WELLINGTON?A 7.0 magnitude earthquake rocked New Zealand's second largest city Christchurch early Saturday, causing widespread damage including the collapse of some buildings and power outages, witnesses and the US geological survey said.

The quake struck at a depth of 16.1 kilometers (10.0 miles) some 30 kilometers (19 miles) northwest of Christchurch at 04:05 am (1635 GMT Friday), the USGS said.

There was no major tsunami warning.

Aftershocks were ongoing and damage widespread in Christchurch, local newspaper The Post reported, while there appeared to be a power outage throughout the city of about 340,000.

Local resident Colleen Simpson said that many people had run out onto the streets in fear, while the mobile phone network was failing, according to the Post.

"Oh my God. There is a row of shops completely demolished right in front of me," Simpson said.

Kevin O'Hanlon, from Mairehau in Christchurch, said: "Just unbelievable. I was awake to go to work and then just heard this massive noise and, boom, it was like the house got hit. It just started shaking. I've never felt anything like it," the newspaper reported.

The damage included structural harm to houses, broken glass and damaged bridges and vehicles, the Post said.

Civil defense staff were deciding whether to activate the national crisis centre, Radio New Zealand reported.

An aftershock of magnitude 5.7 was felt at 1653 GMT (0453 local time), the USGS said, while local media said a series of shocks continued to roll across the city.

The quake was felt through much of New Zealand's South Island where Christchurch is located on its east coast, the Post said. 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.

"Earthquakes of this size sometimes generate local tsunamis that can be destructive along coasts located within a hundred kilometers of the earthquake epicenter," the centre said.

"Authorities in the region of the epicenter should be aware of this possibility and take appropriate action."

The magnitude was revised downwards from an initial estimate of 7.4 and a subsequent one of 7.2, the USGS said.