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Around 1860, a naturalist on a railroad survey noted many dead trunks of Western Redcedar standing along the shores of Willapa Bay on the southern Washington coast. More than a hundred years later, in the late 1980’s, scientists discovered what killed the trees. Their roots had been submerged in saltwater when forested land dropped below sea level during a great earthquake (magnitude 8 or more) about 300 years ago. With that discovery came the realization that a great earthquake could strike the region again. Pacific Northwest Quakes The Pacific Northwest sits atop the Casca- dia subduction zone, where two great slabs of the Earth’s crust—called tectonic plates— are converging. The Juan de Fuca plate on the west, made up of dense oceanic crust, is shoving eastward beneath the North Ameri- can plate, made of lighter, continental crust. The grinding of plates past each other in subduction zones sometimes generates deep earthquakes—originating tens of kilometers beneath the earth’s surface. Subduction-zone earthquakes can be huge; the magnitude 9.2 earthquake that struck Alaska in 1964 is an example. Until recently, scientists consid- ered the Cascadia subduction zone to be moving smoothly and unlikely to generate big earthquakes. But by the late 1980’s, evi- dence such as drowned forests, tsunami deposits, and sediments liquefied by shaking showed that the Cascadia subduction zone had produced great earthquakes in the past and could do so again. Added to that was evidence in the Puget Sound area for past large earthquakes generated on faults near the surface, such as the Seattle fault (see map and figure on back page). These discov- eries inspired the U.S. Geological Survey (USGS) and other institutions to study the Pacific Northwest more intently, to assist decision makers in assessing and mitigating earthquake hazards there. Experiments in Puget Sound Earth scientists from the USGS and eight cooperating institutions (see list on back page) are currently conducting a study called SHIPS, or Seismic Hazards Investigation in Puget Sound. Though scientists cannot yet predict when earthquakes will strike, they can deter- mine where earthquakes will strike and where the shaking will be most violent. To this end, SHIPS scientists are using sound waves to build up images of the sediment Scientists Use Sound Waves to Probe Earthquake Hazards in the Puget Sound Area U.S. Department of the Interior U.S. Geological Survey USGS information sheet August 1999 “Geological evidence for past great earthquakes is clear, but basic information about regional earthquake processes is lacking. Knowing more about the structures underlying the region should give a clearer picture of the places most susceptible to violent shaking, landslides, and structural damage during earthquakes. City planners will have a rational basis for allocating scarce resources for such things as strengthening bridges, schools, and hospitals.”—Michael A. Fisher, U.S. Geological Survey E merging evidence shows that damaging earthquakes of magnitude 8 or greater could strike the Pacific Northwest, a region that stretches from northern California through Oregon and Washington into British Columbia, Canada. This region includes large population centers such as Portland, Seattle, and Vancouver, Canada. To study the earthquake threat in and around Seattle, the U.S. Geological Survey (USGS) and eight cooperating institutions launched SHIPS—Seismic Hazards Investigation in Puget Sound. SHIPS scientists are using sound waves to probe sediment and rock layers beneath the Puget Sound area. The faults and other features they uncover will help them determine where the worst earthquake damage is likely to occur. University of Washington’s Research Vessel Thomas G. Thompson towed an array of airguns (to left of photo) through Puget Sound beside downtown Seattle in March 1998. Bursts of sound from the airguns bounced off rock layers deep beneath the earth’s surface. Returning echoes built up a picture of the rock layers and faults that cut them. These data will be combined with on-land SHIPS data collected in September 1999 to create a complete three-dimensional view of the sediment and rock layers beneath the Seattle area. (Photograph by David L. Carver, U.S. Geological Survey)