Year 8 · Science · 2 minutes
Why does New Zealand have so many earthquakes?
Moving plates, a boundary through Aotearoa, and stuck rock that slips.
Key terms
- tectonic plates
- Giant pieces of the Earth's hard outer shell that are always slowly moving.
- plate boundary
- The place where two tectonic plates meet and grind against each other.
- subduction
- When one plate dives underneath another and sinks slowly deep into the Earth.
- Alpine Fault
- A huge break in the rock in the South Island where the plates slide sideways past each other.
- fault
- A break in the rock where the sides can lock together and then suddenly slip.
- earthquake
- The shaking of the ground when stuck rock suddenly slips and releases stored energy.
Transcript
Every year, scientists record around twenty thousand earthquakes in and around New Zealand. Most are too small to notice, but about two hundred and fifty are big enough to feel. So why does our country shake so much?
The answer is under our feet. The Earth's hard outer shell is cracked into giant pieces called tectonic plates. They're always moving, but slowly. Under New Zealand, they move about five centimetres a year, roughly as fast as your fingernails grow.
And New Zealand sits right on the boundary between two of them. The Australian Plate to the west, and the Pacific Plate to the east, grinding against each other.
Off the east coast of the North Island, the Pacific Plate dives underneath the Australian Plate, and sinks slowly deep into the Earth. Scientists call this subduction.
In the South Island, the plates mostly slide sideways past each other, along a huge break in the rock called the Alpine Fault. The Pacific side is also pushed up, and that built the Southern Alps. The rock rises about seven millimetres a year, but rain, ice and rivers wear it down almost as fast.
But the plates don't slide smoothly. The rocks along a fault lock together, and stress builds up, like bending a ruler further and further. When the rocks suddenly slip, all that stored energy shakes the ground. That's an earthquake.
In twenty sixteen, the Kaikōura earthquake broke more than twenty faults in one go. The Alpine Fault last had a major earthquake in seventeen seventeen. Scientists estimate about a seventy five percent chance of another in the next fifty years. Quakes can strike anywhere here, so we all practise Drop, Cover and Hold.
So, the Earth's surface is made of moving plates. New Zealand sits where two of them meet, diving under in the north, and sliding past in the south. When stuck rock suddenly slips, the ground shakes. And now you know what's moving under your feet.
Sources
Every fact in the video, and where it comes from.
- 1geonet.org.nz/earthquake/faq
- GeoNet locates around 20,000 earthquakes in and around New Zealand each year; most are small but around 250 are big enough to be felt.
- Under New Zealand, the Pacific Plate is moving to the west-south-west at about 50 mm a year, about the same rate that your fingernails grow.
- 2pubs.usgs.gov/gip/dynamic/understanding.html
- Earth's rigid outer layer (lithosphere) is broken into plates that move relative to each other; where one plate sinks under another is a subduction zone.
- Parts of a subducting plate become locked in place for long periods before suddenly moving to generate large earthquakes; near the surface there is a lot of friction between the moving plates.
- 3sciencelearn.org.nz/resources/330-the-alpine-fault
- New Zealand lies above a subduction zone where the rocks of the Pacific plate are pushing under the Australian plate; in the North Island the subduction zone is under water off the east coast.
- In the South Island the plate boundary is on land: the Alpine Fault is a strike-slip (transform) fault; the Australian plate slides horizontally north-east while the Pacific plate is pushed up, forming the Southern Alps.
- The Southern Alps are rising at about 7 millimetres a year, but erosion wears them down at a similar rate.
- 4geonet.org.nz/earthquake/story/2016p858000
- The 14 November 2016 Kaikōura earthquake was magnitude 7.8; it involved more than 20 different faults (a world record) and lifted parts of the coast by up to about 6.5 m.
- 5wgtn.ac.nz/news/2021/04/alpine-fault-quake-risk-higher-th…
- The Alpine Fault's last big earthquake was in 1717 (estimated magnitude 8.1); a 2021 study puts the probability of a magnitude 7+ Alpine Fault earthquake at about 75 percent in the next 50 years, with about an 82 percent chance it is magnitude 8 or more.
- 6civildefence.govt.nz/cdem-sector/consistent-messages/earthquake/res…
- In an earthquake, New Zealand Civil Defence advice is to Drop, Cover and Hold: drop to hands and knees, cover head and neck, hold on to your shelter.
- 7teara.govt.nz/en/map/6896/plate-boundary
- Te Ara: the boundary between the Australian and Pacific plates runs through New Zealand (Kermadec Trench and Alpine Fault); the Pacific Plate converges obliquely on the Australian Plate at about 40 mm a year.
- 8af8.org.nz
- AF8 (2026): 75% probability of an Alpine Fault earthquake in the next 50 years, with a 4 in 5 chance it is magnitude 8 or more.
- 9gns.cri.nz/news/research-finds-alpine-fault-quake-more-li…
- GNS Science release on the 2021 Alpine Fault probability study (75% in 50 years); the fault produces a large earthquake about every 300 years.
- 10geonet.org.nz/data/tools/FDSN
- Map data: GeoNet earthquake catalogue, all located events 1 Jan to 31 Dec 2025 between 33S and 49S, 165E and 180E (20,721 events). Plate boundaries: Bird (2003) PB2002 via Ahlenius/Nordpil (ODC-BY).
Image and sound credits
- “The Southern Alps, from the skies - SouthernAlps9300” by lumoplank (CC0)
- “Aoraki Mount Cook and Mount Tasman, Southern Alps, New Zealand, 22 July 2005” by Phillip Capper from Wellington, New Zealand, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)
- “Alpine Fault SRTM” by NASA/JPL/NGA (Public domain)
- “Powerful Earthquake Exposes New Land Near Kaikoura” by NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey and modified Copernicus Sentinel data (2016) processed by the European Space Agency (Public domain)
- Earthquake data: GeoNet (Earth Sciences New Zealand), CC BY 3.0 NZ (https://creativecommons.org/licenses/by/3.0/nz/)
- Plate boundaries: P. Bird (2003) PB2002, GIS version by Hugo Ahlenius, Nordpil — Open Data Commons Attribution License (https://opendatacommons.org/licenses/by/1-0/)
- Map data: Natural Earth (public domain)
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