A 6.8 earthquake rattled northern California slightly early Monday, 70km west-northwest of Ferndale, California and about 240 miles northwest of Napa valley.

The tremors were felt in San Francisco, but no damage has been reported from the earthquake, which had a depth of 7 km.

Sonoma County had a smaller earthquake in January that was felt by several people in the area, the local newspaper reported.

The Napa Valley online newspaper carries an online page on preparing for earthquakes, which are not uncommon in the region.