Making sense of Kaikoura's quake-raised coast

Scientists have revealed the extent of uplift on the South Island's northeastern coast, observing that in some places the land was raised 2m.

The tide gauge at Kaikoura has been lifted up by 1m, and continuous GPS monitoring sites at Kaikoura and Cape Campbell were also raised by 70cm to 90cm.

At this stage, GeoNet estimated the coast was raised between 50cm and 2m from about 20km south of Kaikoura north to Cape Campbell.

Coastal uplift occurs when the land is raised above the sea by tectonic forces, and could happen gradually over geological timescales or suddenly by an earthquake.

Sudden coastal uplift occurred as the result of large earthquakes.

Vertical movement on a fault could cause land to be pushed up - a type of movement that built many of New Zealand's mountain ranges.

When such vertical movement on a fault happened near the coast, land was raised above sea level.

"Sea level is a powerful horizontal marker for measuring tectonic movement because it is very obvious which land used to be under water before the earthquake," GeoNet noted in a new blog post.

Rock lifted above sea level by Monday's earthquakes on State Highway One near Kaikoura. Photo / Supplied
Rock lifted above sea level by Monday's earthquakes on State Highway One near Kaikoura. Photo / Supplied

Coastal uplift was normal for large earthquakes near the coast that included vertical movement on a fault.

Most movement on the faults that ruptured in the Kaikoura 7.8 earthquake was horizontal, but, as there was vertical movement as well, it was not surprising there had been coastal uplift.

In New Zealand, evidence preserved in the landscape showed many parts of the coast had been repeatedly uplifted through time.

Source / GNS Science
Source / GNS Science

Raised marine beaches and terraces along the Kaikoura Peninsula, Wairarapa coast, Cape Kidnappers, Mahia Peninsula, north of Gisborne and East Cape offered evidence of former beaches that were uplifted from the sea by earthquakes in prehistoric times.

Many of these had been or were being studied to find out the size and age of past earthquakes.

"In historical times we have had several examples of earthquakes causing coastal uplift that, although devastating in the short term, have led to some benefits."

In 1931 the 7.8 Hawke's Bay earthquake raised land around Napier 1-2m above sea level.

Land that was once an estuary now provided space for Napier airport.

And the road from Wellington city to the Hutt Valley became viable as a result of the new land around the harbour's edge raised by the 8.2 magnitude Wairarapa quake in 1855.

Source / GNS Science
Source / GNS Science

It was likely the newly raised ground at Kaikoura would become a permanent feature.

This would mean a major shift of marine species, as many seaweeds and animals that would normally be permanently covered by water would now be struggling to exist in a zone of transition between air and sea.

Some of the creatures that had been raised would be accustomed to air exposure for short periods of time, but not the full tidal cycle they would now experience.

"As the animals not suited to this environment die from the reef, they will be replaced by seaweed and animals appropriate for the new tidal level," Geonet noted.

Source / GNS Science
Source / GNS Science

Geologists and biologists would be undertaking surveys and studies over coming weeks.

"Recording this information will help us to understand which faults ruptured in the earthquake, and how much slip occurred on those faults."

People could help by sharing their pictures of displaced coastal features such as the uplifted reefs and rock platforms; displaced marine life especially before and after photos if animals are being rescued and removed from the reefs, and uplifted man-made structures such as jettys and boat ramps particularly if they have tide level markers on them.

"We are also interested in people's observations, particularly residents who have familiarity with the coastline pre and post-earthquake," GeoNet stated.

"To be of most use, we would like to know the location, date, time of the photograph and if possible, context and scale.

"As always your safety comes first, please do not put yourself at risk to collect this information."

• Pictures and information could be sent to k.clark@gns.cri.nz

- NZ Herald

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