Earthquakes

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.

tectonic-plate-boundaries.PNG
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 earthquakes.

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.

Other Sources

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.

Shaking amplification

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.

Liquefaction

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.


Page reviewed: 06 May 2016 3:34pm