At least five people are known to have died and tens of thousands of people have been evacuated.
The quake struck at 20:46 local time (23:46 GMT) about 86km (52 miles) north-west of the mining area of Iquique, the US Geological Survey said.
Waves of up to 2.1m (6ft) have hit some areas and there have been power cuts, fires and landslides.
The government said the declaration of a disaster in the Arica, Parinacota and Tarapaca regions was aimed at “avoiding instances of looting and disorder”.
President Michelle Bachelet said the country had “faced the emergency well” and called on those in affected regions “to keep calm and follow instructions from the authorities”.
She is due to visit the affected areas on Thursday.
Chilean TV broadcast pictures of traffic jams as people tried to leave the affected areas.
Officials said the dead included people who were crushed by collapsing walls or died of heart attacks