North Island slow-slip follows South Island quake

There has been a lot evidence of land movement during and after what is now referred to as the M7.8 Kaikoura earthquake – it was initially said to be centred close to Culverden and Geonet still shows it as ’15 km north-east of Culverden’ (Kaikoura is about four times further away).

It is thought that the initial quake caused a chain reaction along other fault lines in the South Island, and Wellington was also affected.

Geonet has now revealed that since this movement happened there has been a ‘slow slip’ occurring further north, along the Hawkes Bay and Gisborne coast. Slow slips have been detected there before but only after North Island earthquakes.

Today Geonet posted on Coastal Uplift: How has the Kaikoura Coastline Changed which shows the extent of land movement over a large area of the South island.

Much of the northeastern coast of the South Island was uplifted during the 14th of November 2016 earthquake. We know this from photos of rock platforms covered in seaweed and marine animals such as crayfish and paua stranded above tide levels.

Our records measured the tide gauge at Kaikoura was lifted up by 1 m, and continuous GPS monitoring sites at Kaikoura and Cape Campbell were also raised by 0.7-0.9 m. At this stage we can estimate that the coast was raised between 0.5 m and 2 m from about 20 km south of Kaikoura all the way north to Cape Campbell.

The startling uplift of ~5.5 m at Waipapa Bay is a localised block pushed up between two traces of the Papatea Fault and is thankfully not representative of the whole coastline.

There was greater horizontal movement, reported to up to 10 metres in places. All those ground movement happened along a long stretch of coastline up the north east of the South Island.

kaikouraearthquake_uplift_21nov2016

Uplift and horizontal movement happened on a long stretch of coastline up the north east of the South Island.

Also today Geonet posted Gisborne and Hawke’s Bay slow-slip event follows M7.8 Kaikoura Quake

GPS stations have detected a slow-slip event under the Hawke’s Bay and Gisborne regions in the days following the Kaikoura M7.8 Earthquake.

These silent earthquakes or slow-slip events are undetectable by both humans and GeoNet’s seismographs. They can move faults the equivalent of magnitude 6+ earthquakes over a period of weeks to months, without any detectable shaking.

The ongoing slow-slip event off the North Island’s east coast has moved some GPS stations up to 2-3 centimetres.

So far. It’s only a week since the M7.8

This movement is similar to what has been observed in previous East Coast slow-slip events over the last 15 years, so is not necessarily abnormal. We see events in this area usually every 1-2 years.

We have also observed other slow-slip events happening in response to large earthquakes.

The last slow-slip event offshore of Gisborne followed the Te Araroa earthquake in September 2016 (related GeoNet story http://info.geonet.org.nz/x/ZIAvAQ).

A slow-slip event also occurred following the 2007 M6.7 Gisborne earthquake.

But this time the slow-slip began after a more distant quake.

It is possible that passing seismic waves from the M7.8 earthquake caused stress changes that triggered the slow slip event. GNS Science and GeoNet and scientists are keeping a close eye on the event as it evolves.

So the Culverden quake may have triggered the Kaikoura and Seddon quakes (and three other fault line breaks), nudged across Cook Strait to Wellington and may rearranged stresses enough up the east coast of the North Island to start the slow-slip.

slowslip_hb_and_gisb

This instability covers a large area in which many of New Zealand’s significant earthquakes have occurred.

nz_faults

The initial Culverden quake was on the Hope fault line which reaches back to the top of the Alpine Fault where it breaks apart into Marlborough’s mess of mountains.

Also today Geonet updated it’s statistics based scenarios and forecasts which includes the probability of aftershocks:

  • 99% M6.0-6.9  in the next year (89% within 30 days)
  • 38% >=M7 in the next year (20% within 30 days)

There is no way of knowing, if another large quake occurs, where it would be. There is a lot of uncharted territory here.

What to do about it?

If you feel an earthquake:

Don’t run outside, many injuries are caused by things falling from buildings. Beware of breaking glass.

And if you are close to sea level near the coast don’t wait for a knock on your door or a warning siren, move inland or to higher ground.

And hope that the slow-slip eases the pressure gradually so nothing major gives suddenly.