Site predicts more aftershocks
earthquakes in the aftershock area continued to drop, Geonet said.
For earthquakes of magnitude 7.0 and above, the probability for the year ahead is put at 3.9 per cent, with a range of 0-1 in the next year.
The chance of a magnitude 5.0-5.9 event in the next year in the aftershock area is put at 99 per cent, with an estimated range of 0-14 events of that size, and an average forecast of 5.2 such events.
While earthquakes could not be scientifically predicted, forecasts of future aftershocks could be provided based on probabilities, Geonet said.
‘‘The aftershocks of the magnitude 7.8 Kaiko¯ ura earthquake are mostly occurring throughout a broad area from North Canterbury
through to Cook Strait that surrounds the faults that ruptured in that earthquake, although a few have occurred in the lower North Island.’’
Geonet has also calculated the probability of strong and severe shaking from aftershocks, using the modified Mercalli intensity scale, which describes the intensity and impacts of shaking, which is partly dependent on the type of ground involved.
At MM6 intensity shaking levels, walking steadily is difficult; furniture and appliances may move on smooth surfaces; and objects fall from walls and shelves; glassware and crockery breaks; slight non-structural damage to buildings may occur.
At MM7 intensity shaking levels, it is difficult to stand; furniture and appliances move; contents are damaged; there is minor building damage and liquefaction can occur in susceptible sediments.
The Geonet calculations show the probability of MM7 shaking in Wellington in the next year, resulting from a Kaiko¯ ura earthquake aftershock, to be about 3 per cent. In Kaiko¯ ura it is 10 per cent, while the largest probability of MM7 shaking is about 15 per cent at Cape Campbell.
In the year after the M7.8 Kaiko¯ ura earthquake there were more than 20,000 aftershocks, Geonet said.
From November 14, 2017 to November 14, 2018, there were 4604 aftershocks, of which 289 were magnitude 3.0 and above.
From November 14, 2018, to November 13, 2019, there were a further 3500 aftershocks, 162 of which were magnitude 3.0 and above.
Geonet also has aftershock forecasts for part of Canterbury following the September 2010, 7.1 magnitude earthquake centred 40km west of Christchurch City near Darfield. Those forecasts are being updated and are expected to be published soon.