Some large Pacific Northwest quakes could be limited in size by their location

November 13, 2003

Large, deep earthquakes have shaken the central Puget Sound region several times in the last century, and nerves have been rattled even more often by less-powerful deep quakes. New University of Washington research suggests the magnitude of these temblors might depend on just where beneath the Earth's surface they occur.

Events such as the 2001 Nisqually earthquake and large quakes in 1965 and 1949 happened in what is called the Wadati-Benioff zone, an area deep below the surface where the Juan de Fuca tectonic plate is sliding eastward beneath the North American plate. The two plates first meet on the ocean floor off the coast of Washington, Oregon and British Columbia.

If it turns out that such earthquakes are confined to the uppermost part of the Juan de Fuca plate, in its crustal layer, that means the magnitude of such quakes probably is limited to about 7, said Kenneth Creager, a University of Washington Earth and space sciences professor. However, the plate's crust is only about 3 miles thick, and the cold mantle layer that lies just beneath is much thicker, so a quake that occurs in both layers, in theory, could reach a magnitude of 8, he said.

For a more-detailed examination than previously possible, new tools were devised to analyze data from an experiment called Seismic Hazards Investigations in Puget Sound, or SHIPS. Leiph Preston, now a post-doctoral researcher at the University of Nevada, Reno, created the tools as part of his doctoral research at the UW.

Preston is the lead author of a paper detailing the new analysis published in the Nov. 14 edition of the journal Science. Co-authors are Creager; Robert Crosson, also a UW Earth and space sciences professor; Thomas Brocher with the U.S. Geological Survey in Menlo Park, Calif.; and Anne Tréhu of Oregon State University. The work was funded by the USGS and the National Science Foundation.

In 1998, the SHIPS experiment measured airgun explosions in Puget Sound and the Strait of Juan de Fuca. The detonations generated seismic waves that rebounded from underground structures and then were measured by 200 seismic recorders to generate a subsurface picture.

The boundary between the crust and the mantle in the Juan de Fuca plate created a natural reflector for the sound waves, Preston said, which helped establish a precise location for the boundary. When the depths of various earthquakes were superimposed, it turned out those east of the Olympic Mountains, where the reflector's depth reaches about 30 miles, occurred mainly in the Juan de Fuca plate's crust. Quakes west of the Olympics, where the reflector is shallower, occur primarily below it in the plate's mantle.

"The earthquakes and the reflector are so close to each other that it's taken us five years to be confident of this interpretation," Creager said.

The crust is largely composed of basalt, but when the plate reaches a depth of about 30 miles the basalt sheds water and is transformed into a denser rock called eclogite.

"The fluids seem to be the key, as they provide the lubricant that allows the two sides of a fault to slip past each other to produce an earthquake," Creager said.

If such an earthquake occurs both in the crust and the mantle, thus allowing it to reach a greater magnitude, it would pose a bigger risk to the heavily populated Interstate 5 corridor west of the Cascades. A magnitude 8 earthquake releases 30 times more energy than a magnitude 7 event, and even though the Juan de Fuca plate goes deeper into the Earth as it moves to the east, it also gets closer to being directly beneath the population centers of western Washington.

The Nisqually earthquake in 2001, which occurred beneath the Nisqually River delta near Olympia, measured magnitude 6.8, while the 1949 earthquake, also near Olympia, measured 7.1. The 1965 earthquake between Seattle and Tacoma registered 6.5.

Western Washington also can encounter subduction zone earthquakes, which occur infrequently offshore where the two plates come together and could perhaps measure magnitude 9. There also are major faults, such as the Seattle fault, that typically produce much shallower quakes with more pronounced shaking, so a high magnitude could bring widespread damage.

The new understanding of the nature of earthquakes in the Wadati-Benioff zone means more work like the SHIPS experiment is needed farther south, "particularly in the area of the Nisqually delta, where we have experienced these large earthquakes," Crosson said. "That will help us to understand the effects of such earthquakes on the entire region."
For more information, contact Preston at 775-784-1684 or; Creager at 206-685-2803 or; Crosson at 206-543-6505 or; Brocher at 650-329-4737 or; or Tréhu at 541-737-2655 or

University of Washington

Related Earthquake Articles from Brightsurf:

Healthcare's earthquake: Lessons from COVID-19
Leaders and clinician researchers from Beth Israel Lahey Health propose using complexity science to identify strategies that healthcare organizations can use to respond better to the ongoing pandemic and to anticipate future challenges to healthcare delivery.

Earthquake lightning: Mysterious luminescence phenomena
Photoemission induced by rock fracturing can occur as a result of landslides associated with earthquakes.

How earthquake swarms arise
A new fault simulator maps out how interactions between pressure, friction and fluids rising through a fault zone can lead to slow-motion quakes and seismic swarms.

Typhoon changed earthquake patterns
Intensive erosion can temporarily change the earthquake activity (seismicity) of a region significantly.

Cause of abnormal groundwater rise after large earthquake
Abnormal rises in groundwater levels after large earthquakes has been observed all over the world, but the cause has remained unknown due to a lack of comparative data before & after earthquakes.

New clues to deep earthquake mystery
A new understanding of our planet's deepest earthquakes could help unravel one of the most mysterious geophysical processes on Earth.

Fracking and earthquake risk
Earthquakes caused by hydraulic fracturing can damage property and endanger lives.

Earthquake symmetry
A recent study investigated around 100,000 localized seismic events to search for patterns in the data.

Crowdsourcing speeds up earthquake monitoring
Data produced by Internet users can help to speed up the detection of earthquakes.

Geophysics: A surprising, cascading earthquake
The Kaikoura earthquake in New Zealand in 2016 caused widespread damage.

Read More: Earthquake News and Earthquake Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to