North Island's east coast sliding away in large slow-motion earthquake

Monday, 29 April 2019

Slow slip events on Hikurangi Subduction Zone explained.

A month-long, slow-motion earthquake has pushed parts of the North Island's east coast 3 to 4 centimetres further east without anyone other than scientists knowing.

GNS Science geophysicist Dr Laura Wallace said a slow-slip event (SSE) from north of Gisborne to the southern part of Hawke's Bay had so far released about the same amount of energy as the magnitude 7.1 Darfield earthquake on September 4, 2010.

Swarms of small earthquakes around Waipukurau and Porangahau in the southern Hawke's Bay in recent weeks and the magnitude 4.8 shake near Dannevirke on April 24 were probably linked to the slow quake.

Black arrows show horizontal displacement of GPS sites around Gisborne and Hawke's Bay over the last month during the current slow-slip earthquake. The colours represent estimated slow-slip movement in centimetres along the Hikurangi subduction plate boundary.

While GeoNet GPS (global positioning system) sites showed several centimetres of slip at the surface, at the Hikurangi subduction zone plate boundary about 12 kilometres down off Gisborne there had been movement of up to 20cm, with 30cm of slip offshore of Hawke's Bay.

**READ MORE:

Hawke's Bay earthquakes during the past month. From March 28 to midday on April 29 there have been 202 quakes within this box - including two swarms, at Waipukurau and Porangahau, with the magnitude 4,8 Dannevirke quake on April 24 showing on the immediate left. Most of these quakes were too small to feel, but of the 202 quakes, only nine were of more than magnitude 3.0.

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Ocean bottom seismometers used to monitor the Hikurangi subduction zone near Gisborne.

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During the past week, southern Hawke's Bay GPS sites, such as those at Cape Kidnappers and Pawanui, had also started moving, she said.

Dr Laura Wallace, a geophysicist at GNS Science and co-science leader of the project to drill into the Hikurangi subduction zone off Gisborne.

'This suggests that the rupture of this slow-slip event is propagating south now too. Propagation of slow-slip events from offshore Gisborne into the Hawkes Bay region was also observed in 2016.'

It was unlikely the rupture would continue further south under the Wairarapa and Marlborough, Wallace said.

Waimarama Beach, in Hawke's Bay. The entire east coast of the North Island would be at risk if a tsunami was triggered by an earthquake offshore in the Hikurangi subduction zone, and coastal communities are encouraged to become familiar with emergency evacuation routes in their area.

It was difficult to know if this event, now similar in size to the largest yet recorded off the east coast, in March 2010, would set a record. It was probably near its end, and was unlikely to carry on for more than another couple of weeks.

'It has more to do with the displacements recorded by the GPS sites and what these mean for the size of the SSE, not necessarily just the duration of the event. Some can last for years.'

An earthquake from the Hikurangi subduction zone could generate a devastating tsunami for the east coast of New Zealand.

In a slow-slip quake, Earth's crust moves up to tens of centimetres over weeks rather than rupturing suddenly in seconds as in a normal quake.

These events have been possible precursors for giant, 'megathrust' quakes and appear to have occurred on the undersea plate boundary off Japan two months before the magnitude 9.0 quake in March 2011, which generated a deadly and destructive tsunami.

The Hikurangi subduction zone – which runs about 50 to 100km offshore from Gisborne to Marlborough – is arguably New Zealand's most hazardous fault system, generating 10 major earthquakes and several damaging tsunami in the past 7500 years.

A recent study found the last large quake it generated, of about magnitude 8.0, was between 470 and 520 years ago and on its southern section. The next most recent major quake, between 815 and 870 years before now, was much stronger and may have had a magnitude of more than 8.5.

Wallace said there was often an increase in the number of small to moderate quakes during an SSE, even if in the long-term they reduced the chance of large quakes.

One of the most exciting things for scientists about this event was how well recorded it would be.

'We have a large number of different types of sea-floor instruments out there right now, capturing what is going on. This is the first time that we've had such good coverage

'Once we get those data back, we will be able to gain a better understanding than ever of how these events work. It is very exciting to have such a large event at the same time that we've got so many instruments deployed.'