The US Geological Survey said the quake struck at 20:46 local time (23:46 GMT) about 86km (52 miles) north-west of the mining area of Iquique.
Waves of up to 2.1m (6ft) have hit some areas in Chile, and there have been power cuts, fires and landslides.
Tens of thousands of people were evacuated in affected areas, where a state of emergency has been declared.
Chilean TV broadcast pictures of traffic jams as people tried to leave.
