GeoNet has been busy crunching the numbers based on all the seismic activity in the aftershock area of November’s Kaikoura Earthquake. So what’s the big news? The expected numbers of earthquakes have dropped a lot since the last forecast. There is now a 25 percent chance of one or more M6.0-6.9 earthquakes occurring within the next month; this has decreased from 54 percent from their last forecast on 19 December 2016. The downward movement in their forecast is good step in the right direction!
But does this mean we are all in the clear and don’t need to worry about more big earthquakes? No, absolutely not. Another big earthquake is still well within the probabilities in their models. A 25% chance in a month is still a concerning probability. We need to continue to be prepared for earthquakes as these will go on for years to come. The ongoing Canterbury Earthquake Sequence is an example of aftershocks that can last for years after the initial mainshock (which was the M7.1 Darfield quake in 2010).
The scenarios specifically address the probabilities of what we might see happen within the next year and were estimated in mid-December 2016. The scenarios cover a wider geographic area than the aftershock probability forecast area. The probability numbers in the table above are slightly different to the scenarios. This is because we have used new information we have gathered from the slow-slip events, and their potential impact on the plate interface and other faults, to help define our probabilities in scenario three.
There are very different probabilities for each scenario; some of these may be more unsettling to you than others. We recognise that while these scenarios may increase anxiety the best thing is to be prepared. Remember: To drop, cover and hold in an earthquake. If you feel a long or strong earthquake and you are on the coast, evacuate immediately.
Scenario One: Likely (approximately 70% within the next year)
The most likely scenario is that aftershocks will continue to decrease in frequency (and in line with forecasts) over the next year and no aftershocks of magnitude 7 or larger will occur. Felt aftershocks (e.g. over magnitude 5) can occur in the area from North Canterbury to Cape Palliser/Wellington.
Scenario Two: Unlikely (approximately 25% within the next year)
An earthquake smaller than the mainshock and between magnitude 7.0 to magnitude 7.8 will occur. There are numerous mapped faults in the North Canterbury, Marlborough, Cook Strait and Southern North Island areas capable of such an earthquake. It may also occur on an unmapped fault. This earthquake may be onshore or offshore but close enough to cause severe shaking on land. This scenario includes the possibility of an earthquake in the Hikurangi Subduction Zone. Earthquakes originating from here or in the Cook Strait have the potential to generate localised tsunami. The Hawke’s Bay earthquake sequence in 1931 provides an analogy to scenario two, as a magnitude 7.3 aftershock occurred approximately 2 weeks after the initial magnitude 7.8 earthquake.
Scenario Three: Very unlikely (5% within the next year)
A much less likely scenario than the previous two scenarios is that recent earthquake activity will trigger an earthquake larger than the magnitude 7.8 mainshock. This includes the possibility for an earthquake of greater than magnitude 8.0, which could be on the plate interface (where the Pacific Plate meets the Australian Plate). Although it is still very unlikely, the chances of this occurring have increased since before the magnitude 7.8 earthquake, and have also been also been slightly increased by the slow-slip events.
Initially our scenarios covered what might happen over the next 30 days, but we are now shifting to covering what might happen over the next year. This is because the aftershocks are generally becoming smaller and less frequent (decaying) over time, and this lower aftershock rate increases the uncertainty of what might happen over shorter time periods. The change in forecast does not hugely affect the scenarios at the moment; we will review these again later in the year. While we will continue to update the aftershock probabilities regularly, we will not update the scenarios as often.
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