'Silent quakes' on the move

Slow-slips may trigger moderate earthquakes between 4 and 5, but link to large quakes still unknown.

Slow-slips may trigger moderate earthquakes between 4 and 5, but link to large quakes still unknown.

Earthquake damage on State Highway One and the main trunk railway line north of Kaikoura, 14 November 2016. The Kaikoura earthquake has been linked to a series of slow-slip events across the North Island, including in this region. New Zealand Herald photograph by Mark Mitchell

THE Kaikoura earthquake has been linked to a series of slow-slip events across the North Island, including in Gisborne-Hawke’s Bay.

Over the past two weeks an area on the Hikurangi subduction zone (where the Pacific and Australian plates meet) in Gisborne-Hawke’s Bay has slipped up to 15 centimetres, equivalent to a slow-motion magnitude 7.2 earthquake.

While the East Coast slow-slip appears to be slowing down, more slow-slip movement on the Hikurangi subduction zone has been detected by GeoNet GPS sites in the Kapiti and Manawatu regions.

The plate boundary has slipped between five and seven centimetres, equivalent to a magnitude 6.8 earthquake spread over the last two weeks, and is still moving.

It is the first time scientists have monitored this many slow-slip events at the same time across the North Island since they were discovered in early 2002.

Slow-slip events, sometimes called “silent earthquakes”, are similar to earthquakes, involving more-rapid-than-normal movement across a fault, but they happen over a matter of weeks to months rather than seconds.

GNS Science geophysicist Laura Wallace says the Gisborne-Hawke’s Bay slip probably triggered the moderate earthquakes offshore Porangahau, and the magnitude 5.1 earthquake north of Wairoa on Saturday.

Larger movement than in the past

The movement of 15cm offshore Porangahau is a bit bigger than they have previously seen in slow-slip events there. They typically involve slips of 10cm along the plate boundary.

This is the first time a large earthquake has triggered slow-slip events in multiple parts of the North Island.

“Most areas where we have observed slow-slip in the past are slipping right now. We think the Kaikoura earthquake set them off. Very small changes in stress may have triggered the slow-slip events.

“We have never monitored slow-slip following a central New Zealand earthquake as big as the magnitude 7.8 Kaikoura earthquake, so it might be typical following big earthquakes in the centre of the country,” said Dr Wallace.

The Hikurangi subduction zone, where the Pacific plate is thrust under or “subducted” beneath the Australian plate east of Poverty Bay, is one of a number of locations worldwide notable for slow-slip events.

September 2016 slip

The last slow-slip event offshore of Gisborne followed the Te Araroa earthquake in September 2016.

A slow-slip event also occurred following the 2007 magnitude 6.7 Gisborne earthquake.

Dr Wallace says while slow-slip events may trigger moderate earthquakes of between magnitude 4 and 5, the link to large earthquakes is still unknown.

“It is a pretty new area of research.”

Research is under way on slow-slip events in the area under a recent MBIE Endeavour Fund project led by GNS. In 2018 a major international research project will involve drilling boreholes into the Hikurangi fault and install monitoring equipment.

“Both efforts will help us to greatly understand the link between slow-slip events and earthquakes.”

THE Kaikoura earthquake has been linked to a series of slow-slip events across the North Island, including in Gisborne-Hawke’s Bay.

Over the past two weeks an area on the Hikurangi subduction zone (where the Pacific and Australian plates meet) in Gisborne-Hawke’s Bay has slipped up to 15 centimetres, equivalent to a slow-motion magnitude 7.2 earthquake.

While the East Coast slow-slip appears to be slowing down, more slow-slip movement on the Hikurangi subduction zone has been detected by GeoNet GPS sites in the Kapiti and Manawatu regions.

The plate boundary has slipped between five and seven centimetres, equivalent to a magnitude 6.8 earthquake spread over the last two weeks, and is still moving.

It is the first time scientists have monitored this many slow-slip events at the same time across the North Island since they were discovered in early 2002.

Slow-slip events, sometimes called “silent earthquakes”, are similar to earthquakes, involving more-rapid-than-normal movement across a fault, but they happen over a matter of weeks to months rather than seconds.

GNS Science geophysicist Laura Wallace says the Gisborne-Hawke’s Bay slip probably triggered the moderate earthquakes offshore Porangahau, and the magnitude 5.1 earthquake north of Wairoa on Saturday.

Larger movement than in the past

The movement of 15cm offshore Porangahau is a bit bigger than they have previously seen in slow-slip events there. They typically involve slips of 10cm along the plate boundary.

This is the first time a large earthquake has triggered slow-slip events in multiple parts of the North Island.

“Most areas where we have observed slow-slip in the past are slipping right now. We think the Kaikoura earthquake set them off. Very small changes in stress may have triggered the slow-slip events.

“We have never monitored slow-slip following a central New Zealand earthquake as big as the magnitude 7.8 Kaikoura earthquake, so it might be typical following big earthquakes in the centre of the country,” said Dr Wallace.

The Hikurangi subduction zone, where the Pacific plate is thrust under or “subducted” beneath the Australian plate east of Poverty Bay, is one of a number of locations worldwide notable for slow-slip events.

September 2016 slip

The last slow-slip event offshore of Gisborne followed the Te Araroa earthquake in September 2016.

A slow-slip event also occurred following the 2007 magnitude 6.7 Gisborne earthquake.

Dr Wallace says while slow-slip events may trigger moderate earthquakes of between magnitude 4 and 5, the link to large earthquakes is still unknown.

“It is a pretty new area of research.”

Research is under way on slow-slip events in the area under a recent MBIE Endeavour Fund project led by GNS. In 2018 a major international research project will involve drilling boreholes into the Hikurangi fault and install monitoring equipment.

“Both efforts will help us to greatly understand the link between slow-slip events and earthquakes.”

Your email address will not be published. Comments will display after being approved by a staff member. Comments may be edited for clarity.

Poll

  • Voting please wait...
    Your vote has been cast. Reloading page...
    Do you support Labour’s plan, if elected to government later this year, to co-invest up to $20 million in a prefabricated building plant in Gisborne?