Some Large Pacific Northwest Quakes Could Be Limited In Size By Their Location

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."