Thousands of Californians received earthquake warnings on Thursday after a 4.7-magnitude tremor shook Northern California.
The US Geological Survey (USGS) issued a ShakeAlert after the quake struck at 4:49am PT (7:49am ET), about 70 miles offshore of Petrolia.
ShakeAlert messages are triggered when seismic sensors detect a quake of magnitude 4.5 or higher expected to cause significant shaking.
The epicenter was located 190 miles west of Chico, a city of about 121,000 residents, and 65 miles west-southwest of Fortuna, which has roughly 12,000 residents.
Residents in these and nearby communities reported feeling the tremor.
Seismic data suggested it struck along the Cascadia Subduction Zone, a 700-mile-long, dangerous fault stretching offshore from Northern California to Vancouver Island, Canada.
The Cascadia Subduction Zone has historically produced 8.0 or larger magnitude earthquakes every 400 to 600 years, with the last striking in 1700.
This suggests the ‘sleeping giant’ is poised for another big one soon.
The epicenter was located 190 miles west of Chico, a city of about 121,000 residents, and 65 miles west-southwest of Fortuna, which has roughly 12,000 residents
The USGS reported shaking in Eureka, Indianola and Kneeland in northern California, along with towns to the south, including Hacketsville and Rio Dell.
An assessment from Michigan Tech University showed that people typically do not feel quakes with a magnitude of 2.5 or less. Those from 2.5 to 5.4 are often felt but only cause minor damage.
No injuries or damages have been reported.
A 2025 study found that an 8.0 magnitude or higher quake along the Cascadia Subduction Zone, combined with rising sea levels, would cause coastal land to sink up to 6.5 feet within 30 minutes of a major tremor.
The team generated tens of thousands of earthquake models to estimate the potential range of earthquake-driven subsidence — sinking land — that can be expected from the next large Cascadia earthquake.
They determined that the most severe effects would hit southern Washington, northern Oregon and northern California, densely populated areas in the region.
This event would result in a significant expansion of the coastal floodplain — an area with a one percent chance of flooding each year — increasing it from 35 square miles to 116.
If such a quake struck today, the researchers estimate that an additional 14,350 residents, 22,500 structures and 777 miles of roadway would fall within the post-earthquake floodplain, more than doubling flood exposure.
