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09 Aug 13 13:23

On February 22nd 2011 a magnitude 6.3 earthquake struck the city of Christchurch. From a seismological perspective it was a very interesting event since it is one of the few where we gathered a lot of data very close the the rupture of the earthquake. Further it was the first event where Liquefaction (the phenomenon that soils lose their coherence under earthquake loading) was not only a localized phenomenon but was observed over large areas.

The large surprise in was that liquefaction can also add significantly to an individual Earthquake loss. Liquefaction was previously considered to affect only small regions and not contribute significantly to the total loss of an event. This view changed with Christchurch. Many will remember the large residential areas being literally flooded by the earthquake.

In addition, we learned that the cost of demolishing a building can actually be higher than constructing a new one. This especially holds true if the building is affected by liquefaction. Such a building might seem completely intact - but could be tilted by only 3 degrees which renders the building uninhabitable. One can now not simply tear down such a building but it has to be deconstructed. This becomes especially complex and costly in the case of tall buildings that neighbor each other closely.

Did now our models learn from this? Are we now - 2.5 years later in a better position to mimic the losses from liquefaction? Can it in fact be modeled?

Christchurch revealed that liquefaction should be included in earthquake models as it can add significantly to the total loss of an individual event. Liquefaction has always been implicitly present in most earthquakes models, but Christchurch revealed a need to expand our understanding of this risk. The key challenge is that most existing methods for modeling liquefaction require in-depth knowledge about the local conditions (such as depth to ground-water table and local soil composition). Data sets that provide this information are rare, and limited to a handful of cities around the world. As the earthquake insurance industry grows into new markets, models are increasingly limited by availability and quality of data. Nevertheless, modeling of liquefaction risk with limited data is still an attainable goal. SwissRe just developed it's own worldwide approach to quantify liquefaction risk also in regions with limited data availability.

Although Christchurch drew our attention to liquefaction risks, it remains a single example in the history of earthquake loss events. The key contribution to the loss-costs will remain the losses we expect from ground-shaking. This is because the areas exposed to shaking are far larger than those exposed to liquefaction. The experience with Christchurch should serve to remind us that many second-order effects of earthquakes are captured by our models only approximately, if at all. Other examples are potential losses from a dam-failure following an earthquake, or disruption of complex supply-chains in the manufacturing industry.

So besides sharpening our view to a newly arising risk we should not forget to sharpen our capabilities on the existing ones. We still have much to learn about earthquakes. And liquefaction is a small aspect in this.

Category: Climate/natural disasters: Earthquakes

Location: Christchurch, Canterbury, New Zealand


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