the time and place of the Kingdome demolition. According to Brocher, "A movie made from our detailed recordings shows that the seismic waves produced by the demolition were prolonged and increased by the young river bed deposits along the Duwamish River. As we all know, there was concentrated damage and soil liquefaction along the Duwamish River during the Nisqually earthquake, which didn’t surprise us based on the movie we had made from our Kingdome SHIPS recordings."

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