A 6.4-magnitude earthquake struck off the coast of northern California early Tuesday but there was no tsunami threat, seismologists said. The relatively shallow quake hit 25 miles (40 kilometers) southwest of the port city of Eureka, in Humboldt county, the US Geological Survey said. In a preliminary assessment, the USGS said there was a low likelihood of casualties but some damage was possible. The National Tsunami Warning Center said a tsunami was not expected. Power supplies appeared to have been hit, with nearly 60,000 customers without electricity in Humboldt County, according to poweroutage.us. “Power is out across the county. Do not call 911 unless you are experiencing an immediate emergency,” Humboldt Country Office of Emergency Services tweeted. Around a dozen small aftershocks were reported in the area, USGS said. California is regularly shaken by tremors and seismologists say a quake capable of causing widespread destruction is almost certain to hit the state in the next 30 years. A 6.7-magnitude earthquake in 1994 in Northridge, northwest of Los Angeles, left at least 60 people dead and caused an estimated $10 billion in damage, while a 6.9 quake in San Francisco in 1989 claimed the lives of 67 people.