Aftershocks to hit California for “months” after 7.0 earthquake—expert

Northern California residents could feel aftershocks as high as magnitude 6.0 for months following Thursday morning’s magnitude 7.0 earthquake that hit offshore in Eureka, California.
The main quake hit offshore Cape Mendocino at approximately 10:45 a.m. local time. In the hours that followed, at least 34 aftershocks have also occurred in the region. The strongest aftershock was a 4.3 magnitude quake that hit near Petrolia, California, at around 12:04 p.m. local time, according to Tim Clements, a research geophysicist at the U.S. Geological Survey’s (USGS) Earthquake Science Center.
“We expect aftershocks to occur in the area for weeks to months,” Clements told Newsweek. “According to our aftershock forecast, the chance of a M6-plus earthquake in the week is about 1 in 20,” or 5 percent.
Aftershocks are smaller earthquakes that follow a larger earthquake. Many of the aftershocks felt on Thursday originated off the coast, although some occurred inland, according to a USGS map.
U.S. Geological Survey
Milder aftershocks have a greater chance of happening, according to the USGS aftershock forecast.
There is a more than than 99 percent chance of additional magnitude 3.0 or higher earthquakes happening. The forecast anticipates an 84 percent chance of magnitude 4.0 or higher quakes, a 31 percent chance for magnitude 5.0 and less than a 1 percent chance of magnitude 7.0.
Damage usually doesn’t occur until an earthquake measures around a magnitude 4 or 5, the USGS said.
The earthquake that hit offshore in northwestern California occurred in the Mendocino Triple Junction, likely near the Mendocino Fracture Zone, Clements said.
“The Mendocino Triple Junction is where the Gorda, North America and Pacific plates meet,” he said. “This area is particularly active with several M6-plus earthquakes over the past decade,” including a M6.4 on December 20, 2022, a M6.2 on December 20, 2021, and a M6.6 on December 8, 2016.
However, the last time a magnitude 7.0 or greater earthquake struck the area was a magnitude 7.2 that hit on April 25, 1992.
Eight earthquakes with a magnitude 7.0 or larger have hit the area since 1922, including the one that occurred on Thursday.
That quake was strong enough to trigger a tsunami warning for around 600 miles of California and Oregon coastline, but no tsunami occurred.
Clements advised people in the affected area to check buildings and structures for potential damage before reoccupying them.
Should strong shaking related to aftershocks occur, the USGS recommends that people “drop, cover, and hold on.”
Residents said they felt shaking in populated areas such as Sacramento, Eureka and Monterey, according to the Los Angeles Times. However, Eureka city officials said no major damage had been reported a few hours after the quake, although officials had little time to assess damage before evacuating because of the tsunami warning.