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Mapping Faults Beneath Puget Sound

Story by
Nancy Joseph

Just as doctors use ultrasound to diagnose what can't be seen, earth scientists now are using sound waves to reveal geological features and faults. Using this approach, the UW is participating in the largest, most complex experiment ever mounted by the U.S. Geological Survey (USGS) to investigate earthquake hazards in an urban setting--the Puget Sound region.

A student installs a seismometer and related equipment as part of the SHIPS project, which seeks to identify regional fault patterns.

A student installs a seismometer and related equipment as part of the SHIPS project, which seeks to identify regional fault patterns.

"Although geological evidence for past great earthquakes is clear, basic information about regional earthquake processes is lacking," explains Chief Scientist Michael Fisher with the USGS. "Knowing more about the structures underlying the region should give a clearer picture of the places most susceptible to violent shaking, slides, and damage during earthquakes. City planners will have a rational basis for allocating resources for such things as strengthening bridges, schools, and hospitals."

The study, known as the Seismic Hazards Investigations in Puget Sound (SHIPS), seeks to unravel complicated regional fault patterns from Olympia to Vancouver, B.C. Over a two-week period in March, researchers used the UW's 274-foot research vessel, a 240-foot Canadian ship, an airplane, and 240 seismometers spread over a 15,000-square-mile region, to collect data for the study.

As the UW's ship traversed the Puget Sound, it emitted bursts of compressed air that created sound waves able to penetrate sediments and deep rock layers. The signals rebounding off of deep rock bodies were then recorded in three ways: by sensitive devices on a mile-and-a-half streamer towed behind the ships, by monitors on the ocean floor, and by an extensive network of land-based seismometers, many of which were placed in basements and backyards of area residents. The seismometers are about the size of a soda can and come with a recorder and battery pack.

This massive undertaking required considerable preparation, including gaining cooperation from the community. "People readily agreed to placing seismometers on their property," say Ken Creager and Robert Crosson, professors of geophysics, who were responsible for deploying 60 seismometers across the greater Seattle/Tacoma area and east to the Cascades. Throughout the project, great care was taken to lessen potentially negative effects on marine life.

The effort involved 60 faculty and students, with one third of the students concurrently taking an undergraduate course tailored around the USGS experiment. "I saw this as an opportunity to teach students about earthquakes hazards in this area while giving them a sense of how science is done," says Creager, who taught the course.

Students were required to spend at least one full day in the field deploying the seismographs, collecting data, and making adjustments as necessary. They were paired with graduate students or faculty for the field work.

Although some researchers already have reported preliminary findings from the data, this effort is viewed as a long-term project. In fact, it may take six years to completely analyze the data collected in March.