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Wellington Faults.  Source: GNS Science

Wellington Faults. Source: GNS Science

CERA Meeting Scares Media

Some media outlets have put a rather negative spin on GNS Science’s most recent earthquake forecast for the central Canterbury region, which was discussed at a recent meeting of the Canterbury Earthquake Recovery Authority (CERA). GNS’s latest forecasting model estimates the probability of a “large” (magnitude 6.0 to 6.9) event occurring within the region over the next 12 months to be 23%, dropping to 10% over the following year.

The forecast figures provoked a bit of a media feeding frenzy. “Strong chance of another big shake” trumpeted the New Zealand Herald yesterday morning. “Big earthquake risk put at 23 percent” read yesterdays’ headline on Stuff.co.nz. Clearly such headlines are not good for the progress of Christchurch’s recovery, or indeed for the nation’s $9 billion tourism industry. Enticing overseas visitors to a damaged city is hard enough without telling them that there is a 1 in 4 chance of another major quake in the next year. Christchurch Mayor Bob Parker was forced into damage control mode, claiming on TVNZ that GNS’s forecasting figures have been taken out of context.

Hazard is Not the Same as Risk!

There is clearly some validity to mayor Parker’s claim. In the scientific sense, GNS’s figure of 23% does not relate directly to “risk”, as portrayed by some media outlets. The probability of a large earthquake occurring somewhere within the model area within the next year is 23%. That simply reflects the regional hazard, which is very different than the risk to the largely urban population. The risk associated with a natural hazard is nearly always highest where the hazard intersects with population centers. So if we are actually going to talk about risk, we should be looking at the probability of another large earthquake occurring very close to Christchurch (or Wellington for that matter).

Adding Fuel to the Fire

Unfortunately, the media haven’t done a great job of informing the public of what the GNS forecast means in terms of risk, and have arguably succeeded in generating unnecessary fear and anxiety. Those sentiments are clearly evident in the comment stream on Stuff.co.nz, which include:

” I appreciate getting this info. although I feel quite down about it. I would rather know than not know. Now we can make decisions about whether to stay here or not. At this stage I,m thinking of moving away for a year or so, just to see what pans out. I have been on edge since Sept. 4th. + don,t really fancy living another year like this.”

Which sums up the emotional state of many residents of Christchurch and surrounding area, who are tired of waiting for the aftershocks to end. Tired of waiting for information on EQC and insurance claims. Wondering if homes will survive the next big aftershock. Wondering if it makes sense to stay in the Christchurch area. Many Cantabrians are on edge. But is the negative press actually justified?

What Does the Science Actually Tell Us?

Let’s take a closer look at the GNS figures, and ask:

What is the science actually saying about the future risk of another large earthquake in the greater Christchurch region?

One needs to put the GNS figures in the correct context, which means acknowledging that the GNS model calculates the probability of future earthquakes in the entire post-4-September aftershock area, which covers roughly 10,000 square kilometers of the central Canterbury plains and Banks Peninsula. The aftershock area encompasses Christchurch City (including the former Banks Peninsula District), and large portions of the Waimakariri and Selwyn Districts. In terms of population, approximately 460,000 people live in the area encompassed by GNS’s forecast, with some 370,000 (or 80%) of those people living in Christchurch City. By my very rough calculations, the urban area of Christchurch City is approximately 200 square kilometers, a small fraction of the area that the GNS forecasts cover.

Source: GNS Science (aftershock area boundary and urban area added)

Source: GNS Science (aftershock area boundary and urban area added)

Even if we consider the broader Christchurch City area to be 25% of the overall GNS forecast area, then we are more realistically looking at a 1 in 20, or 5% chance of a similar event to 22nd February occurring again in the next year. How does this compare to other urban areas in New Zealand? According to GNS scientists,

“What we’ve actually calculated is the 23% probability for the entire aftershock region. So that’s looking much beyond Christchurch.
We don’t really have any specific way to narrow that down to Christchurch, but as a kind of guide based on some basic information, that’s roughly a quarter of that probability. In rough terms, it means the quake probability for Chch has become similar to many other parts of NZ such as Wellington, Hawke’s Bay, Wanganui, and Poverty Bay where quakes are more frequent.’ (M. Gerstenberger and J. Callan, GNS).

“Turn it around the other way, there’s a 95% chance that nothing will happen and I think that is the space that I would rather exist in than sitting around going ‘there’s a 5% chance it will happen in my city” (Christchurch Mayor Bob Parker)

So, based on GNS’s latest forecasts, Christchurch’s earthquake risk in the near future is similar to that for other seismically active urban regions in New Zealand. The 22 February earthquake caused so much devastation in Christchurch (even though it was within the realm of aftershocks that were expected following 4 September), because it unfortunately occurred very near the highest density of population and infrastructure. Could this happen again? Yes, it could, but the chances are much less than 1 in 4, and probably no more likely than than a similar event in Wellington.

Where Would the Fault Lie?

It is also important to note that it takes a significant fault to generate an earthquake of M 6-6.9. Such a fault would have to be at least as long as the Port Hills fault, and/or connected to other faults that could rupture at the same time. Computer models simply cannot tell us definitively whether or not that is the case. We need to know conclusively if such a scenario is possible, and if so, what the risks are. Which is why scientists are currently working very hard to generate a better picture of the faults and past seismicity of the greater Christchurch region. This work includes subsurface geophysical investigations, which can be focused on certain priority areas based on the distribution of aftershocks following the 4 September earthquake. Such areas include the oft-mentioned “gap” between the Greendale and Port Hills faults. Some media stories have led concerned people to believe that a fault in the gap “may rupture and link the two bigger faults” (Stuff.co.nz, 31 May, 2011). This is somewhat misleading, as the two bigger faults have already ruptured, so even if a connecting fault in the gap ruptured in the near future, most of the potential energy from the longer end sections has already been released. Furthermore, the highest density of aftershocks has been in the gap, suggesting that much of the energy there has also been released. Of course, that does not preclude the possibility of other faults near Christchurch, a question that current and future scientific investigation will help to resolve.

Vast amounts of data have already been collected, and leading scientists are processing and interpreting that data now. These data will represent one of the most extensive pre-to-post earthquake data sets in existence. In the near future we will have a better idea of the earthquake history, and fault distribution and connectivity in central Canterbury. In the meanwhile, New Zealanders, as a matter of course, should be preparing for future earthquakes. We live on a plate boundary, and Christchurch will not be the last place that is affected by earthquakes.