As geologists learn more about the Alpine Fault, emergency managers are learning more about the consequences of major earthquakes in our future. Read more...
Te Ara Encylopedia of New Zealand is a good general on-line resource for natural hazard information. Links to Active Faults (which includes the Alpine Fault), Earthquakes, Historic Earthquakes (including experiences of those who have experienced large earthquakes), and Plate Boundary help gather a quick overview of the New Zealand situation.
GNS Science has lots of information about natural hazards in New Zealand from a scientific perspective and links to Geonet which is a scientific monitoring programme with public access.
West Coast Regional Council has information about West Coast specific natural hazards.
NZ Alpine Fault Blog gives an account of a teacher fellow’s journey in the preparation of the ShakeOut.
The Science Behind the Risk
New Zealand lies on the boundary of two of the main plates of the Earth’s crust. We are part of the ‘Pacific Ring of Fire’ that defines the areas that suffer the most significant volcanic and earthquake risks in the world. It is not a place to settle down for a quiet life, at least not in a geological timeframe.
North Island is to the West of the plate boundary, and sits entirely on the Australian Plate. The less buoyant Pacific Plate dips underneath the North Island and this action (subduction) caused hot spots under the surface in the Earth’s mantle that create volcanic activity above it.
Off the South of the South Island, the process is reversed, with the Australian Plate dipping beneath the Pacific Plate. The boundary between these conditions is clearly going to be an interesting place and it is known as the Alpine Fault, running most of the length of the South Island, 650km, and one of the longest natural straight lines in the world, visible from space. (Learn more: How the Plates are Moving)
Scientists have been studying the Alpine Fault since it was first discovered and named by Harold Wellman in the 1960’s during a mineral hunting expedition. A simplified geological map of New Zealand show clearly what he noticed, with similar types of rocks present, in the same sequence, at either end of the South Island, and apparently shifted in a North East – South West axis along what could only be the Alpine Fault.
The 450km movement of rocks has occurred over the last 23 Million years or so, and scientists now believe that it has always done so in episodic movements of about 8m every 200 – 400 years. Each movement would be associated with large (magnitude 7 -8) or even great (magnitude >8) earthquakes.
Scientists have found out that the last earthquake occurred in 1717, and can even specify that it was in the growing season of that year. Combinations of techniques such as studying trenches dug across the fault trace, aging river terraces, aging tree stands, and finally studying the rings of individual trees have allowed detailed and undisputed evidence of the last event. Before this it is difficult to be so precise, but it seems that earthquakes have occurred in 1620, 1450, 1220, and 950. The average gap here is the 200 years noted in the advertising for the Shake Up, but to base earthquake predictions on this would be foolish because of the relatively small sample size given the 23 Million year history. To say that they occur every 200-400 years is a better estimate of the realistic situation when we now know, through accurate GPS measurements, that the movement of the Pacific Plate relative to the Australian Plate is about 25-30 mm per year in the central section of the Alpine Fault.
The Alpine Fault is responsible for the uplift of the Southern Alps as well as the large geological offset. The rate of uplift is much less than the horizontal (or strike-slip) movement, but with 1.0 – 1.5m movements at each earthquake we may expect the Southern Alps to be about 20 000m high during their growth in the last 12 Million years. The only reason that they are no much bigger is that they are being constantly eroded, with earthquakes themselves being the most severe cause of large landslips and hence erosion. The rainfall does not help matters, but that comes of having a mountain range in the midst of a moist air stream. Add to that the poor rock structure near to the fault line, and the over-steepened slopes as the mountains continue to be pushed up from the East, and the rate of erosion is keeping pace with the uplift.
Investigation continues into the Alpine Fault and a recent meeting of 50 or so of the worlds leading geoscientists in Franz-Josef have initiated a proposal to drill into the fault at a depth of about 4km to study the nature of the rocks at depth and compare them with those that are exhumed at the surface. The Alpine Fault has a number of unique features that make it ideal for study and this may well be a long-term research interest on the West Coast. (Learn more: Scientific Drilling)
For a more full understanding of the science behind the Alpine Fault visit the GNS website and follow the links connected with earthquakes and natural hazards.
An excellent simple account of the essential geology of the Southern Alps can be found in “The rise and fall of the Southern Alps” by Glen Coates (Canterbury University Press).
A more full account of New Zealand’s geological history can be found in “In search of ancient New Zealand” by Hamish Campbell and Gerard Hutching (Penguin and GNS Science).
A full description of New Zealand geosciences and the current research developments can be found in “A continent on the move – New Zealand geoscience into the 21st Century” edited by Ian J Graham (GNS Science)
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