A major earthquake struck 170 miles off Alaska early Tuesday, prompting a tsunami warning that forced people in coastal communities to flee to higher ground.

The magnitude 7.9 quake was reported at 12:31 a.m. local time in the Gulf of Alaska, according to the United States Geological Survey. There were no immediate reports of damage.

The United States National Tsunami Warning Center reported that a small tsunami with a wave height of less than eight inches was observed in the Alaska towns of Old Harbor, Seward and Kodiak.

About three hours after the quake hit, the center canceled tsunami warnings for British Columbia and parts of Alaska and also canceled a watch order that was in place from Washington State to Mexico. An advisory remained in effect for a 400-mile-wide swath of southwest Alaska from east of Seward to Chignik Bay.

Tsunami sirens went off in Kodiak after the earthquake. “Evacuate inland or to higher ground above and beyond designated tsunami hazard zones or move to an upper floor of a multistory building depending on your situation,” the authorities warned on Tsunami.gov. “Move out of the water, off the beach, and away from harbors, marinas, breakwaters, bays and inlets.”

Stephanie Wyzkoski, 42, who co-owns the Cranky Crow Bed and Breakfast in Kodiak with her husband, John, said emergency workers “told everyone to evacuate in the lower areas of town.”

After urging residents to seek higher ground, the police in Kodiak said that one popular area — Pillar Mountain — was at capacity. The police urged people to go instead to the local high school or to the Safeway or Walmart parking lots.

The warnings included San Francisco, where the authorities told people living within five blocks of the bay that they might need to evacuate.

The earthquake struck in the Gulf of Alaska, about 175 miles southeast of Kodiak Island. The region is part of a large subduction zone, where one large piece of the earth’s surface, or plate — in this case the floor of the Pacific Ocean — is slowly sliding under another — the North American continent. The Alaska subduction zone is the source of many earthquakes, including one in 1964 that, with a magnitude of 9.2 was the second-largest quake ever recorded.

But Peter J. Haeussler, a research geologist with the United States Geological Survey in Anchorage, said that the 7.9-magnitude quake most likely did not occur directly on a fault where the two plates meet. Rather, he said, it appeared more likely that the slip occurred on the ocean plate, at a point where it bends as it starts to slide under the continent.

The direction of the fault movement in this case would be horizontal, more like the San Andreas fault in California, and would be less likely to generate large tsunamis, Dr. Haeussler said.

Source: nytimes