Dozens of Earthquakes Strike Off Oregon Coast, but Experts Say Not to Worry

Early Tuesday morning, a 4.2-magnitude earthquake struck off the coast of Oregon. It was nothing groundbreaking, because quakes happen offshore all the time.

An hour and a half later, another tremor rippled through the seafloor.

And then another earthquake struck. And another. And another.

By now, seismologists were really paying attention. Almost 30 earthquakes happened that day along the westernmost segment of the Blanco Fracture Zone, a roughly 200-mile-long plate boundary off the state’s coast, according to data from the U.S. Geological Survey. The strongest recorded tremor had a magnitude of 5.8, the smallest 3.4.

By Wednesday afternoon, at least 66 quakes had been recorded in the area, Susan Hough, a seismologist with the U.S.G.S. in Pasadena, Calif., said that day. And the quakes had not let up by Wednesday night.

If that many earthquakes had struck along a different area, such as the formidable San Andreas fault in California, there could have been chaos and destruction.

But this series of small and moderate quakes, known in earthquake parlance as a swarm, was nothing to worry about, said Don Blakeman, a geophysicist at the U.S.G.S.’s National Earthquake Information Center.

“This is just how the earth works in that spot,” he said, adding that the area was “a fairly active zone.”

Someone on the beach 250 miles from the fault might feel the ground shake, he said, but would not need to worry about a tsunami or a powerful quake occurring much closer to them.

The National Weather Service wearily reminded people of that fact on Twitter on Wednesday morning.

“For the 7th time in the last 16 hours … tsunami not expected with earthquake off the southern Oregon coast,” the agency wrote.

In fact, it is extremely unlikely that the Blanco Fracture Zone would generate a tsunami, Douglas Toomey, a geophysics professor at the University of Oregon, said on Wednesday.

The Blanco Fracture Zone is what is known as a strike-slip fault, which means its two sides move alongside each other horizontally. Think of when someone “rubs two hands together,” Dr. Toomey said. For a tsunami to happen, the seafloor would need to shift up or down.

(Other types of faults have vertical movement, which could generate a tsunami.)

While a tsunami is an impossible occurrence along the Blanco Fracture Zone, earthquakes are fairly frequent there.

“If they had an ocean-bottom seismometer out there, it would be recording earthquakes every week,” Dr. Toomey said.

He said he was not “entirely surprised” to hear about the swarm but added that the number and size of the tremors were “a bit unusual.”

A 2005 swarm might have eclipsed this one, however, because its most powerful quake reached a magnitude of 6.6, said Dr. Hough at the U.S.G.S. in Pasadena. The largest tremor from this week’s swarm reached a magnitude of 5.8.

While there might have been swarms in the past that generated numbers of tremors similar to those this week, the U.S.G.S. did not monitor the swarms as closely as it does now.

It was not clear what caused this swarm, but Dr. Hough said there was a theory that swarms in general could be caused by fluids that “move around” once they get into the earth’s crust.

“We don’t really understand why swarms get started,” she said.

It is hard to predict when a swarm will end, but one can last a week in some areas of California, she said.

In an earthquake, stresses that have built up along a fault reach a breaking point, releasing huge amounts of energy. That can set off nearby faults, similar to tipping a row of dominoes.

It is possible, though “exceedingly unlikely,” that the swarm from the Blanco Fracture Zone could set off the Cascadia Subduction Zone, which is 100 miles to the east, Dr. Hough said.

The Cascadia zone, a fault running from Northern California to Vancouver Island, is like the Blanco Fracture Zone’s bigger and scarier cousin. It is closer to the coast and much longer, and it would move vertically in an earthquake. A powerful quake there could devastate the Pacific Northwest, seismologists say.

But, again, experts have said that there is no reason to be concerned about the swarm along Blanco Fracture Zone. Its neighbor to the south, however, is more worrisome.

The Blanco zone is the same type of fault as the San Andreas fault in California, but that is really all they have in common. The San Andreas is a long fault that runs up the spine of California, and there is a risk of a significant earthquake along parts of it, according to the U.S.G.S. In California lore, that long-feared quake is known as the Big One.

The Blanco Fracture Zone is not expected to generate such a powerful quake because the earth’s crust is much thinner there than in the area of the San Andreas fault, Dr. Hough said.

“It’s like a fault going through a piece of tissue paper,” she explained, “as opposed to cardboard.”

Henry Fountain contributed reporting.

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