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06 Dec 17 08:49

Yes, you read it correct, this blog is all about a complex earthquake. Can an earthquake be complex? Yes, though we may not have heard about it many a times. We might be frequently hearing about large or highly damaging earthquakes, but not about the complexity of an earthquake. May be it is bit of a coincidence that this complex earthquake also comes from a region, which has produced one of the most surprisingly high earthquake losses in the past - New Zealand. Yes, by now you might have guessed it, I am talking about the Kaikoura earthquake which hit New Zealand during second week of November 2016.

Why there is so much complexity for this earthquake? Earthquakes are nothing but the ground vibration created due to fracturing of faults. Usually, during an earthquake, a few number of faults will be ruptured - normally one or two. Soon after the Kaikoura earthquake, GNS (Geological and Nuclear Science, New Zealand) estimated that there were around 5 faults ruptured during the earthquake, which itself is a very high number. However in the following months, researchers found that it is not 5 but the number can be as high as 21 faults. The rupturing happened through stages and this has prolonged the duration of shaking (the shaking lasted for several minutes). The earthquake which originated near the tourist center of Kaikoura, progressed towards northern direction due to this multiple fault rupturing.

The fault rupture jumped from one fault to the other, which are separated by many kilometers. The jumping of fault rupture has occurred even in the cases where the faults were separated by around 15 km. This huge jump is uncommon and is being observed for the first time. As a result of this, the maximum ground shaking was observed around 100 km away (in the northern direction) from the epicenter.

Can this happen in other parts of the world? If yes, what can be the implications? The answer is a clear yes - it is quite possible. The scientific fraternity till now thought that the jumping of rupturing of faults separated by more than 5 kilometers were not possible. But Kaikoura earthquake has proved it wrong. Most of the present Catastrophe models do not consider this complexity in fault rupturing. This means that the risk assessment dome by models can go wrong in many regions, thereby leading to surprises.

There are events in the past like Tohoku, where the cascading of fault rupture were observed – the fault rupture occurred in several segments, which were considered as independent faults. This jumping of fault rupture has caused an earthquake with larger than expected magnitude. The latest USGS earthquake model for California limits the extent of fault rupture jumps to 5 kilometer. This means that a rupture mechanism like the one that took place in Kaikoura can cause earthquakes of much higher magnitudes in California (and other parts of the world) than what we anticipate. These things clearly point towards one thing – catastrophe models should go beyond the basic fault rupture assumptions for the right scientific reasons. It is high time that we take the "Kaikoura effect" into our catastrophe modelling.

Category: Climate/natural disasters: Disaster risk, Earthquakes, Resilience

Location: New Zealand


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