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Scientists Describe Upcoming Earthquake Research Project Set for Puget Sound
Released: 5/27/1997

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: 415-329-4000

Scientists from the U.S. Geological Survey, the Geological Survey of Canada and several universities in both countries are preparing for a 1998 scientific project that will greatly advance the understanding of hazards from shallow earthquakes in the region of Puget Sound and Georgia Strait.

The study, known as Seismic Hazards Investigations in Puget Sound (SHIPS), is part of a comprehensive research effort to unravel the complicated regional fault pattern. The SHIPS project involves a marine airgun survey of many of the waterways of Puget Sound and Georgia Strait, giving United States and Canadian scientists an unprecedented opportunity to investigate crustal earthquake problems in an urban setting.

During the SHIPS cruise, scheduled for March 1998, the Canadian research ship, Tully, and the University of Washington ship, Thomas Thompson, will record seismic reflection and refraction data from Olympia northward into Georgia Strait and then westward through the Straits of Juan de Fuca. "There are few major urban areas where crustal faults can be studied with as much detail as in the Seattle area," noted USGS researcher Mike Fisher.

Although geologic evidence for past great earthquakes in the Pacific Northwest is clear, basic information about crustal earthquake processes is lacking. Recent research by the USGS and other organizations has shown that the Seattle area was rocked by a major earthquake about 1000 years ago. This earthquake, which scientists believe occurred on the Seattle fault, raised a beach on Bainbridge Island 22 feet, caused landslides in Lake Washington, and generated a tsunami in Puget Sound that surged northward to West Point and Whidbey Island. The main question for residents of the Seattle area is: How often do these major earthquakes occur? A major earthquake has a magnitude greater than magnitude 7.

"Large earthquakes occur along the Pacific Coast because rocks under the Pacific Ocean west of Washington and southern British Columbia are thrust eastward beneath the North American continent, in a process called subduction," Fisher said. "Two direct consequences of this relentless process are volcanic eruptions and earthquakes. As oceanic rocks move beneath the continent, shallow rocks shudder and lurch to accommodate earth forces that are deep seated and affect broad areas."

In this geologic environment earthquakes occur in three source zones: 1) the subduction interface, which is the contact between the bottom of the continent and the top of the downgoing oceanic rocks; 2) a deep zone beneath central Puget Sound where the downgoing oceanic rocks push beneath the North American continent; and 3) a shallow zone that includes faults just beneath urban areas.

Earthquakes generated along the Cascadia subduction interface can attain enormous magnitudes (8.5 to greater than 9) because this fault has a huge area. The last subduction-zone earthquake occurred on January 25, 1700. This earthquake generated a tsunami that has been dated by its arrival at several sites in Japan. In addition, precise dating of trees killed along the coast has shown that they died in the winter of 1699-1700.

The Puget Sound basin was hit a major earthquake in 1949 when a magnitude 7.1 earthquake occurred in the deep zone beneath Puget Sound. Eight people died as a result of the shaking from that earthquake. Other damaging earthquakes could occur along faults that are known to pass directly under urban areas. The Seattle fault, for example, threatens downtown Seattle and neighboring port facilities. Three other magnitude-7 earthquakes have struck the region of Puget Sound and Georgia Strait (Seattle and Tacoma in 1949, Vancouver Island in 1918 and 1946). All were similar in size to the January 15, 1995 Kobe, Japan, earthquake, which demonstrated the vast destruction possible in a major urban area when an earthquake occurs on a shallow, crustal fault.

The degree of seismic hazard to the region of Puget Sound and Georgia Strait is thought to be high, but there continues to be much uncertainty surrounding the role of crustal faults. Much of the diverse earthquake environment around Puget Sound lies obscured beneath young glacial deposits, water, city and forest. In particular, the configuration of deep faults that could generate large earthquakes is largely unknown. To close this information gap and understand more thoroughly the earthquake processes in this complex region, the scientists will collect deep seismic data by artificially produced sound waves that travel through deep rock layers and are recorded on the surface by sensitive instruments. The technique is similar to ultrasound scans that doctors use to examine the human body.

During the SHIPS project seismic data will be collected along three generally east-west and two north-south lines. For the east-west lines through Seattle and Tacoma, the source of seismic waves will be explosives detonated in holes drilled onshore, whereas for the north-south lines and the east-west line through the Strait of Juan de Fuca, the seismic source will be bursts of compressed air in the water. In addition, the scientists will obtain three-dimensional data from offshore seismic sources and onshore receivers, both of which will be widely distributed around the sound.

SHIPS will use a marine airgun array, and to lessen potentially negative effects on marine biota in general, and marine mammals in particular, the airgun array’s main power emission will be at frequencies that are lower than the sensitive hearing range of marine mammals. In addition, the survey will be conducted in March, when the fewest number of marine mammals are in the sound. The SHIPS project will be conducted according to requirements established by the National Marine Fisheries Service of the National Oceanic and Atmospheric Administration (NOAA).

Fisher said the main goals of SHIPS are to: 1) improve our understanding of crustal faults in the region of Puget Sound and Georgia Strait, 2) link the known, shallow structure of the Seattle and other faults to features at great depth where many earthquakes occur, 3) better determine the geometry of the Seattle sedimentary basin, 4) provide an improved understanding of the seismic velocities in rocks at depth (this information will then be used by researchers at the University of Washington to improve estimated earthquake locations), and 5) examine the effect of the deep sediment-filled Seattle basin on amplifying strong ground motions at sites across the region. From these observations, USGS scientists will be better able to assess the degree of hazard from crustal earthquakes in Puget Sound. Fisher noted that hazard mitigation is the ultimate goal of the project; no attempt is being made to predict where and when future earthquakes will occur.

Fisher added that information from onshore seismometers will reveal where deep rocks may focus earthquake waves at the surface and where surface sediment may be expected to amplify ground motion. >From these data it will be possible to produce a map showing potential ground motions corrected for local geologic site conditions. These maps will be of interest to those struggling to set priorities for seismic retrofitting of existing buildings and structures.

The USGS and many university collaborators began investigating regional earthquake hazards in 1991 by determining the detailed geologic structure of the Cascade foothills east of Seattle (where the 1995 Duvall earthquake occurred). In 1995 the USGS led a major seismic investigation to examine possible faults along a seismic line across the White Pass highway. In addition, several previous marine experiments in Puget Sound have identified the Seattle fault and the South Whidbey Island fault as possible locations of major crustal earthquakes. Investigation shifted offshore in 1996, when the German research ship Sonne hosted a cooperative experiment involving USGS and German scientists. Finally during summer, 1997, the USGS will conduct a detailed aeromagnetic survey of Puget Sound that will help pinpoint the location of possible crustal faults.

"The large earthquakes that we know have occurred along the Pacific Coast in the past have ignored political boundaries," said Roy Hyndman, a geophysicist with the Canadian Geological Survey. "When the next large earthquake occurs, it too will not discriminate along national borders, so it is in all of our interests to work together on this project."

In addition to scientists from the USGS, SHIPS team members are from Oregon State University, the University of Washington, the Geological Survey of Canada, the University of British Columbia, the University of Victoria and the University of Texas, El Paso.

Editors: For more background information on Pacific Northwest earthquakes, the SHIPS project and similar experiments conducted by the USGS and other scientists in various parts of the world, call the USGS Public Affairs Office at 415-329-4000, or Mike Fisher at 1-800-403-0624.

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