How worried should Tacoma be about the ‘Big One,’ the looming M-9.0 Cascadia earthquake?

Many Western Washington residents live in fear of the natural disaster to end all natural disasters: the “Big One.”

Scientists say that the Cascadia subduction zone off the coast of the Pacific Northwest has the potential to spark a magnitude-9.0+ earthquake , plus a subsequent tsunami. That scenario last occurred here in the year 1700.

Inquiring minds want to know: When will it happen again?

September was recently declared “National Preparedness Month” by both the Tacoma City and Pierce County councils. So, The News Tribune spoke with experts to learn what such a disaster would look like and how to get ready for it.

What happens during ‘Big Ones’?

Nearly a decade ago, The New Yorker published a piece that would become etched in the minds of many. Titled “ The Really Big One ” in the July 20, 2015 print edition, it reads like the stuff of nightmares.

In 2011, more than 18,000 people lost their lives amid a magnitude-9.0 earthquake and tsunami in Tohoku, Japan, The New Yorker noted. That catastrophe wound up costing approximately $220 billion.

Back in North America, in 1700, a 9.0 earthquake and monster tsunami rocked the Cascadia region , an area that stretches along what would become Western Washington, Oregon and northern California, as well as British Columbia and Vancouver Island, according to the U.S. Geological Survey. Indigenous oral histories recounted deaths, violent shaking, serious flooding and building damage.

One such account from southern Oregon described the tsunami, one with enormous waves that rushed over the land and swallowed entire villages. Some inhabitants tried to secure their canoes to tree tops before the boats were ripped away.

“After the tidal wave, they found tree tops filled with limbs and trash,” the account goes, according to USGS. “The big flood and tidal wave tore up the land and changed the rivers. Nobody knows how many [people] died.”

What would a 9.0 earthquake mean for the area?

Big One-related research efforts made national headlines this summer, including in The Washington Post . NBC News recently reported that between the earthquake and subsequent tsunami, some 14,000 people in Washington and Oregon would perish.

That event would certainly spell bad news for Tacoma, said Tieka Adeogun, the city’s chief emergency management officer. Even after the shaking stops, we’ll be left to deal with the aftermath.

“As much as we don’t want something like that to happen, the reality is one day it will happen, and it’ll be pretty devastating for the city and its residents,” Adeogun said.

Damage to the port, which acts as a major hub, would hamper the ability to send and receive resources, she said. Cargo ships would either be damaged or destroyed, and that area would require a rebuild. Officials would work to get it back up and running as soon as possible.

It’s unknown how many waves would occur during a tsunami, nor how high they’d be, Adeogun said. If we see a 9.0 earthquake, a “series of aftershocks” could trigger additional tsunamis.

Much of the city rests on hilly areas, and there could be post-quake landslides, plus buildings slipping off their foundations, Adeogun said. The city of Tacoma wouldn’t have power for quite some time, and substations would likely be damaged.

Although Adeogun didn’t provide a firm estimate on the number of potential casualties in Tacoma, she did say it would probably exceed 100. On top of that, the Big One would cost the city at least $1 billion in damages.

“We’ve talked about that the Cascadia [earthquake] is like the thing that keeps you up at night, right?” she said.

When will the Big One strike?

When people discuss the Big One in this region, they’re talking about an earthquake on the plate boundary between the Juan de Fuca and North American tectonic plates, said Paul Bodin, a University of Washington research professor with the Pacific Northwest Seismic Network. The boundary between the two is known as the megathrust, or the Cascadia subduction zone, fault.

“That plate-boundary fault is normally locked, and stresses build up, and then when it lets go, there can be tens of meters … of slip on that fault, and that’s what causes all the shaking and damage,” Bodin told The News Tribune.

No one knows exactly when the Big One will rear its head. Bodin explained that on average, such 9.0-ish earthquakes happen about every 500 years.

In terms of probabilities, Bodin said, that equates to a 14%-15% chance in a 50-year period of a megathrust earthquake occurring.

How long will the shaking last?

A small earthquake might feel like a sharp jolt or last a few seconds, he said. Shaking during the Big One will extend for much longer — 5 or so minutes . Because of that long period, there could be damage to bridges and tall buildings, as well as infrastructure like sewers and gas and water pipes.

The 2015 New Yorker piece quoted an expert who claimed that “everything west of Interstate 5 will be toast.” Bodin disagrees.

“One misconception, I think, is that the coast is going to be a smoldering ruin from tsunamis,” he said. “I think that’s highly, highly unlikely, just because of the differences [in terrain] between Japan and here.”

There won’t be a uniform impact, Bodin continued. Rather the region would witness “pockets of extreme damage.” Some buildings might collapse amid the shaking while more modern structures could emerge more or less unscathed.

Injuries are to be expected as residents step on broken glass or get hit by falling items, he said.

Bodin did offer a bit of hope for certain homes.

“One thing I want to be clear about is that the structures that most of us live in, which are small, single- or two-, or most, three-story wood-frame, flexible houses, the external damage to those probably will be quite light and localized,” he said.

Liquefaction

Liquefaction could occur during the Big One, meaning that the soil loses its strength and acts like quicksand , according to the Washington State Department of Natural Resources (DNR).

“Liquefaction, we believe, will provide the most significant impact into the Puget Sound region, because we have a lot of critical infrastructure in areas that might be impacted by that,” said Amy Gillespie, deputy director of the Pierce County Department of Emergency Management.

For much of Tacoma, the liquefaction susceptibility is considered “very low,” according to a map available on DNR’s website. That risk is elevated to “high” in the Dune Peninsula and Point Ruston area, as well as the Tideflats.

Other parts of Pierce County in the moderate or high-risk range are located in Orting, Sumner and Puyallup, per the map.

What comes after the Big One?

Even after the Big One is done, aftershocks should be expected, Bodin said. They could go on for days, weeks or months — possibly even years . Such quakes might be large themselves, with magnitudes in the 6- and 7-range.

The post-Big One tsunami would take hours to work its way into Puget Sound, Bodin said. Tacoma is a bit insulated because it’s further inland.

“There’s no ambiguity of what to do” in terms of a tsunami, Bodin said. “You get to the highest ground that you can,” and bring your family and pets.

Bodin noted another worry: fires.

Gas lines might break, and gas water heaters could fall over and set homes ablaze, he added. On top of that, if the water infrastructure is wrecked, it would make fighting fire even more difficult.

How to prepare for a 9.0 earthquake

The city wants residents to have enough supplies on hand for at least two weeks should the Big One arrive, Adeogun said. However, it could take three or four weeks before officials have an idea of what resources would be arriving from outside the state.

“The reality of this situation is our infrastructure, when it comes to transportation, will be so damaged that our citizens will have to largely rely on what resources we do have and what neighboring jurisdictions can get in just by road transport,” she added.

When the shaking has stopped, turn off your gas line, Gillespie said.

It’s a good idea to keep a wrench near your gas meter so it’s readily available to shut off, added Mike Halliday, spokesperson for the Pierce County Department of Emergency Management.

To build an emergency kit , stock up on items like non-perishable food, medications and pet supplies. Gillespie recommends starting small: Grab one item at a time, such as water or a flashlight.

For many, preparedness is front of mind. Nearly 100,000 participants have registered so far in Pierce County for the Great ShakeOut , a worldwide earthquake drill, on Oct. 17.

Gillespie advises residents to establish an out-of-state point of contact if possible. That way, if the Big One strikes, that person can be responsible for relaying any updates to inquiring loved ones.

She also wants people to know about the county’s neighborhood emergency-response teams, whereby residents are trained how to take care of one another during disasters . You can also check out the Emergency Operations Center — and pick up items for your own emergency kit — during an open house Sept. 28 at the department’s Tacoma headquarters, 2501 S. 35th St., Suite D.

Gillespie offered a key takeaway: Pierce County, Washington state and federal agencies have been planning for the Big One. When it arrives, they’ll work together to get resources into communities as quickly as they can.

Don’t panic, Bodin said: Be prepared.

“Rather than being spooked beforehand and scared about it, just [realize] that this comes with the beautiful terrain and area that we live in,” he added. “And we can be ready for it.”

What do I do if an earthquake strikes?

Get out of whatever building you’re in and head outside as soon as the ground stops shaking, Adeogun said. Many lives are lost when a structure collapses on itself.

If you live in a location that’s at risk of tsunami, get to higher ground once the earthquake is through.

Adeogun urges people to secure their TVs and refrigerators to the wall ahead of time, too.

Remember this saying: Drop, cover and hold on.

• Drop to your knees and hands

• Cover your neck and head, crawling beneath a nearby table or desk if possible, or next to an interior wall

• Hold on until the ground stops moving. If you’re under a desk, hold onto it with one hand while protecting your head and neck with the other

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