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Living on shaky ground: Quakes a reminder of potential threat

Graphic Courtesy of Oregon Emergency Management
 

The ground continues to shake off the Oregon Coast, with a 4.5 earthquake recorded off the central coast Thursday, a 4.9 in the same area Wednesday, a 4.4 magnitude in the ocean 89 miles west of Eureka on Thursday and a cluster of 14 off the coast between Brookings and Gold Beach two weeks ago.

The area is considered seismically silent, said Ryan Sandler, a meteorologist with the National Oceanic and Atmospheric Association, but this flurry of rattles has people taking note.

Every year, the two plates that make up Cascadia’s subduction zone – the place where the proverbial “Big One” will occur – should slip about 3 centimeters to ease the pressure that builds up when plates slide past one another, Sandler predicted.

 

Those little slips result in minor earthquakes that aren’t felt by anyone. Or, the two plates could slide past each other in larger lurches, resulting in “moderate” quakes in the 5.0 magnitude range. 

The Pacific and North American plates haven’t moved much in 300 years. And that silence will prove deadly, Sandler said.

The 3-centimeter annual slippage needed hasn’t kept up, and will result in an earthquake that “unzips” from the Crescent City area to Alaska, Sandler said.

That’s one big earthquake, he told a group of about 20 Oregon and California residents who came to Southwestern Oregon Community College in Brookings Thursday afternoon to learn the basics of tsunami behavior.

Not all quakes, he emphasized – result in tsunamis, usually just those 7.0 or higher. And then, the alignment of land and water must be right to determine if and where a tsunami will form.

“Brookings is in a better place than many places,” Sandler said. “If you look at the Gold Beach (tsunami inundation) map, basically, that town is destroyed. Gold Beach will be gone.”

The severity of earthquakes and the tsunamis that can follow depend on a wide array of factors: where and how much the plates slip, how far offshore and how deep the subduction zone is in the ocean, and the kind of soil through which it travels, among others.

“The time of day and the time of year matter,” Sandler said. “The worst case would be in the summer, in the middle of the night, the big one hits and all those people at the state parks – people from Kansas who don’t even know what a tsunami is.”

Those worst case scenarios are most likely to result after a big earthquake in Alaska, such as the quake, and then tsunami, that deluged Crescent City in 1964, the 9.2 temblor in Japan last year that wiped out harbors along the California and Oregon coasts – or along the subduction zone about 30 miles offshore that parallels the western coast of North America.

Coastal residents are pretty well aware of the danger that lurks around them, Sandler said. But people underestimate the possible severity because they don’t know how tsunamis work.

For instance, regular waves are created by the wind and arrive in rolling circles – ask any surfer about the curl. But a tsunami wave, created by a sudden vertical thrust when the earth displaces ocean water, doesn’t have a steep backside; it is a steady wall of water. That water represents millions of cubic feet, with each cubic foot weighing about 62 pounds.

The energy behind those waves is tremendous, Sandler said.

People in Japan experienced three times the force of gravity – only back and forth, instead of up and down – when the tsunami struck there March 11, 2011.

He said if forecasters hear about a 6-inch rise in ocean waters, that gets their attention. A foot puts them in action. Japan witnessed an overall sea rise of 3 feet, but a tsunami wave 70 feet tall.

“It’s like getting a tide in, but instead of six hours, it’s here in five or six minutes,” Sandler said. “It just rushes in, it gets higher and higher. As it goes down the river, you can see it pushing down.”

Tsunami inundation maps released this month weren’t as detailed as many residents would have liked.

In a worst-case scenario – a 9 quake off the coast – the Chetco River would fill to the banks and likely take out the bridge. Waters would subsequently fill creeks that feed the river, and likely flood low-lying homes. Water would crash over the steep cliffs ringing downtown; Harbor would be flooded to the 101.

“If people (in the Brookings-Harbor core) could make it to the 101, they’d be golden,” Sandler said.

Because Brookings is so close to the subduction zone, waves could reach heights of 80 to 110 feet, Sandler said. Harbor would see a wave of 105 to 115 feet; people closer to the Winchuck River could see a wave 150 high.

A distant tsunami could result in a wave 35 to 40 feet high, which is said, “is pretty darn high.”

The wave will pick up everything in its path and rush it back out to the sea. Other, and sometimes more powerful, tsunami waves could follow. And earthquake, and probably not a tsunami, would be what would take out the Chetco bridge.

And even if a tsunami or earthquake didn’t affect those on high seaside cliffs, landslides could take them out.

“There is going to be severe shaking at the coast,” Sandler said, adding that when a 9.0 quake rattles the Oregon coast, it will result in an estimated 400 bridges destroyed and another 600 damaged.

“Everyone in the community will be a little island on the coast,” he said. “It will look like a war zone. It’ll be a nightmare.”

While it could take up to three weeks for help to arrive here, but it will probably take three to 12 months to rebuild highways linking the coast and the Interstate 5 corridor – and three to five years to reconstruct 101 and I-5, Sandler said.

Oregon’s role in a disaster, he added, is notification and education.

Even today, sirens are becoming passé, in lieu of alerts on smart phones, electronic signs on highways and radio emergency announcements. And those will only be broadcast if electrical infrastructure is intact and cell towers haven’t collapsed – and people are in cell range.

The Emergency Alert System – most often heard as that screeching “test” on radio stations – went wireless in June, but only new phones have the capability for the program.

Alerts remain the same: Tsunami “warnings” are the worst, with waves of at least 3 feet expected; “advisories” indicate a wave of 1 to 2 feet is on its way and can cause extensive damage; a “watch” is not used much anymore; and “info” means there is no danger.

“The ‘warning’ is the trickiest,” Sandler said. “You don’t want to be down near the ocean. If you feel shaking for more than 20 seconds, that’s certainly an earthquake here. That’s your warning.

“If it’s really the big one offshore, there’s not much you can do,” he added. “At least, try not to get killed. It’d be a mess. You definitely don’t want the worst-case scenario.” 

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