A Portland-area study finds single-family homes do well and that upgrades to older commercial buildings could slash both casualties and damage.

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There are plenty of grim statistics in a new analysis of potential damage from a Cascadia megaquake, but the report also includes several surprisingly hopeful findings.

Among them:

• Most single-family homes, with their flexible wood frames, should hold up well to the type of moderate ground shaking expected.

• The bulk of injuries will be minor, which means training more people in basic first aid could have a big payoff.

• Even moderate seismic upgrades to older commercial buildings would significantly reduce the number of people killed or injured and the level of property damage.

The report, released Thursday from the Oregon Department of Geology and Mineral Industries, is the most detailed damage estimate for a magnitude 9 Cascadia Subduction Zone earthquake in a major urban area, said Ian Madin, the agency’s deputy director. It focuses on the three-county Portland area, but experts say many of its conclusions apply to Seattle and surrounding communities.

Ground motions are expected to be similar in both areas, though Seattle sits on a deep basin that can amplify shaking. Both cities also have a comparable mix of building types and ages, though Oregon was slower to factor earthquakes into its construction codes than Washington.

The Portland-area analysis is one of the first to incorporate detailed information on buildings, including location, construction date and construction type.

“We can look at the damage on a building-by-building basis,” Madin said. “As far as we know, nobody has ever done this for more than 600,000 buildings.”

The scenario also examines the dramatic differences between a quake that strikes during the dry season and one that occurs in winter or fall, when the ground is saturated and more vulnerable to landslides and liquefaction.

In the “dry” scenario, the costs to repair damaged buildings in the three counties could total about $24 billion. In the “wet” scenario, the figure soars to about $37 billion. More than 85,000 people could be displaced from their homes by a wet-season quake, five times the number when the ground is dry.


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No similar analysis has yet been done for Seattle, where steep slopes are landslide-prone even when the ground isn’t shaking. A 2015 University of Washington study estimated a powerful quake on the shallow Seattle Fault, which would rattle the ground much more violently than a quake on the offshore Cascadia Subduction Zone, could trigger more than 30,000 landslides if it strikes when the ground is wet. Another UW team is working on local landslide projections for a Cascadia quake.

One thing that might surprise people about the Portland-area scenario is that the total loss of building stock is only estimated to range between 9 and 14 percent, Madin pointed out.

“I think a lot of people have heard all of the hype and the (2015) New Yorker article and just assumed that it’s going to be a total wasteland of smoking ruins the morning after the quake, but it’s not,” he said.

Washington’s and Oregon’s urban corridors are more than 100 miles from the offshore Cascadia fault, and the level of ground shaking from a magnitude 9 quake is only expected to be moderate — though it will last for several minutes.

“Widespread, moderate damage is what we expect,” Madin said.

But the loss of thousands of buildings, coupled with damage to roadways, utilities and other infrastructure, will be enough to disrupt the region’s society and economy.

“All you have to do is knock out key parts of the power system and water lines and you’re hauling water with a bucket and living in the dark,” Madin said.

The new analysis focused mainly on building damage but estimated that up to 12 percent of major electrical-transmission towers are at risk, along with more than three-quarters of roadways that will be crucial to emergency response.

The resilience of single-family homes is a bright spot. The analysis finds that nearly two-thirds are not expected to suffer any structural damage, and that only about 5 percent would collapse, even during wet conditions.

Older homes that aren’t bolted to their foundations are the most vulnerable, along with houses on landslide-prone slopes and unstable soils.

“The single most effective thing you can do to minimize disruption after a quake is to secure your house to the foundation,” Madin said. “If your house falls off the foundation, you’re going to a refugee camp.”

The new analysis also underscores the benefit of seismically retrofitting vulnerable commercial buildings.

Both Seattle and Portland have been pushing for retrofit requirements for the most dangerous type: Old brick structures, also called unreinforced masonry. Seattle has about 1,100 unreinforced masonry buildings, while Portland has more than 1,650.

The analysis found that moderate seismic upgrades to older buildings, including unreinforced masonry structures, could slash building repair costs from $24 billion to $6.5 billion, and reduce the number of people killed or injured from more than 18,000 to about 2,000.

The death toll in the three-county area from a Cascadia quake could range from 169 to more than 1,400, depending on the time of year and time of day. In a worst-case scenario, as many as 25,000 people could be injured, but more than three-quarters of those injuries are expected to be minor.

“Those can be dealt with through basic first aid, so we want to get as many people possible trained,” said Dan Douthit, of Portland’s Bureau of Emergency Management.

Scenarios are a good way to bring earthquake risks to life, and spur residents to take steps like retrofitting their homes or putting together emergency kits, said Matt Auflick, public education and outreach coordinator for Seattle’s Office of Emergency Management.

A 2005 scenario outlined some of the expected effects in Seattle of an earthquake on the shallow Seattle Fault, and a 2013 scenario mapped out some of the damage expected across the region from a Cascadia megaquake. But neither report is as detailed as the new Portland version.

Washington’s Department of Natural Resources has been working to incorporate building inventories in hazard analyses, but hasn’t extended that work to King, Pierce or Snohomish counties, said assistant state geologist Tim Walsh.

King County residents can get a general idea of their own earthquake, landslide and liquefaction risks by using a new tool called Seattle and King County Ready, which can search for specific addresses.