Folds at surface show ancient seismic stresses still at work in Washington


Scientists know that tectonic stresses have left dips and folds deep within the Earth's crust across a large swath of the Puget Sound region called the Seattle uplift. New research shows the same stresses have caused surface sediment deposits to mimic what has happened far below them.

In the last decade, geoscientists used sophisticated imaging techniques to understand how stress has accumulated from one to more than 20 miles deep in the Seattle uplift, an area that closely corresponds to the north-south extent of Vashon Island. The island lies in Washington state's Puget Sound, between Seattle and the Kitsap Peninsula. The images show how the crust folded back on itself because of the growing stress in the area.

Those images also provided the first evidence of the likelihood of a Tacoma fault zone at the southern edge of the uplift that is a counterpart to the Seattle fault zone at the northern edge. The new work also reinforces the notion of a Tacoma fault zone.

"I think our basic picture is correct. We're squeezing Washington north-south, things are buckling and breaking, and it's been happening for a long time," said Derek Booth, a University of Washington research associate professor of Earth and space sciences and of civil and environmental engineering.

He is the lead author of a paper describing the surface folds in the Seattle uplift, published in the June edition of Geology, a journal of the Geological Society of America. Co-authors are Kathy Troost, a UW research scientist in Earth and space sciences, and Jonathan Hagstrum of the U.S. Geological Survey in Menlo Park, Calif.

The Seattle fault zone cuts across Puget Sound through the southern part of Seattle, crossing just north of Vashon Island, and the apparent Tacoma fault zone runs just south of the island, in the area of Point Defiance. In between, for thousands of years the crust has continually been squeezed and buckled upward to relieve stress. The sediments that continued to accumulate on top of the folded crust were originally deposited as horizontal layers, but the stresses that buckled the deeper crust also have caused the surface to deform and tip.

It is not uncommon, Booth said, to see the surface crust tipped in one direction in one place and tipped in a different direction a half-mile away. Presumably there is a fold in between two such sites, one that coincides with a fold in the deeper crust. The researchers made such observations the length and breadth of the Seattle uplift, he said, focusing on beach cliffs where waves have left the deposits well exposed.

"We walked every bit of that coastline, all around Vashon, all around Maury Island, all the way from West Seattle to Point Defiance. On the Gig Harbor side we walked from the Tacoma Narrows to Southworth. It took several years," he said.

The researchers found that deformation of the Seattle uplift has been progressing for at least the last few hundred thousand years, and that older sediment deposits during that period showed greater deformation than the youngest exposed deposits. They calculated that during that time the north-to-south squeezing of the crust has been absorbed by the folding across the Seattle uplift at a rate of 0.25 to 1 millimeter per year, or one to four inches per century.

The presence of more pronounced deformation in the Point Defiance area of Tacoma also corresponds with previous work that projected the existence of the Tacoma fault zone.

The current work will add to the knowledge of the seismic hazards in the Puget Sound region by providing greater understanding of the region's tectonic stresses, the rate at which the stresses are accumulating, and the geologic processes at work, Booth said.

"It is information that is most directly related to our understanding of the Seattle fault, and for what appears to be a similar structure in the Tacoma area," he said.

The work also provides evidence that the fault in the Tacoma area has been active in the last million years, which is relatively recently on geological time scales, Booth said.

"It does not mean that our assessment of earthquake hazards over human time scales has suddenly increased," he said. "It does remind us, however, that we live in a seismically active area, where preparation for future earthquakes will always be warranted."

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Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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