Fifty simulations of the 'Really Big One' show how a 9.0 Cascadia earthquake could play out

October 23, 2017, University of Washington
Seismogram being recorded by a seismograph at the Weston Observatory in Massachusetts, USA. Credit: Wikipedia

One of the worst nightmares for many Pacific Northwest residents is a huge earthquake along the offshore Cascadia Subduction Zone, which would unleash damaging and likely deadly shaking in coastal Washington, Oregon, British Columbia and northern California.

The last time this happened was in 1700, before seismic instruments were around to record the event. So what will happen when it ruptures next is largely unknown.

A University of Washington research project, to be presented Oct. 24 at the Geological Society of America's annual meeting in Seattle, simulates 50 different ways that a magnitude-9.0 earthquake on the Cascadia subduction zone could unfold.

"There had been just a handful of detailed simulations of a magnitude-9 Cascadia earthquake, and it was hard to know if they were showing the full range," said Erin Wirth, who led the project as a UW postdoctoral researcher in Earth and space sciences. "With just a few simulations you didn't know if you were seeing a best-case, a worst-case or an average scenario. This project has really allowed us to be more confident in saying that we're seeing the full range of possibilities."

Off the Oregon and Washington coast, the Juan de Fuca oceanic plate is slowly moving under the North American plate. Geological clues show that it last jolted and unleashed a in 1700, and that it does so roughly once every 500 years. It could happen any day.

Wirth's project ran simulations using different combinations for three key factors: the epicenter of the earthquake; how far inland the earthquake will rupture; and which sections of the fault will generate the strongest shaking.

Simulated ground motions and seismometer readings in four cities for the scenario that generated the most shaking in Seattle. In this scenario the hypocenter was far away, but the rupture spread inland and up the coast, causing waves to pile up toward Seattle. Credit: Nasser Marafi/University of Washington

Results show that the intensity of shaking can be less for Seattle if the epicenter is fairly close to beneath the city. From that starting point, seismic waves will radiate away from Seattle, sending the biggest shakes in the direction of travel of the rupture.

"Surprisingly, Seattle experiences less severe shaking if the epicenter is located just beneath the tip of northwest Washington," Wirth said. "The reason is because the rupture is propagating away from Seattle, so it's most affecting sites offshore. But when the epicenter is located pretty far offshore, the rupture travels inland and all of that strong ground shaking piles up on its way to Seattle, to make the shaking in Seattle much stronger."

The research effort began by establishing which factors most influence the pattern of ground shaking during a Cascadia earthquake. One, of course, is the epicenter, or more specifically the "hypocenter," which locates the earthquake's starting point in three-dimensional space.

Another factor they found to be important is how far inland the fault slips. A magnitude-9.0 earthquake would likely give way along the whole north-south extent of the subduction zone, but it's not well known how far east the shake-producing area would extend, approaching the area beneath major cities such as Seattle and Portland.

The third factor is a new idea relating to a subduction zone's stickiness. Earthquake researchers have become aware of the importance of "sticky points," or areas between the plates that can catch and generate more shaking. This is still an area of current research, but comparisons of different seismic stations during the 2010 Chile earthquake and the 2011 Tohoku earthquake show that some parts of the fault released more strong shaking than others.

Wirth simulated a magnitude-9.0 earthquake, about the middle of the range of estimates for the magnitude of the 1700 earthquake. Her 50 simulations used variables spanning realistic values for the depth of the slip, and had randomly placed hypocenters and sticky points. The high-resolution simulations were run on supercomputers at the Pacific Northwest National Laboratory and the University of Texas, Austin.

Simulated ground motions and seismometer readings in four cities for the scenario that generated the least shaking in Seattle. In this scenario the hypocenter was fairly close to Seattle, beneath the northwest tip of Washington state. Credit: Nasser Marafi/University of Washington

Overall, the results confirm that coastal areas would be hardest hit, and locations in sediment-filled basins like downtown Seattle would shake more than hard, rocky mountaintops. But within that general framework, the picture can vary a lot; depending on the scenario, the intensity of shaking can vary by a factor of 10. But none of the pictures is rosy.

"We are finding large amplification of ground shaking by the Seattle basin," said collaborator Art Frankel, a U.S. Geological Survey seismologist and affiliate faculty member at the UW. "The average duration of strong shaking in Seattle is about 100 seconds, about four times as long as from the 2001 Nisqually earthquake."

The research was done as part of the M9 Project, a National Science Foundation-funded effort to figure out what a magnitude-9 earthquake might look like in the Pacific Northwest and how people can prepare. Two publications are being reviewed by the USGS, and engineers are already using the simulation results to assess how tall buildings in Seattle might respond to the predicted pattern of shaking.

As a new employee of the USGS, Wirth will now use geological clues to narrow down the possible scenarios.

"We've identified what parameters we think are important," Wirth said. "I think there's a future in using geologic evidence to constrain these parameters, and maybe improve our estimate of seismic hazard in the Pacific Northwest."

Explore further: Study confirms large earthquakes along Olympic Mountain faults

Related Stories

Learning from past quakes

July 31, 2015

Computer simulations of ground shaking which replicates the historic, destructive 1811-1812 New Madrid seismic zone (NMSZ) earthquakes on the Tennessee-Missouri border suggest that future activity in the region could produce ...

Recommended for you

Slow flow for glaciers thinning in Asia

December 11, 2018

Providing water for drinking, irrigation and power, glaciers in the world's highest mountains are a lifeline for more than a billion people. As climate change takes a grip and glaciers lose mass, one might think that, lubricated ...

Increased snowfall in Antarctica buffers sea-level rise

December 11, 2018

A team of scientists from NASA and British Antarctic Survey (BAS), describes how analysis of 53 ice cores collected from across Antarctica reveals snowfall increased during the 20th century and mitigated sea-level rise by ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

BendBob
1 / 5 (2) Oct 23, 2017
Gee whiz, Batman, that was earth-shaking news. Should we dig a new tunnel for the Batmobile? Maybe down near the US/Mexico border might work, no one would look there!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.