Big unknown: Where Dunedin is in quake cycle

Otago is one of the least seismically active areas in the country, so what is the actual risk that an earthquake will topple the Dunedin Courthouse tower?

An earthquake strong enough to collapse buildings is estimated to strike Dunedin once every 3135 years - the problem is, no-one knows where the city is in that cycle.

In the latest investigation of seismic risk in Otago, Opus International Consultants' 2005 assessment for the Otago Regional Council, which has regulatory responsibility for managing natural hazard risks in Otago, reports that on average there are three potentially damaging earthquakes of magnitude 5.5 on the Richter scale in Otago every decade.

However, they do not all cause damage because much of the region is sparsely populated and most of the epicentres are outside Otago, to the northwest of the region.

The Richter scale measures the size of the earthquake at its epicentre, but the Modified Mercalli (MM) Intensity scale measures the intensity of shaking at a specific location by the effect it has on people and the natural and built environment.

On the internationally accepted MM scale, shaking rated MM6 is strong enough to cause items to fall off shelves and slight damage to some buildings, and weak chimneys may fall.

An MM7 shake would cause masonry to crack and fall, and chimneys and unbraced parapets to fall.

The weakest buildings would first begin to collapse in an MM8 shake.

The last earthquake to cause damage in Dunedin was a Richter scale magnitude 5 quake just off Dunedin on April 9, 1974, that dislodged grocery stock from shelves and damaged chimneys in South Dunedin.

Assessments based on the damage observed placed that earthquake at between MM6 and MM7, Opus reports.

It estimates that in Dunedin a quake that causes MM6 shaking has a 110-year return period, and an MM7 shake a 536-year return period.

An MM8 quake in Dunedin is estimated to recur every 3135 years. The difficulty, GNS seismologist Ken Gledhill said, was that the science was not exact, and based on probabilities, so there was no way of knowing where in the cycle a particular location was, or when an earthquake might occur in the cycle.

In New Zealand, the Building Act requires building strengths to be at least 33% of the current building standard, to ensure they can withstand a moderate earthquake.

The Christchurch earthquake, in which 185 people were killed in buildings or by debris falling from buildings, prompted widespread reviews of the government's building stock, including the Dunedin Courthouse, which was assessed by Opus for the Ministry of Justice in 2011.

Opus found the presently closed north wing of the building was at 35% to 45% of the code - not technically earthquake prone under the Building Act.

However, the tower, because its ground floor was not strengthened (although the upper levels of the tower were strengthened in 1993) was only at 15% to 20% of the standard.

If buildings are under 33%, the Act requires local authorities to put in place processes to have the building strengthened or demolished. However, until recent years local authorities had fairly passive policies, often only acting when a building's owner wanted to do something with it.

Many councils, including Dunedin's, are now being proactive in requesting that information from owners and setting timeframes for the work to be done.

With regards to the Dunedin Courthouse, Opus reported the ground floor holding up the tower could potentially fall in a moderate earthquake, damaging most of the northern wing of the building.

''We are of the opinion all of these areas of the northern part of the building would be classified as posing high risk to users of the building in the event of a moderate earthquake and should be classified as earthquake prone''.

The building's parapets and ornaments were also expected to be below the required standard, and so also posed a risk to the occupants, the report said.

The ministry consequently decided to close the northern part of the building.

The general manager of the district courts has made ''no apologies'' for the closure.

Tony Fisher has previously acknowledged it is a bad situation.

But, while he accepted it was frustrating, Christchurch had shown the ministry ''all too clearly'' what was at stake when buildings were not capable of withstanding earthquakes, he said.

The ministry's overriding concern was the safety of staff, court users and the general public.

''We have a report saying the tower block could fall, so until the building is strengthened, we will not re-enter the premises.''

- debbie.porteous@odt.co.nz