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
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
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
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
''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
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
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