Pacific

Did you feel it? The earthquake that rumbled beneath the ocean floor off the Oregon coast earlier this month?

© Mark Graves/The Oregonian/OregonLive

Unless you’re a crab or some other bottom-dwelling sea creature, the answer is almost certainly no.

That’s because the magnitude 3.5 quake struck roughly 150 miles west of Port Orford. It was one of nearly 3,500 seismic tremors of that magnitude 2.5 or greater that have roiled the fault zone off the Oregon coast since 1970.

While 3,500 quakes may seem like a lot, Chris Goldfinger, an Oregon State University paleoseismologist, said the Cascadia Subduction Zone is actually far quieter than similar fault zones around the world.

“Considering that you’re looking at 50 years of data, the map is nearly blank,” Goldfinger said of the series of faults that runs parallel to the West coast from Northern California to Canada. “There’s very little activity onshore. If you were to take a similar map for somewhere like Southern California, the whole thing would be lit up.”

This chart shows the frequency of major quakes along the Cascadia Subduction Zone over the last 10,000 years. © Mark Graves/The Oregonian/OregonLive This chart shows the frequency of major quakes along the Cascadia Subduction Zone over the last 10,000 years.

That inactivity may be both a blessing and a curse, Goldfinger said.

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The Cascadia Subduction Zone is capable of creating a massive earthquake, up to magnitude 9.0 or higher. So while residents thankfully aren’t jolted by frequent seismic events, the relative dearth of strong quakes has potentially created a false sense of security for some in the Pacific Northwest.

That’s why every year on the third Thursday in October, local and state leaders encourage everyone to participate in an earthquake drill known as The Great Shakeout. More than 500,000 Oregonians have registered to take part, promising to “drop, cover and hold on” as part of a rehearsal for a major quake.

That advice may be misguided for some, however, as Goldfinger noted that the Northwest is littered with buildings that could collapse in a major quake.

“When you tell people to get under their desks in a building that’s going to collapse, you are going to have a problem,” Goldfinger said, adding that in some specific situations, evacuating the building would be a better strategy.

A seismograph at Portland State University shows a 1993 earthquake centered in Oregon. © The Oregonian A seismograph at Portland State University shows a 1993 earthquake centered in Oregon.

For others, the gravest danger may not come from the earthquake itself, but the massive tsunami that will swamp the coast in its wake.

A MATTER OF WHEN, NOT IF

The earthquake that Northwesterners have long known about and feared is one caused by a rupture of the Cascadia Subduction Zone, an area off the Oregon coast where several smaller tectonic plates slide beneath the massive North American plate.

In shallower areas of subduction zones, less than 19 miles beneath the earth’s surface, the plates are held in place by friction as tension builds while one slides beneath the other. When the accumulated force exceeds the friction’s ability to hold the plates, they slip and produce what are known as “megathrust” earthquakes.

Subduction zones are the only types of faults capable of producing megathrust quakes of magnitude 8.5 or greater and are responsible for some of the largest quakes in recent history, including the magnitude 9.1 Tohoku quake that hit Japan in 2011 and the quake of similar strength that rocked large parts of the Indian Ocean in 2004.

The last time the whole Cascadia Subduction Zone ruptured was January 26, 1700, and at least 40 large-magnitude earthquakes have occurred along the fault in the past 10,000 years.

Seismologists studying core samples from the ocean floor peg the likelihood of a major quake occurring on the whole fault at 10 to 14% percent in the next 50 years.

Those numbers are slightly higher for the central and northern Oregon coast, where experts say the chances of a large quake in the next 50 years are between 16% and 22%.

An estimate for damage and casualties in the Portland area alone found that in a worst-case scenario, the metro area could see more than $80 billion in building damage, tens of thousands of people wounded or killed and more than 250,000 people facing long-term displacement.

And that estimate doesn’t include the havoc wrought by a potential tsunami.

‘SEASIDE IS IN THE BULLSEYE’

Seaside, a popular tourist destination that’s home to about 6,500 full-time residents, sits between Cannon Beach and Astoria and boasts an aquarium, a carousel and scores of hotels and restaurants that have proven a draw to visitors from all over the state.

The city will also be ground zero for casualties stemming from a tsunami that hits the Oregon coast.

“We’ve been keenly aware that Seaside is in the bullseye of the Cascadia event,” said Jay Barber, the city’s mayor. “Anything from a (magnitude) 8.2 to 9.0 is basically going to destroy Seaside.”

An analysis prepared for the Oregon Health Authority found that as many as 18,667 people could be killed in a tsunami following a magnitude 9.0 earthquake along the entirety of the Cascadia Subduction Zone. Warrenton could see more than 1,000 deaths. Rockaway Beach could see 1,600.

Seaside, representing more than half the total deaths estimated to occur in a major tsunami, could see 10,734 people killed, according to the report.

The estimate is based on a near-worst-case scenario: a magnitude-9.0 quake that strikes in the middle of the night during the middle of summer, when homes and hotels are nearly all occupied. The number of deaths could be much lower if the quake were to strike in the winter but could also be dramatically higher if it struck during the middle of a summer day, when thousands are on the beach.

Barber will be participating The Great Shakeout, but beyond the aspects of the drill everyone will be taking part in, he’s also planning to go for a walk.

He’ll be walking, rapidly, he said, from his home in the city’s inundation zone to an assembly area on higher ground — one of five the city has designated east and south of the low-lying downtown core. It’s a walk he’s taken many times before and one he can complete in about 10 minutes. He recommended all Seaside residents practice their escape route as part of the yearly earthquake drill.

Escape routes in the city are marked by signs and decals on the ground, installed by local high school students last year, so that tourists will be able to quickly identify the fastest route to safety. The assembly areas are stocked with survival supplies, packed into barrels, so that those who are forced to leave their homes with nothing are able to get by for up to four days after a tsunami.

Four years ago, voters in Seaside approved a $100 million bond to build a new high school on higher ground, where it now serves as one of the assembly points.

Barber is well aware there will be little he can do to protect the city itself, but he’s confident that the plans put in place to evacuate the most susceptible areas can prevent a catastrophic loss of life.

“If you can pull that off, you very well may survive even if the city itself does not,” Barber said. “You can replace buildings and infrastructure, but you can’t replace lives, and that is the most precious thing worth preserving.”

TO DROP OR NOT TO DROP

For Oregonians who don’t live in a tsunami zone, the messaging will be ubiquitous on Thursday: If you feel an earthquake, drop, cover and hold on.

But Goldfinger, who has been studying earthquakes and was in Japan during the 2011 event, said that is not the best course of action for everyone.

Thousands of Oregon buildings were built before a Cascadia quake’s scope of devastation was fully understood, and many of them were constructed using unreinforced masonry, which poses a risk of collapse.

For his own earthquake plan, Goldfinger looked up the building where he works and saw that it was constructed in the 1960s, well before more stringent building codes were in effect. Knowing that his building could fall down, he figured out the fastest route outside and then looked for other risks — adjacent buildings that could fall over, large windows that could shatter or utility poles that could topple — so he could identify the safest place to be when the shaking starts.

“It took me a total of 10 minutes to figure out, and you don’t have to be an earthquake geologist,” Goldfinger said. “Every town in Oregon has a ton of (unreinforced masonry buildings) and each person needs to assess the susceptibility of their structure. There isn’t a one size fits all solution for everyone.”

So while Goldfinger commends state leaders on the scope of their messaging, he thinks telling everyone to “drop, cover and hold on” is misguided.

Jay Raskin, an architect and former chair of the Oregon Seismic Safety Commission, disagreed. He said that in many unreinforced masonry buildings, the walls can be the first thing to fail, falling away from the building and causing great danger to people near the perimeter of the structure.

“For unreinforced masonry, if you don’t know anything about it, the best course of action is to drop, cover and hold on,” Raskin said.

Both Raskin and Goldfinger stressed that knowing what kind of building you live or work in is paramount to deciding what to do when an earthquake hits.

“It may be more complicated than what we have now, but it’s easier than lots of other complicated things we learn these days,” he said. “We owe it to people to give them the straight answer because I think most people are averse to being crushed by a building.”

— Kale Williams; [email protected]; 503-294-4048; @sfkale

Source : https://www.msn.com/en-us/weather/topstories/is-e2-80-98drop-cover-and-hold-on-e2-80-99-the-best-advice-in-a-major-pacific-northwest-earthquake/ar-AAPNeLN

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