Just after midnight on Tuesday, an earthquake measuring 4.2 on the Richter scale hit the Oregon coast. It was nothing groundbreaking because earthquakes happen offshore all the time.
An hour and a half later, another quake rippled through the seabed.
And then another earthquake struck. And another. And another.
By now, seismologists were really paying attention. According to data from the US Geological Survey, nearly 30 earthquakes occurred that day along the westernmost portion of the Blanco Fracture Zone, an approximately 200-mile plate boundary off the coast of the state. The strongest recorded quake had a magnitude of 5.8, the smallest 3.4.
By Wednesday afternoon, at least 66 earthquakes had been recorded in the area, Susan Hough, a seismologist with the USGS in Pasadena, Calif., said that day. And the tremors had not stopped Wednesday night.
If there had been so many earthquakes in another area, such as the formidable San Andreas fault in California, there could have been chaos and destruction.
But this series of minor and moderate earthquakes, known in earthquake usage as a swarm, was nothing to worry about, said Don Blakeman, a geophysicist with the USGS’s National Earthquake Information Center.
“This is exactly 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 could feel the ground shaking, he said, but wouldn’t have to worry about a tsunami or a powerful earthquake happening much closer to them.
The National Weather Service wearily reminded people of that Twitter on Wednesday morning.
“For the 7th time in the past 16 hours … tsunami not expected with earthquake off the southern Oregon coast,” the agency wrote.
It’s not even possible for the Blanco Fracture Zone to generate a tsunami, Douglas Toomey, a professor of geophysics at the University of Oregon, said Wednesday.
The Blanco Fracture Zone is a so-called strike-slip fault, which means that the two sides move horizontally next to each other. Think of when someone “rubs two hands together,” said Dr. toomey. For a tsunami to occur, the seafloor would have to slide up or down.
(Other types of faults have vertical movement, which could trigger a tsunami.)
While a tsunami is an impossible event along the Blanco Fracture Zone, earthquakes there are quite frequent.
“If they had a seismometer on the ocean floor, it would record earthquakes every week,” said Dr. toomey.
He said he wasn’t “completely surprised” to hear about the swarm, but added that the number and size of the tremors were “a bit unusual,” he said.
However, a 2005 swarm could overshadow it, as the most powerful earthquake hit a magnitude of 6.6, said Dr. Hough of the USGS in Pasadena. The swarm’s largest quake this week reached a magnitude of 5.8.
While there may have been swarms in the past that caused some tremors similar to this week’s, the USGS hasn’t monitored the swarms as closely as it does now.
It was not clear what caused this swarm, but Dr. Hough said there’s a theory that swarms in general could be caused by fluids “moving around” once they get into the Earth’s crust.
“We don’t really understand why swarms start,” she said.
It’s hard to predict when a swarm will end, but in some parts of California it can last for a week, she said.
In an earthquake, stresses that have built up along a fault reach a breaking point, releasing enormous amounts of energy. That can cause nearby errors, similar to how you would shake an unopened soda can.
It’s possible, though “extremely unlikely,” that the swarm from the Blanco Fracture Zone could trigger the Cascadia Subduction Zone, which lies 100 miles to the east, said Dr. hough.
The Cascadia Zone, a fault line that runs from Northern California to Vancouver Island, is like the bigger and scarier cousin of the Blanco Fracture Zone. It’s closer to shore and much longer, and it would move vertically in an earthquake. A powerful earthquake there could devastate the Pacific Northwest, seismologists say.
But again, experts have said there’s no need to worry about the swarm along the Blanco Fracture Zone. Its southern neighbor, however, is more worrying.
The Blanco Zone is the same type of fault as the San Andreas fault in California, but that’s really all they have in common. The San Andreas is a long fault line that runs along California’s spine, and there is a risk of a significant earthquake along parts of it, according to USGS lore. In California, that long-feared earthquake is known as the Big One.
The Blanco Fracture Zone is not expected to cause such a powerful earthquake because the Earth’s crust is much thinner there than in the San Andreas Fault area, said Dr. hough.
“It’s like a flaw going through a piece of tissue paper,” she explained, “as opposed to cardboard.”
Henry Fountain reporting contributed.