A 6.4-magnitude earthquake rocked a rural Northern California county early Tuesday, leaving at least two dead and a dozen injured as the shaking cut off power to tens of thousands of people.

The quake occurred near Ferndale, a small city in Humboldt County some 200 miles north of San Francisco, just after 2:30 a.m. local time, the United States Geological Survey reported. Authorities were still assessing the extent of the impact, and the county sheriff's office said it had fielded reports of "widespread damages," including buckled roads, cracks in a historic bridge and some collapsed homes.

Officials initially reported just a few minor injuries, but by the early afternoon, Humboldt County's emergency management office said two people, ages 83 and 72, had died "as a result of medical emergencies occurring during and/or just following the earthquake." The number of people reported hurt increased to 12 and was expected to rise further.

The area's power infrastructure also suffered significant damage. For about 12 hours, 70,000 customers in Humboldt - more than two-thirds of the tracked population - were without electricity, according to the website PowerOutage.us. By the evening, 50,000 were still in the dark. Officials said it would take at least one to two days to fully restore power.

The county warned residents to "be prepared for aftershocks," the smaller tremors that follow large earthquakes. The USGS has observed at least 17 aftershocks of a magnitude 3.0 or higher, which were strong enough to be felt in the surrounding area. The largest aftershock was magnitude 4.6, state officials said.

"We live in earthquake country," Mark Ghilarducci, director of California's office of emergency services, said at a news briefing late Tuesday morning. "This is another example of the fact that earthquakes can occur at any time without notice."

While there is no way to predict an earthquake with enough time for people to evacuate, a new government-sponsored earthquake early warning system sent alerts to the phones of 3 million people in Northern California about 10 seconds before the quake began.

In the aftermath, rattled residents posted photos and footage of toppled furniture and jumbled rooms.

"That was a big one," Ferndale-based journalist Caroline Titus said on Twitter. "House is a mess."

The lingering effects of seismic activity were on full display Tuesday afternoon in Rio Dell, a city of about 3,400 that was the epicenter of the earthquake. As local leaders gathered in City Hall to address the media, another aftershock shook the building, startling the assembled speakers.

"For many of us, this is a tough day," Rio Dell Police Chief Greg Allen said. "It started at 2:37, when most of us were asleep. That jolt hit us pretty hard."

So far, city investigators have deemed 15 houses unsafe for occupancy, and as many as 150 Rio Dell residents could be displaced, said City Manager Kyle Knopp. Most have found temporary shelter through family or friends, he said.

The more dire problem is the city's water supply, which has been completely cut off - taps running dry and toilets not flushing. Residents could go 48 hours without running water, Knopp said.

Some of the most striking damage was reported about 12 miles away at Fernbridge, a concrete arch bridge that has spanned the Eel River since 1911 and has survived a barrage of floods and earthquakes.

At least one crack was visible on the bridge in a photo from the region's arm of California's Department of Transportation. Mike McGuire, who represents the area in the state Senate, said the bridge would be closed "out of an abundance of caution" while engineers assessed the structure. Emergency vehicles were still allowed to traverse it, and Ferndale residents were able to get in and out of their city via another route.

Elsewhere, a structural fire was reported, along with breaks in water and gas lines. Pacific Gas & Electric said it was sending emergency response crews to the area.

Humboldt is a county of about 136,000 that sits on the state's rugged northern coast and is famous for its vast stretches of old-growth redwood forests and its miles of protected parkland. The region is also known for a frontier spirit and has become a capital of the country's cannabis industry.

In a state always on guard for the "big one," north coast locals have much experience with temblors. Tuesday's earthquake came exactly one year after a 6.2-magnitude quake hit a few dozen miles from Eureka, Humboldt's county seat and largest city. That bout of shaking shattered glass in homes and stores, but did not trigger the widespread power outages seen on Tuesday.

"It's not unusual for us to have earthquakes of this size in this region," Cynthia Pridmore, of the California Geological Survey, said at the state's morning briefing.

The earthquake occurred about two miles offshore on the Gorda Plate. It's a geologically complex area, Pridmore said, home to several other plates, most notably the North American and Pacific plates. Over the last century, there have been roughly 40 earthquakes there between magnitudes 6 and 7.

The USGS predicts there is a 13 percent chance of an aftershock of magnitude 5 or above sometime in the next week, which could deal a devastating one-two blow to buildings already damaged by Tuesday's quake.

"People do need to be prepared," Pridmore said. "Especially if they're in weakened structures."