NCEA Level 2 · Year 12 · Earth and Space Science, Geography · 3 minutes

The Alpine Fault: New Zealand's next big earthquake

A plate boundary, a mountain range, and a fault that is loading up again.

Key terms

Alpine Fault
The plate boundary running over six hundred kilometres through the South Island, where the Australian and Pacific plates meet.
Recurrence interval
The average gap between big earthquakes on a fault, about three hundred years for the Alpine Fault.
Uplift
The upward movement of land as plates push together, which raised the Southern Alps.
Rupture
The moment a locked fault suddenly slips, letting the ground lurch sideways.
Magnitude
A measure of a quake's energy, where each step up releases about thirty two times more.

Transcript

0:00

Along the West Coast, a fault is slowly loading up like a spring. The last time it let go was in seventeen seventeen. On average, it ruptures about every three hundred years. Do the maths.

0:13

Here's the setup. New Zealand straddles a plate boundary: the Australian Plate to the west, the Pacific Plate to the east. They grind past each other about four centimetres a year, as fast as your fingernails grow. In the South Island, they meet along the Alpine Fault, over six hundred kilometres long.

0:32

And they're not just sliding past. They're pushing together, shoving the Pacific side of the fault upward. Over twelve million years, that uplift has raised the Southern Alps about twenty kilometres. Erosion wears them down almost as fast, so Aoraki Mount Cook is under four kilometres tall, not twenty.

0:52

Most of the time, the fault is stuck. The plates keep moving, so the rocks bend and store stress, until they snap, and the ground can lurch sideways by up to about ten metres.

1:05

So how do we know the timing? Big quakes shake landslides and mud into swamps and lakes, leaving layers. In Fiordland, scientists have counted twenty seven ruptures in eight thousand years. That average gap is the recurrence interval: about three hundred years.

1:24

But faults don't keep a timetable, so scientists talk in odds. A twenty twenty one study of West Coast lake mud found a seventy five percent chance of an Alpine Fault earthquake in the next fifty years, and about four in five that it's magnitude eight or bigger.

1:42

Magnitude measures a quake's energy, and each step up is about thirty two times more. A magnitude eight means around two minutes of shaking, countless landslides, and routes over the Alps cut for weeks. So be ready to look after yourself for at least three days, realistically a week.

2:01

Three things for your exam. One: the Alpine Fault is the Australian and Pacific plate boundary, and its uplift built the Southern Alps. Two: its recurrence interval is about three hundred years, and the last rupture was seventeen seventeen. Three: a seventy five percent chance of a quake in the next fifty years. Not a reason to panic. A reason to be ready.

Sources

Every fact in the video, and where it comes from.

  1. 1
    wgtn.ac.nz/news/2021/04/alpine-fault-quake-risk-higher-th…
    • The last major rupture of the Alpine Fault was in 1717 (estimated magnitude 8.1, about 380 km of the fault ruptured).
    • AF8: there is a 75% probability of an Alpine Fault earthquake occurring in the next 50 years, and a 4 out of 5 chance it will be a magnitude 8+ event (2021 study, Howarth et al., Nature Geoscience: about 82%; 20 earthquakes recorded in sediments of four West Coast lakes over 4,000 years).
  2. 2
    af8.org.nz/explore-the-science
    • The Alpine Fault has ruptured 27 times over the last 8000 years, every 300 years on average; evidence from two Fiordland sites, John O'Groats and Hokuri Creek.
    • The Alpine Fault is about 600 km long (Te Ara: 650 km onshore from Blenheim to Milford Sound; AF8: over 800 km) — script says 'more than six hundred kilometres'.
    • GNS scientists calculate the Southern Alps have been raised by 20 km over the last 12 million years.
    • The fault moves horizontally up to about 10 m at a time during an earthquake (AF8 scenario: c. 9 m dextral-reverse displacement).
  3. 3
    sciencelearn.org.nz/resources/330-the-alpine-fault
    • The Alpine Fault is the boundary between the Australian and Pacific plates, visible on land in the South Island; the uplift of the hanging wall (east side) over millions of years has formed the Southern Alps; erosion wears them down at a similar rate.
  4. 4
    teara.govt.nz/en/node/10656
    • The Pacific plate moves relative to the Australian plate at about 40 mm a year, roughly the rate fingernails grow.
  5. 5
    nzase.org.nz/app/uploads/2019/05/2019-05-Alpine-Fault-earth…
    • The Pacific plate moves relative to the Australian plate at about 40 mm a year, roughly the rate fingernails grow.
  6. 6
    en.wikipedia.org/wiki/Alpine_Fault
    • The Alpine Fault is about 600 km long (Te Ara: 650 km onshore from Blenheim to Milford Sound; AF8: over 800 km) — script says 'more than six hundred kilometres'.
  7. 7
    en.wikipedia.org/wiki/Aoraki_/_Mount_Cook
    • Aoraki / Mount Cook, New Zealand's highest peak, is 3,724 m (since a 2014 University of Otago GPS survey adopted by LINZ).
  8. 8
    af8.org.nz/af8-scenario
    • The fault moves horizontally up to about 10 m at a time during an earthquake (AF8 scenario: c. 9 m dextral-reverse displacement).
  9. 9
    af8.org.nz
    • AF8: there is a 75% probability of an Alpine Fault earthquake occurring in the next 50 years, and a 4 out of 5 chance it will be a magnitude 8+ event (2021 study, Howarth et al., Nature Geoscience: about 82%; 20 earthquakes recorded in sediments of four West Coast lakes over 4,000 years).
  10. 10
    usgs.gov/programs/earthquake-hazards/earthquake-magnitu…
    • Each whole-number increase in magnitude is about 32 times more energy released.
  11. 11
    orc.govt.nz/managing-our-environment/natural-hazards/earth…
    • An Alpine Fault earthquake will last about two minutes; countless landslides; transalpine routes impassable for weeks; some areas isolated immediately; plan to be self-reliant for at least three days, a week or more is realistic.

Image and sound credits

  • Alpine Fault shaded relief (SRTM): NASA/JPL/NGA, public domain, via Wikimedia Commons
  • South Island and Southern Alps satellite images (MODIS): MODIS Land Rapid Response Team, NASA GSFC, public domain, via Wikimedia Commons
  • The Southern Alps from the skies; Westland landscapes: lumoplank, CC0, via Wikimedia Commons
  • Aoraki / Mount Cook: CC0, via Wikimedia Commons
  • Lake Mapourika: Paul Stewart, CC0, via Wikimedia Commons
  • Fiordland National Park: Bernard Spragg, CC0, via Wikimedia Commons
  • Photographs of Īnangahua Earthquake, 1968: Archives New Zealand Te Rua Mahara o te Kāwanatanga, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)
  • Difficulties of transport and other earthquake scenes, West Coast, 1929 (Auckland Weekly News): Auckland Libraries Heritage Collections, no known copyright restrictions
  • Reconstructing the Buller Gorge road after the 1929 earthquake, 1930 (Auckland Weekly News): Auckland Libraries Heritage Collections, no known copyright restrictions
  • Map data: Natural Earth (public domain) · Plate boundary, cross-section, sediment core and timeline drawn in code
  • Paper textures: ambientCG (CC0)
  • Sound effects: Freesound contributors (CC0)
  • Music: generated with ACE-Step · Voice: Gemini TTS

Teaching this next week?

Kuraplan turns this topic into a lesson plan, slides and a worksheet for your class in about a minute.

Plan a lesson on this topic

Watch next

2:28

How we discovered the shape of DNA

NCEA Level 2 · Biology

2:38

The Great Depression in New Zealand

NCEA Level 2 · History

2:31

The ozone hole: the chemistry that saved the sky

NCEA Level 2 · Chemistry