Plates slipping under Marlborough
The magnitude-7.8 November earthquake set in motion a slow-slip event under the upper South Island that has so far released the same amount of energy as a magnitude-7.3 earthquake.
The plate boundary in the area, about 25 kilometres beneath the Earth’s surface, has slipped up to half a metre since the November 14 quake, GeoNet public information specialist Caroline Little said yesterday.
‘‘This movement was most active during the first month after the earthquake and has slowed down considerably since the start of the year.’’
Most of the land movement in Marlborough was caused by the Hikurangi subduction plate boundary – where the Pacific plate meets the Australian plate – slipping beneath the Marlborough region. That was similar to slow-slip events in the North Island started by the Kaikoura quake.
Slow-slip events had only been recognised fairly recently and it had only been possible to detect them in New Zealand in the past 15 years, Little said.
‘‘Even though the slow-slip events relieve built-up stress between the tectonic plates without earthquakes, they can transfer stress onto other faults around them.
‘‘We’ve detected dozens of slow-slip events in the past that haven’t caused large earthquakes, although it is possible that the Cook Strait earthquake sequence in 2013 was triggered by the Kapiti slowslip event, which was occurring at the time.’’
Because so much slow-slip had been happening since the Kaikoura quake, scientists were trying to work out how slow slips might influence the likelihood of future earthquakes.
That included updating calculations on the likelihood of a quake of similar or greater size to the Kaikoura quake within the next year. Current models suggested a 5 per cent chance of that happening. While that was small, it was about six times greater than before the Kaikoura event.
Other slow slips set off by the Kaikoura quake and identified earlier were offshore from the east coast of the North Island and beneath Kapiti.
The event offshore from the east coast had stopped, after up to 15cm of movement that was mostly finished by the end of November, Little said.