5 Things To Know About CA's Tsunami Warning, Earthquake

SAN FRANCISCO — A magnitude 7 earthquake rattled Northern California Thursday morning, triggering a tsunami warning for millions on the West Coast as coastal communities braced for the worst. Over the next hour, cities from Eureka to San Francisco urged their residents to seek higher ground and stay away from the coast.

The warning was canceled shortly before noon after forecasters detected no significant change to water levels.

According to the Tsunami Warning Center, warnings are usually issued when coastal earthquakes reach magnitude 6.5 or higher. Speaking at a news conference Thursday, Gov. Gavin Newsom said it was state protocol to issue warnings whenever a quake of magnitude 5 or greater struck a coastal region.

Here are five things to know about Thursday's earthquake and tsunami warning:

Where was the epicenter?

According to the United States Geological Survey, the magnitude 7.0 Offshore Cape Mendocino earthquake happened roughly 62 miles southwest of Ferndale, off the coast of California, in an area known as the Mendocino Fracture Zone.

"This earthquake occurred in the vicinity of the Mendocino triple junction — the region where the Pacific, North America, and Juan de Fuca/Gorda plates meet," the USGS said Thursday. "Focal mechanism solutions indicate that rupture occurred as a result of strike-slip faulting on a steeply dipping fault striking either east-southeast or north-northeast."

How often do earthquakes happen in the area?

Federal geologists consider the "triple junction" a common region for earthquakes. The USGS noted that the magnitude 6.4 Ferndale earthquake was recorded on Dec. 20, 2022, roughly 47 miles east of Thursday's magnitude 7 event. A 6.2 magnitude earthquake happened approximately 56 miles east in 2021. Even stronger quakes have occurred in prior decades, but Thursday's magnitude 7 is in somewhat rarer company.

"In the past century, there have been at least 40 other earthquakes of M6 or larger, including five earthquakes M7 or larger, within (155.3 miles) of the December 5, 2024, earthquake," the USGS said. "These prior earthquakes primarily occurred along the Mendocino transform fault, in the Cascadia subduction zone, or within the Juan de Fuca/Gorda plate."

How many aftershocks will follow?

A cluster of more than two dozen aftershocks were recorded offshore in the wake of the magnitude 7 earthquake, and two on land in Petrolia in the early afternoon. According to the USGS, aftershocks are smaller earthquakes that typically happen in the same general area after a larger event and continue until the "background seismicity level" has resumed.

"As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock," the USGS said. "The frequency of these aftershocks decreases with time."

What were the impacts of Thursday's earthquake?

According to the USGS ShakeMap, shaking was felt strongest in communities along the Humboldt County coastline, including Petrolia, Ferndale and Fortuna, south of Eureka. Milder tremors rippled through much of Northern California and across the Bay Area. In San Francisco, residents reported feeling a rolling motion for several seconds, followed by several aftershocks.

There were no immediate reports of major damage or injuries. In Ferndale, the community nearest the quake's epicenter, a fire official told the New York Times only minor property damage was reported.

The outsized impact of the earthquake was a broad tsunami warning, which was in place for more than an hour along hundreds of miles of coastline, stretching from southwest Oregon down to the edge of Monterey Bay. Approximately 5.3 million people were within the warning zone, prompting localized alerts, brief evacuations and urgent calls to leave coastal areas for higher ground. The warning was canceled a few minutes before noon.

What do you do during a tsunami warning?

According to the National Weather Service, an active tsunami warning indicates the imminent danger of widespread coastal flooding and powerful currents. Those living within the warning area are instructed to avoid coastal and low-lying areas and move to higher ground or further inland. Tsunami watches, advisories, and warnings are updated and canceled as actual conditions become clearer.

In California, officials urge residents who live within a tsunami hazard zone to develop an emergency plan, know their evacuation routes, and ensure they are signed up to receive local emergency notifications. Residents can find more information about the specific hazards to their property by searching their address on the Office of Emergency Services website.

The Associated Press contributed to this report.