An ‘oddball’ 6.0 earthquake is recorded almost 800 miles off Southern California’s coast

A 6.0 magnitude earthquake was recorded almost 800 miles off the Southern California coast Tuesday night, an “oddball” event that made little impact but drew intrigue from scientists because of its location.

“It’s a decent size, but that’s a location that is just not common,” said Susan Hough, a seismologist with the U.S. Geological Survey in Pasadena. “I’ve been watching earthquakes for going on 40 years, and I don’t remember anything that big in that location.” The temblor occurred roughly in line with San Diego.

Since 1900, there have been only 10 other recorded earthquakes in the north Pacific Ocean off Southern California’s coast — and of the temblors more than 500 miles from the coast, all measured under 5.0 magnitude, according to the USGS.

Tuesday’s earthquake occurred just before 10 p.m., and as of Wednesday morning, no aftershocks had been recorded, Hough said. She said because of its location, the seismic activity is unlikely to have caused any damage or had any other effects, though she said nearby boats could have felt the movement.

The earthquake’s location is considered odd because it did not fall near or along tectonic plate boundaries, but in the middle of a large plate — a phenomenon known as an “intraplate earthquake,” Hough said. The vast majority of earthquakes occur along plate boundaries, like along California’s San Andreas fault, though intraplate earthquakes do sometimes occur.

“It’s a weird earthquake,” Hough said. “There really isn’t an overarching explanation for intraplate earthquakes, except that they happen.”

She called the earth’s plates “fairly stable,” as they don’t have major fault lines that shift or move, but she said stress can occur mid-plate, such as the cooling of oceanic crust as it ages.

“There’s stresses in the crust for all sorts of reasons, so you can have these oddball events that pop up,” Hough said. “It’s not unprecedented to have an intra-plate oceanic earthquake in an oddball location.”

Although some offshore earthquakes can become a concern, depending on their proximity to communities or their ability to form a tsunami, Hough said this one was too far and didn’t move the seafloor in such a way.

“Being a significant underwater earthquake is not sufficient to generate a tsunami,” according to a tweet from the USGS Earthquakes account Wednesday morning. “The quake also has to move the seafloor vertically, which is not what this quake did.”

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Hough said a handful of people in California reported to USGS feeling “weak shaking” around the time of the earthquake, but she said it’s hard to confirm if that was a direct result of the oceanic quake. She said she expects this earthquake could trigger more research about intraplate seismic activity and any lingering effects, but otherwise she called it “not that big a deal.”

“The expectation is that it’s just an oddball, kind of black swan event that’s in the middle of the Pacific,” Hough said. “But people will be keeping an eye on it.”

She said in 2018 oceanic seismic activity off the east coast of eastern Africa led to scientists finding an underwater volcano, but Hough said that was after detecting repeated earthquakes and movement, which she has seen no sign of in this case.

But she said it does underscore how some earthquake activity remains difficult to predict or explain, noting that a 6.0 earthquake near Chicago or on the East Coast could cause real damage.

“These oddball, intraplate earthquakes can pop up where we haven’t seen them in human lifetimes,” Hough said. “An unlikely place isn’t a problem if it’s in the middle of the north Pacific or well offshore — an unlikely quake close to people is a problem.”