The two biggest threats to Southland are earthquakes on the
Alpine Fault and earthquakes within the Puysegur subduction zone.
Southland's most significant recent earthquake activity
occurred 100 km north-west of Tuatapere on the 15 July 2009 and had a magnitude
of 7.8 It must be noted this earthquake did not originate from the Alpine
Fault. Fortunately because of the remote location and minimal built and social
environments in this area the impact on the community was minimal.
Southland lies adjacent to the boundary of the Pacific and
Australian tectonic plates. The movement of the sub ducting Australian plate
and the overlying Pacific plate together with movement along the fault line
are the source of frequent earthquakes in Fiordland.
Map of the South Island of New Zealand showing the tectonic
plate boundary. SZ= subduction zone. Source: GNS
The Alpine Fault
The Alpine Fault is a major fault that runs northwards from
south of Milford Sound, across Lake McKerrow and along the length of the West
Coast. Analysis of 24 past ruptures
suggests that the fault gives rise to magnitude 7+ - 8+ earthquakes every 330
years on average. The most recent Alpine
Fault is thought to have occurred in 1717.
Given the timing of the last event, the average interval
between events and the magnitude of the event, the likely duration of the
shaking (two minutes), the Fault obviously presents a significant hazard,
especially to the north west part of the Southland region and Central Otago. There is a 30% chance of a large earthquake
occurring on the Alpine Fault in the next 50 years.
The shaking in an Alpine Fault earthquake is predicted to
last for about two minutes, which is substantially longer than most
The Puysegur Subduction Zone
The Puysegur subduction
zone is the name given to a large area from about Doubtful Sound,
southwards to Solander Island, west of Stewart Island.
It is an area within which one very large chunk of the
earth’s crust (the Australian Plate) slides under another (the Pacific Plate). The clashing of the plates is not a smooth
journey. Strain accumulates and
earthquakes occur when it is suddenly released.
Many earthquakes occur in the Puysegur subduction zone – the last
notable ones being July 2009 (magnitude 7.8) and August 2003 (magnitude
7.2). Magnitude 8+ earthquakes are
regarded as a possibility on the Puysegur subduction zone.
Earthquakes from the south western part of the zone are
considered to be a source of tsunami along the southern coast of Southland.
There are several known (and, most likely, a few unknown)
active faults both within and beyond the Southland region that have the
potential to generate damaging levels of shaking within the region. An active fault is any fault that has moved
in the last 125,000 years.
The amount of shaking felt in any given earthquake varies
with the strength of the subsoil and underlying rock at the location. The soil and rock condition has been mapped
as an indicator of the capacity for amplified shaking.
Generally, floodplain soils have the potential to amplify
shaking. As such, it is likely that in
much of Southland the severity of shaking (and potential damage) will be
greater than what is predicted or modelled for various earthquake scenarios.
The awareness of liquefaction has increased markedly since the Christchurch earthquakes. It is basically an earthquake related process of turning a solid soil into a liquid and weaker state. It is most likely to occur in saturated sands and silts.
Related to liquefaction is a process called lateral spreading, whereby land moves towards lower areas whilst in a semi liquid state.
The Southland areas considered most susceptible to liquefaction include low lying areas of hydraulic fill, peat mires, low lying parts of lake deltas and around the margins of the fjords.
The last two magnitude 7+ earthquakes have resulted in isolated pockets of liquefaction, some as far away as the Manapouri/Te Anau area.
Report - Coastal deformation and tsunami deposit observations following the July 15, 2009, MW 7.8 Dusky Sound earthquake - August 2009
Report - Amplified ground shaking and liquefaction susceptibility, Invercargill City - January 2012
Report - Analysis of Te Anau residents' impacts, awareness and preparedness following the 2003 Fiordland earthquake - December 2004
Report - Geological Hazards – Southland District Council Lifelines Study - July 2006
Report - National Seismic Hazard Model for New Zealand 2010 - August 2012
Report - Surface effects and geological observations following the 1988 Te Anau and 1989 Doubtful Sound earthquakes, Fiordland, NZ - September 2006