A powerful magnitude 6.4 earthquake has jolted the extreme northern coast of California before dawn, crumpling homes and roads, rupturing utility lines and leaving thousands of residents without running water and electricity.

At least a dozen people were injured, and two others died from “medical emergencies” that occurred during or just after the quake, according to the Humboldt County Sheriff’s Office.

The tremor, which struck at 2.30am on Tuesday and was followed by about 80 aftershocks, was centred 350 kilometres north of San Francisco offshore of Humboldt County, a largely rural area known for its redwood forests, local seafood, lumber industry and dairy farms.

The region also is known for relatively frequent seismic activity, although the latest quake appeared to cause more disruption than others in recent years.

Tuesday’s tremor set off one structure fire – by severing the gas line of a hot-water heater – and caused at least two other buildings to collapse, authorities said. The blaze was quickly extinguished, and fire crews rescued a resident briefly trapped in the home, according to fire officials.

About two dozen houses were so heavily damaged as to be “red-tagged,” declared unsafe for habitation. Most were in Rio Dell, a town of about 3400 residents that bore the brunt of the quake.

Water service to the entire community was knocked out, and City Manager Kyle Knopp said he expected 100 to 150 residents would likely end up displaced once housing inspectors had assessed all the structural damage there.

Some 79,000 homes and businesses countywide were without power in the quake’s immediate aftermath

Fire officials said dispatchers fielded about 70 emergency calls.

Details of casualties were sketchy. The two fatalities involved individuals, one aged 72, the other 83, who suffered medical emergencies that coincided with the quake, preventing rescue teams from reaching them in time to render life-saving care.

Most of the 12 survivors known to have sought medical care got themselves to hospitals and sustained relatively minor injuries, many from fallen objects. Two of the most serious cases were a head injury and a broken hip, officials said.

California’s earthquake early warning system sent electronic alerts to the mobile devices of some three million northern California residents 10 seconds before the first shaking was even felt.

While earthquakes are routine in California, tremors at a magnitude 6.4 are less common and potentially dangerous.

Shaking from Tuesday’s quake was felt as far away as the San Francisco Bay area, the US Geological Survey reported. The biggest aftershock registered a magnitude 4.6.