A Geologist's Thoughts on the Pacific Northwest Mega-Quake Story

As the aftershocks of Kathryn Schulz’s article The Really Big One in The New Yorker continue to reverberate across western Washington, Aspect is fielding questions from concerned family members, friends, and clients. Will everything west of I-5 really be “toast”? Should I be worried about a landslide on the hill in my backyard? Is my house going to hold up against a 9.0 quake?

[Source: USGS]

The Science is Sound 

The article has reactivated the region’s awareness of the Cascadia subduction zone fault. Aspect’s Senior Associate Engineering Geologist Dave McCormack says the science presented in Schulz’s article is sound, but that it should be tempered by the understanding that a 9.0 earthquake is a lower probability event in our region than for some parts of the Cascadia subduction zone that Schultz references. Specifically, the northern California to central Oregon section of the subduction zone may be near “due” based on the average rates of re-occurrence, according to McCormack.

For us in western Washington, our section of the subduction zone appears to be just entering the window of higher probability of rupture, and is still several hundred years from the average. Of course, as Schultz points out, averages can have tremendous variability and we can’t actually predict when the next “mega quake” will occur—but it is almost certain to occur sometime between now and the next 150 years. We don’t know when, but we can say that it will have extremely high consequences, as Schulz describes in the ominous scenarios that leave Seattle, Portland, and the coast in ruins. Based on the uncertainty of timing and magnitude of the consequences, scientists, engineers, and planners all agree it is important to take steps to protect our communities and ourselves.

We Have Other Faults Worth Knowing About

Puget Sound Area Faults

Dave notes there are many other faults crisscrossing western Washington that threaten to trigger earthquakes. Some, such as the deep earthquakes that have occurred in 1949, 1965, and 2001, have a much higher rate of reoccurrence than that of the Cascadia subduction fault, but with shorter and less intense shaking and lower consequences. Others include shallow crustal faults such as the Seattle fault, Tacoma fault, and Southern Whidbey Island fault. These have lower rates of reoccurrence than the Cascadia subduction zone earthquakes, but may have strong shaking for a minute or more and have even greater consequences for communities near those faults. Those shallow faults run closer to the surface, some right under urban, densely populated areas. It’s the intensity of the shaking, along with the possibility of frequent aftershocks, which causes most damage in a major earthquake. These quakes, the ones that are more likely to happen within the lifetime of a building or are likely to occur close to our urban areas, are a pressing concern for developers and the geologists and geotechnical engineers supporting their efforts. In 2007, Dave wrote about these other regional faults’ impact on development in an article for the Daily Journal of Commerce, Developers Must Dodge Newly Discovered Fault Hazards.

More Study and a Lot More Preparedness Needed

Dave says the science on seismic hazards in the region is only “in its adolescence” as geologists continue to collect data on the history of the area’s faults, determining their rate of reoccurrence, and magnitude of rupture. Such data will help determine the average probability for a quake to occur and inform design standards and zoning laws.

Hopefully Schulz’s article will spur further geological studies across Washington State and lead to more funding for hazard studies and emergency preparedness. After that, all we can do is try our best to inform others of the hazards and ways that they can protect themselves and strengthen our communities. 

More Resources about Seismic Hazards and Staying Prepared in Our Region