Understanding New Zealand’s biggest seismic hazard


Over the last few weeks there has been coverage of research being done on New Zealand’s biggest seismic hazard, the Alpine Fault. I been surprised by the silliness and ignorance displayed by people on social forums giving the impression that they  know better about the research project. The research currently in progress examining the probable timing and size of the next earthquake is going to potentially save a lot of lives. It will also give the authorities an idea of how long they may have to get New Zealand ready and how big the event will be when the shaking starts. So, I welcome the research being done on the Alpine Fault. It is as exciting to see how the fault works as it is important to understanding it’s long term threat. But it is useful to review what is currently known.

The Alpine Fault is a dextral strike slip fault, meaning if one looks across the fault, the land on the other side is moving to their right. Whereas many faults have a dip component to their movements, forming a scarp when they rupture, the land to east of the Alpine Fault is moving southwest and the land to the west is moving northeast. The fault is made up of four known sections. Whereas the southern and central sections have been considerably researched, the northern section is not so well known. The Alpine Fault has a remarkable record of earthquakes starting to emerge. For 8,000 years it has ruptured at surprisingly regular interviews, with the last rupture being in 1717. So what do we know about the Alpine Fault?

  • It ruptures in magnitude 7.8-8.2 earthquakes
  • The duration of shaking would last 2-4 minutes
  • It ruptures about 330 years with nearly 300 having passed since the last event, suggesting that one in the next 50 years is quite realistic
  • The entire South Island would face significant disruption for an unknown length of time; lesser disruption would occur in the lower North Island
  • In the northern South Island the Alpine Fault transfers its stress loads to a set of roughly parallel faults all striking NE/SW, called the Hope, Clarence and Awatere faults
  • Widespread geomorphological effects would occur, ranging from landslides to liquefaction, landslide dams to lateral spreading
  • There may be up to 8 metres of horizontal displacement

The geophysics of the fault are fascinating. There is quite high heat flow under the fault, suggesting considerable friction along the fault interface. On the surface the fault appears to be locked, but at depth about 30mm of slippage is happening each year. At some point the surface strata has to reconcile with the lower strata.

But the news is not all bad. The Christchurch/Canterbury earthquake sequence has a silver lining that people should know about. This is now probably the safest place in Australasia to be in an earthquake. It will not bring back or justify in any way the deaths of 186 people. At the same time though, these quakes have reduced the probable death toll from an Alpine Fault earthquake by several hundred to possibly a thousand people by not only eliminating the most vulnerable buildings, but forcing the significant reinforcing of many others might have been unsafe post-earthquake. They have instilled lessons about resilience and getting community groups, government agencies and so forth working together. Because Christchurch is so strong now, it will be a really useful base for response and recovery efforts.

 

 

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