Geophysicists from four institutions, including the University of Washington, are launching a second round of the Seismic Hazards Investigations in Puget Sound (SHIPS) project that started last year.
This year’s program, called “Dry SHIPS,” will be entirely land based. A 60-mile “Seattle line” of 1,000 seismic recorders will cross the Puget Sound lowland, from Olympic National Park through Kitsap County, Bainbridge Island, Seattle and Redmond to the Cascade foothills. The recorders will document seismic waves generated by explosions in deep boreholes along the line.
Surveyors have recruited landowners and property managers, such as timber companies or local, state and federal agencies, to place seismic recorders on their property for four or five days.
Plans call for detonations in deep boreholes at 2- to 3-mile intervals along the seismic line. The detonations, carefully placed so the waves will not damage man-made or natural structures or disturb people, will occur in early morning hours to provide the best-quality data. After the work is completed, borehole and detonation sites will be restored to their original appearance along state Department of Ecology guidelines.
The 1998 “Wet SHIPS” experiment aboard the UW research vessel Thomas G. Thompson used 12 air guns to generate underwater seismic waves that were recorded by land- and seafloor-based seismometers. The Thompson mostly cruised north to south to collect seismic images beneath the Puget Sound lowland. Results of last year’s experiment are improving scientists’ understanding of the locations of subsurface crustal faults that can produce earthquakes in the Puget Sound area. They also are giving clues about how local geological structure might increase or reduce shaking in a strong earthquake.
With unprecedented clarity, wet SHIPS mapped the three-dimensional contrasts in the speed of sound produced by earthquake motion along crustal faults in the Puget Sound lowland, said Thomas Brocher of the U.S. Geological Survey in Menlo Park, Calif., who is heading the Dry SHIPS project. Maps produced by the project show that crustal faults found at the surface extend deep into the subsurface, a finding that helps earthquake seismologists define the geometry of subsurface faults.
For example, the Seattle fault is shown in the images to be a complex deformation, several miles wide and generally running east to west, that typically brings harder volcanic rocks over the softer sedimentary rocks that fill the Seattle basin. Wet SHIPS data provided the first evidence that the Seattle basin has at least three different subbasins separated by subsurface structures running north and south, Brocher said. At least two of those subbasins lie along mapped faults.
“Wet SHIPS gave us the big picture of the geological structures in the Puget lowland associated with crustal earthquakes,” Brocher said, “and Dry SHIPS will give us a close up look at the Seattle basin and whether the basin prolongs and enhances strong shaking during local earthquakes.”
SHIPS is a cooperative project among the UW, the U.S. Geological Survey, the University of Texas at El Paso, Oregon State University and the Geological Survey of Canada, the first four of which are conducting the “dry” phase of the project.
Previous seismic, geophysical and geological studies of the Seattle basin, a major geological structure directly beneath the Seattle area, indicate it is as deep as 9 kilometers (5.5 miles) – equivalent to the height of Mount Everest. The Seattle fault, an active fault that has been the source for some major Seattle-area earthquakes, forms the basin’s southern boundary. Other large sediment-filled basins lie beneath Tacoma and Everett. Computer simulations suggest these basins might amplify and prolong strong ground shaking during earthquakes in the Puget Sound area.
Researchers expect the SHIPS data to provide a complete three-dimensional view of the Seattle basin, allowing them to model the effects of strong shaking on the basin over which much of Seattle, Bellevue, Kirkland and Redmond are built. Similar studies have been conducted in the Los Angeles basin and in the San Francisco Bay area.
During eight days of fieldwork during the last half of September, explosions in deep boreholes will generate seismic waves. Echoes from the explosions will be used to produce images of the subsurface layers to help understand the distribution of rock and faults at earthquake depths. Scientists also will measure the speed of sound through the rocks beneath the Puget Sound lowland to see how earthquake waves would travel through those materials.
The seismic signals will be recorded by about 1,000 seismometers, installed by a team of 70 scientists, students, and volunteers at 110-yard intervals along the “Seattle line.”
The sensors will be buried in holes a foot wide and a foot deep to ensure accurate recording of seismic waves generated by the explosions. Most of the seismometers are designed to record man-made seismic waves, but at 40 sites special seismometers will be used in an attempt to record any local earthquakes that might occur during the study. These seismometers are very small (the largest can fit within a large picnic cooler, and most will be smaller) and only record signals from their ground-motion sensor.
For more information, contact Thomas M. Brocher, U.S. Geological Survey, Menlo Park, Calif., (650) 329-4737 or http://rmmcweb.cr.usgs.gov/~dcatts/hazards/reports/ships.html