Closing the Cook Strait gap
The top of the South Island is creeping closer to the North Island as the land continues settling after the November 2016 Kaiko¯ ura earthquake.
The massive forces released by the magnitude 7.8 quake on November 14, 2016, had immediate effects on the North Canterbury and Marlborough landscape. But the changes unleashed have not finished yet, with post-quake monitoring showing Cook Strait is getting narrower as the northern part of the South Island edges northeast.
The difference in the South Island’s shape could be seen if a large enough map of central New Zealand as it is today could be laid over one on November 13 two years ago.
The Kaiko¯ ura quake tore apart 25 faults as it raced northwards from its epicentre close to Waiau, North Canterbury, through to the shores of Cook Strait.
Over about a minute-and-a-half, parts of the Marlborough coast were lifted more than 6 metres in places and dropped more than 2m in others.
Land was shunted horizontally by 12m along the Ke¯kerengu¯ Fault and vertically by as much as 9m along the Papatea Fault.
Global Positioning System (GPS) monitoring stations installed by GNS Science and Otago University across the quake zone in the days after the main shock show the fastest movement in the past two years since then has been at Cape Campbell.
GNS Science geodetic scientist Dr Sigru´n Hreinsdo´ttir, who has been analysing the creep towards the North Island, said Cape Campbell had moved northeast and was now about 35 centimetres closer to Wellington than before the quake.
Other GPS sites showed Kaiko¯ ura had slid about 15cm further east, Blenheim was now 15cm further northeast towards Cook Strait, and Nelson had slipped about 5cm southeast. Sites in the Marlborough Sounds had moved northeast by about 10cm but land around the Waiau epicentre had barely moved.
Wellington had also shifted about 5cm northeast since the quake, she said. It was difficult to quantify how much post-quake creep was due to slip on the subduction zone below the region or ‘‘after slip’’ on the Needles, Ke¯kerengu¯ and Jordan faults, she said.
Test via cellphones
If your phone suddenly makes a ‘‘loud, penetrating sound’’ tomorrow evening don’t worry – the second annual test of the emergency mobile alert system is being carried out. About 3 million phones – half the total in New Zealand – are thought to be capable of receiving the alerts. So as long as they are switched on, they should get the test alert between 6pm and 7pm tomorrow. It’s not possible to opt out. The emergency alert system is administered by the Ministry of Civil Defence and Emergency Management on behalf of a range of agencies. The first test was a year ago, when about 34 per cent of devices received the test alert. The increase to about half of mobile phones is because an increasing number of devices are alert compatible. The test alerts will link to the ministry’s website, where people can give feedback about how they experienced the alert.