CALIFORNIA — (CNN) A 6.8-magnitude earthquake shook the ocean floor about 100 miles off the northern California coast Thursday morning, the US Geological Survey said.

The quake, centered about 102 miles west of Ferndale, California, happened around 6:50 a.m. PT and is believed to have caused weak or light shaking in parts of northern California, including as far south as San Francisco, the USGS said.

There is no tsunami threat from the quake, the Pacific Tsunami Warning Center said.

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