UW Today

October 22, 2013

Study: Quake-triggered landslides a significant hazard in Seattle

The next big earthquake on the Seattle fault could trigger destructive landslides in the city, potentially affecting a much larger area than previously thought, and in areas outside those currently considered to be landslide prone, a new University of Washington-led study shows.

“A major quake along the Seattle fault is among the worst-case scenarios for the area, since the fault runs just south of downtown.” said Kate Allstadt, a UW doctoral student in Earth and space sciences. “Our study shows the need for dedicated studies on seismically induced landsliding.”

Allstadt is the lead author of a paper documenting the research, published online Oct. 22 by the Bulletin of the Seismological Society of America. Co-authors are John Vidale, a UW professor of Earth and space sciences and Allstadt’s doctoral adviser, and Arthur Frankel of the U.S. Geological Survey, which funded the research.

The research offers a framework for simulating hundreds of earthquake scenarios for the Seattle area.

Map shows the path of the Seattle fault.

This map traces the course of the Seattle fault across Puget Sound, through the city and across Lake Washington to the Eastside.

While the region is vulnerable to deep earthquakes, shallow earthquakes and catastrophic quakes in the Cascadia subduction zone off the Pacific Northwest coast, a shallow crustal quake on the Seattle fault would likely be most damaging to the city.

The Seattle fault crosses Bainbridge Island and cuts across West Seattle and Beacon Hill, just south of downtown, then crosses Lake Washington to the eastern suburbs and the Cascade foothills. The last major quake on that fault was about 900 A.D., and scientists have documented that it triggered giant landslides that caused large tracts of forest land to slide to the bottom of Lake Washington.

The Seattle Basin that underlies much of the city amplifies ground motion and generates strong seismic waves that tend to increase the duration of the shaking. Allstadt said the hazard of landslides triggered by earthquakes is very high and warrants greater attention, in Seattle and in other areas prone to the phenomenon.

“This finding is strongly supported by some recent earthquakes such as the Wenchuan earthquake in China in 2008, where thousands were killed by landsliding, including an entire school filled with children,” she said.

Another significant finding is how much ground motion governed the extent and distribution of landslides. In the case of Seattle, ground motion would be amplified by the sedimentary basin beneath the city and in the shallow soils near the surface, and energy would be focused in the direction that the fault breaks.

For the study, the researchers divided the city into a grid of 210-meter cells and simulated ground motion for a magnitude 7 Seattle fault earthquake. Then they subdivided the area into 5-meter cells, applying anticipated amplification of shaking because of shallow soil layers.

“I was surprised to find that a third of the landslides triggered in our simulation were outside of areas currently defined as being prone to landsliding,” Allstadt said.

That is because existing landslide hazard zones are defined primarily by considering only landslides triggered by water, the most important factor when analyzing the potential landslide impact. However, it is also important to look at the details of amplified surface ground motion, since that is the second-most important factor, Allstadt said. The new research suggests more studies are needed to fully understand the hazards from landslides triggered by ground motion.

The authors note their work is just one randomized earthquake scenario of many that could strike the city. The results do not show which areas of the city might be most affected by a Seattle fault earthquake, but they do illustrate the extent of landsliding that could be expected.

The study suggests the southern half of the city and the coastal bluffs, many of which are developed, would be hardest hit. Depending upon the water saturation level of the soil at the time of the earthquake, several hundred to thousands of buildings could be affected citywide. The analysis suggests landslides could also affect some inland slopes, threatening key transit routes and impeding recovery efforts.

“There’s a kind of haunting precedence that tells us that we should pay attention to a large earthquake on this fault because it happened in the past,” said Allstadt.

 ###

This story is adapted from a news release by Nan Broadbent of the Seismological Society of America.