Widespread damage has occurred including the collapse of some buildings and power outages.
Aftershocks were ongoing and damage widespread in Christchurch.
Many people had run out onto the streets in fear, while the mobile phone network was failing.
The damage included structural harm to houses, broken glass and damaged bridges and vehicles.
The quake was felt through much of New Zealand's South Island where Christchurch is located on its east coast.
There was no widespread threat of a tsunami, according to the Pacific Tsunami Warning Centre.
But an earthquake of this magnitude could generate a local tsunami, the centre said.
The magnitude was revised downwards from an initial estimate of 7.4, the USGS said.
