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the series of earthquakes that damaged so much of Christchurch and even changed its landforms were far smaller than a major Alpine fault quake is likely to be. But just like the old real estate maxim of location location location, the locality of a natural catastrophe is just as important as its severity. while Christchurch is one of the three largest population centres in new Zealand, the southern end of the Alpine fault area designated as most likely to unleash a magnitude 8.1 earthquake is relatively lightly populated. the largest town in the region is the tourist centre of Queenstown (right), with a population of around 28,000. however, damage to roads and communications would nevertheless impact severely on communities in the region. the Christchurch earthquakes have cost the insurance industry an estimated $nZ17 billion, the government-operated earthquake Commission some $nZ12 billion and the new Zealand Government directly an estimated $nZ6.7 billion.

area that could rupture during a big earthquake is much larger. “So the energy that could be released would be far greater than people have thought.” “The Alpine Fault when it goes will be almost certainly a big earthquake. “What the research is showing that the area that breaks during that quake will be bigger than previously thought, so that the energy released would be larger and presumably the shaking would be over a wider area, too.” Previous research has shown that a large Alpine Fault earthquake will cause major changes to landforms and rivers. Horizontal movement of the Alpine Fault is about 30 metres every 1000 years, which GNS Science says is “very fast by global standards”. Each time it has ruptured, it has also moved vertically, lifting up the Southern Alps in the process. In the past 12 million years the mountain chain has been lifted by an estimated 20km. “It is only the fast pace of erosion that has kept [the mountains’] highest point below 4000 metres,” GNS Science says. “The glaciers and rivers have removed the rest of the material and spread it out across the lowland plains or onto the sea floor.” 60

An Alpine Fault earthquake will likely rupture along a larger fault length – several hundreds of kilometres rather than several tens of kilometres – over a longer period of time and affect a much larger area than the Darfield earthquake did. Moreover, it is likely that the aftershock sequence following an Alpine Fault earthquake will involve earthquakes of as much as M7. Research conducted at the University of Otago and GNS Science in the last few years has revealed that a large Alpine Fault earthquake will trigger a cascade of environmental effects that could persist for up to 50 years after an earthquake. “Violent shaking along the entire length of the earthquake rupture will trigger large landslides in steep topography and weaken hill slopes, making them more susceptible to landsliding in subsequent storms,” the groups say in a report published by GNS Science. “As west coast rivers and streams transport material produced by this landsliding down stream, alluvial fans and floodplains will aggrade, rapidly causing rivers to change their course abruptly and more frequently. “The cascade of impacts has the potential to chronically affect towns, road, insuranceNEWS

October/November 2015

communications and power infrastructure for decades after the earthquake.” “Additionally, aftershocks triggered by the main earthquake could be expected to be as large as M7 and to continue for many years.” Professor Lamb says his group’s research has established that someone standing many kilometres inland from the line of the fault might in fact now find they are standing right on top of it. The southern “wedge” helps to answer why the mountain ranges are wider and higher in the south, and why there has been a higher frequency of small earthquakes in the southern zone. He says the seismic testing has demonstrated that the crust under the South Island is very thick. “Seismic waves tend to travel faster the deeper they go, but seismic waves get slower under the Southern Alps. That is not what you would expect if the two tectonic plates were just sliding past each other on a near vertical fault. “That makes these regions more vulnerable than we first thought. “I think one of the lessons we have learned is that all of New Zealand is actually * earthquake-prone.”

Profile for Insurance News (the magazine)

OCT/NOV 2015 - Insurance News (the magazine)  

It didn’t rain enough at Monte Carlo during the September Reinsurance Rendezvous, putting paid to the hopes of industry heavyweights who wer...

OCT/NOV 2015 - Insurance News (the magazine)  

It didn’t rain enough at Monte Carlo during the September Reinsurance Rendezvous, putting paid to the hopes of industry heavyweights who wer...