Aftershocks expected to rock NZ's Christchurch for decades

(Reuters) - Two further tremors rattled the New Zealand city of Christchurch on Friday as scientists predicted aftershocks from last year's devastating earthquake would last for decades.

The two latest aftershocks, the largest with a magnitude of 5.0, shook New Zealand's second-largest city just hours after a briefing called to address concerns about a large number of tremors in recent weeks.

"A lot of the stress must have been released in the city area, but around (the surrounding region of) Canterbury there may well be an ongoing sequence over a period of a few decades," Kelvin Berryman of GNS Science said. Christchurch, the largest city in New Zealand's South Island, is still recovering from a February 2011 quake measuring 6.3 which killed 182 people and caused around NZ$20 billion ($15.5 billion) in damage.

Three strong aftershocks on December 23, the strongest with a magnitude of 6.0, have been followed by a host of smaller quakes. Most aftershocks, including the latest two, have been centred to the northeast of the city at shallow depths.

"There is almost certainly still a (magnitude) 5.0 out there and we would guess quite a few 4's and 3s on a daily basis," said Berryman. "We are progressing into a period where quakes are not damaging but they can affect people mentally."

Many Christchurch residents have left the city and those who remain are feeling the strain.

"Every time I walk down the corridor at work, or go to the bathroom I'm thinking what I would do if another big one hit 'now'," one reader on the stuff.co.nz website wrote.

"I just hope that one day we're able to, once again, live our lives without the fear of every building falling down around our ears."

Large areas of the business district are still off limits after the February quake, which toppled the city's famous cathedral, as well as shops, office buildings and homes.

About 6,000 homes in the city were condemned after that quake.

New Zealand straddles the boundaries of the Indo-Australian and the Pacific tectonic plates. Christchurch has suffered more than 9,500 aftershocks since an initial 7.1 magnitude quake on September 4, 2010 which caused extensive damage but no casualties.

- Reuters