What if it happened here?

Invercargill's city centre could be flattened in a large earthquake because very few old buildings have been strengthened.

Geologists say it is only a matter of time before a massive quake occurs on the Alpine Fault, which runs along the western spine of the South Island, and Invercargill is in the firing line.

It is thought there is a 50 per cent chance of an Alpine Fault earthquake happening in the next 50 years.

The city was also at risk of tsunami damage from any earthquakes in the Puysegur fault lines and, like Christchurch, could be surprised by a previously unmapped fault line.

Invercargill City Council building regulations services manager Simon Tonkin confirmed yesterday that most of the old buildings in the CBD were considered "potentially earthquake prone" and had never been properly checked.

Pre-1970 buildings, including hotels, churches, malls, civic buildings and busy retail outlets, do not have to meet the same minimum structural standards that apply to new buildings.

Any concrete or brick structure without steel reinforcing was likely to suffer damage in an earthquake.

Mr Tonkin said owners were free to check their buildings for structural safety but were not required to, and there had been very little retrofitting in the city.

University of Otago professor of geology Richard Norris said the earthquake that hit Christchurch came from a previously unknown fault line and those probably existed under the Southland plains as well.

Prof Norris said he knew that in Dunedin, extensive retrofitting had been done to old buildings to strengthen them.

In Wellington, it was compulsory to retrofit old buildings because of the heightened earthquake risk. "It doesn't stop damage to the buildings but it stops them from collapsing in a heap. If it is done, there is much less risk to life.

"Invercargill is quite a long way from the (main fault lines) but on the negative side, like Dunedin, you have quite a lot of old brick buildings," Prof Norris said.

The magnitude-7.8 earthquake in Fiordland in July last year was thought to have radiated out into the Tasman Sea, he said.

If it had gone the other way there would have been much more damage in Southland.

Likewise, a rupture in the Alpine Fault line would cause more damage in Southland if it started further up the island, building up energy as it moved south.

Most buildings in central Invercargill are considered a "potential earthquake risk" and most old and historic structures have never been strengthened.

Mr Tonkin said councils met last month to work out a unified approach to earthquake-prone buildings.

Once those guidelines came out they would be notifying all owners with buildings considered at risk and they would be able to have an input into new rules.

- © Fairfax NZ News