A moderate 5.1-magnitude earthquake hit an offshore area of northern California early Sunday, but there were no immediate reports of casualties, the US government said. The epicenter of the tremor, which occurred at 2:26 am (1526 PST), was located in the Pacific Ocean, 30 kilometers (18 miles) west of the town of Petrolia, the US Geological Survey said. The reading was based on the open-ended Moment Magnitude scale, now used by US seismologists, which measures the area of the fault that ruptured and the total energy released.