RIO DELL, Calif.: A powerful magnitude 6.4 earthquake jolted the extreme northern coast of California before dawn on Tuesday, damaging homes, roads and water systems and leaving tens of thousands of people without electricity. At least 11 people were reported 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 2:30 a.m. PST and was followed by about 80 aftershocks, was centered 215 miles (350 km) 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 temblor set off one structure fire, which was quickly extinguished, and caused two other buildings to collapse, according to the California Department of Forestry and Fire Protection (CalFire). The department said its dispatchers fielded 70 emergency calls after the quake, including one report of a person left trapped who needed rescuing, spokesperson Tran Beyea said. Details on quake-related casualties was sketchy, but one surviving victim was a child with a head injury and the other an older person with a broken hip, according to local media reports citing the sheriff"s office. "REALLY INTENSE"Police closed a bridge crossing the Eel River into Ferndale, a picturesque town notable for its gingerbread-style Victorian storefronts and homes, after four large cracks were discovered in the span. The California Highway Patrol also said the roadway foundation there was at risk of sliding. Authorities reported at least four other roads in Humboldt County closed due to earthquake damage, and a possible gas line rupture under investigation. One section of a roadway was reportedly sinking, the Highway Patrol said. Ferndale and the adjacent towns of Fortuna and Rio Dell appeared hardest hit, with damage including water main breaks and about two dozen homes "red-tagged" because they were too unstable to be safely inhabited, state emergency services officials said. "The shaking was really intense," said Daniel Holsapple, 33, a resident of nearby Arcata, who recounted grabbing his pet cat and running outside after he was jostled awake in pitch darkness by the motion of the house and an emergency alert from his cellphone. "There was no seeing what was going on. It was just the sensation and that general low rumbling sound of the foundation of the whole house vibrating," he said. Janet Calderon, 32, who lives in the adjacent town of Eureka, saidshe was already awake and noticed her two cats seemed agitated moments before the quake struck, shaking her second-flood bedroom "really hard." "Everything on my desk fell over," she said. California"s earthquake early warning system appeared to have worked, sending electronic alerts to the mobile devices of some 3 million northern California residents 10 seconds before the first rumbles were felt, said state emergency chief Mark Ghilarducci. While earthquakes producing noticeable shaking are routine in California, tremors at a magnitude 6.4 are less common and potentially dangerous, capable of causing partial building collapses or shifting structures off their foundations. Tuesday"s temblor struck in a seismically active area where several tectonic plates converge on the sea floor about 2 miles offshore, an area that has produced about 40 quakes in the 6.0-7.0 range over the past century, said Cynthia Pridmore, a senior geologist for the California Geological Survey. "So it is not unusual to have earthquakes of this size in this region," she told a news conference. Shaking from Tuesday"s quake, which occurred at the relatively shallow depth of 11.1 miles (17.9 km) was felt as far away as the San Francisco Bay area, the U.S. Geological Survey reported. The biggest aftershock registered a magnitude 4.6.Some 79,000 homes and businesses were without power in Ferndale and surrounding Humboldt County shortly after the quake, according to the electric grid tracking website PowerOutage.us. PG&E crews were out assessing the utility"s gas and electric system for any damage and hazards, which could take several days, company spokesperson Karly Hernandez said.