Quake unravelling continues

Friday, 25 November 2016, 3:46 pm
Press Release: Science Media Centre

Quake unravelling continues

Though the aftershocks from last week's Kaikoura earthquake have been quieter this week (knock on wood), science behind the quake continues apace.

Niwa's research vessel Tangaroa was diverted to the Kaikoura coast after last week's earthquake. Surveys of the seabed have confirmed the Needles Fault ruptured 34km offshore. Scientists on board the vessel have collected 61 sediment cores - each measuring about 5.5 metres - from the continental margin between Poverty Bay and Kaikoura (pictured).

It's hoped the cores will provide evidence of submarine landslides. Niwa marine geologist Dr Philip Barnes said the cores revealed a huge turbidity current offshore from Marlborough and Wairarapa - extending at least 300km from Kaikoura. But the locations of underwater landslides that caused the turbidity have not been identified.

Meanwhile back on land, geologists have been looking at evidence of new faulting in Marlborough. University of Canterbury researchers found "spectacular" fault displacements, including an area north of Waiau whihc had 2.2m horizontal displacement and 1.5m vertical.

GPS stations detected a slow-slip earthquake underway in the Hawke's Bay and Gisborne regions. Silent or slow-slip earthquakes are undetectable by both humans and GeoNet's seismographs, but the GPS stations detected movement of up to 2-3cm - similar to that seen in previous East Coast slow-slip events. A magnitude 5.5 earthquake that struck the area on Tuesday fit with previous earthquake "clusters" during slow-slip events in the area.

Kayak guides spotted an interesting by-product of the earthquake - bubbles in the sea off Kaikoura. University of Canterbury's Dr Matthew Hughes said the bubbles were likely due to a mix of carbon dioxide and hydrogen sulphide released through cracks in the seafloor caused by the magnitude 7.8 earthquake.

An aftershock that damaged homes in North Canterbury on Tuesday was within the aftershock forecast, GNS Science's Natalie Balfour told media. But since then, things have been particularly quiet.

GeoNet's Sara McBride wrote on Wednesday that the number of aftershocks was within the forecasted range, but on the low side and cautioned people still to be prepared. "Just because we are in the lower end of the forecast, it doesn't mean that this will stay that way."

In non-earthquake news, an air force flight last week reported floating pumice to the west of Minerva Reef - about 600km south-east of Fiji. GeoNet director Dr Ken Gledhill said satellite images showed the pumice rafts extended for more than 100km. Though scientists have not yet tracked the pumice back to a source location, Dr Gledhill said the nearest active submarine volcano - Monowai - was active on November 10-11.

Finally, some numbers. Writing on GeoNet, Dr Gledhill pulled out some of the data from the Kaikoura earthquake:

• Movement along at least five separate faults, extending about 150km up the coast

• Land movements up to 11m horizontally and 5m vertically

• Maximum ground shaking 1.3 times the force of gravity in Ward

• Tsunami height of 4m at Little Pigeon Bay

• More than 5,500 aftershocks.

Quoted: Stuff.co.nz

"We're 12 days out, which in geological terms is not a long time at all."

GNS Science seismologist Dr Matt Gerstenberger
on aftershock probabilities from the Kaikoura earthquake.

ends

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