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 about 52 miles north-west of the mining area of Iquique, the US Geological Survey (USGS) said.
Waves of up to 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 Chile 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 later.
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.
Iquique Governor Gonzalo Prieto told local media that in addition to those killed, several people had been seriously injured.
While the government said it had no reports of significant damage to coastal areas, a number of homes were reported destroyed in Arica.
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