Deep Fault Drilling ProgrammeDrilling into a live earthquake fault

Overview

The Alpine Fault runs for about 600km up the spine of the South Island. It’s the land boundary between the Pacific and Australian Plates, and one of the world’s major geological features.

Five things to know about the Alpine Fault.

1. It has ruptured four times in 1,000 years – 1717, 1620, 1450 and 1100 – producing an earthquake of about M (magnitude) 8 each time.
2. The likelihood of a major rupture in the next 50 years is much higher than we thought(external link).
3. The fault is moving horizontally by about 30m per 1,000 years — very fast by global standards. It’s also moving vertically – the alps have been uplifted by an amazing 20km in the last 12 million years. It’s only the fast pace of erosion that has kept their highest point below 4,000 metres.
4. The rapid uplift has brought deep faulted rock to the surface, enabling scientific study. That same uplift restricts earthquake activity to depths shallower than normal.
5. Drilling into a live fault overdue for a rupture gives us a rare opportunity to see the state of the fault before it breaks.

Fractured and strongly-layered rocks and extremely hot temperatures put an end to the drilling. While DFDP at Whataroa didn’t achieve all of its technical goals, it made some unexpected discoveries.

The project

A large Alpine Fault rupture could affect us for 50 years

Research from GNS Science and the University of Otago suggests a large Alpine Fault earthquake will trigger a cascade of environmental effects that could last for up to 50 years:

• violent shaking along the entire length of the rupture will trigger large landslides in steep land and weaken hillslopes making them more susceptible to landslides in subsequent storms
• as West Coast rivers and streams transport material from these landslides downstream, rivers will change their courses abruptly and more frequently
• this cascade of impacts may affect towns, road, communications and power infrastructure for decades after the earthquake
• aftershocks reach M7 and continue for many years

Why is the Alpine Fault so interesting?

In the simplest of terms, it’s the way it moves.

When the fault slips, it displaces the rocks on either side horizontally and vertically: rocks on the eastern side of the fault move southwest-wards and upwards. Over time, rocks that were deeply buried beneath the Southern Alps are brought to the surface.

This happens so rapidly – in geological terms – that the rocks don’t have time to cool and many temperature-controlled processes that normally occur at great depth persist to much shallower depths. By studying these processes and the mineralogical and structural signatures they leave behind, we can learn about how the Alpine Fault behaves at the depths at which earthquakes nucleate (approximately 6–12 km). Using rock and fluid samples collected from boreholes enables us to see through the effects of near-surface weathering and erosion.

The Alpine Fault dips downward into the earth at an angle of about 45°. This means we can penetrate the fault more cheaply and more easily than with vertical faults such as the San Andreas Fault(external link).

The Deep Fault Drilling Project (DFDP)

Experts believe the Alpine Fault is ready to fail in a large earthquake within coming decades.

Deep Fault Drilling (DFDP) gives us a rare opportunity to determine the state of the fault in the late phase of its seismic cycle – before it breaks. Most other projects have to drilled into active faults after major earthquakes(external link).

DFDP-1: This first phase was completed in February 2011 with two boreholes intersecting the Alpine Fault at Gaunt Creek, South Westland.

DFDP-2 began in October 2014, targeting the Alpine Fault c1km below the Whataroa River Valley.

These two projects were led by GNS Science, Victoria University of Wellington and the University of Otago. Altogether, they involved nearly 100 scientists from more than a dozen countries. They retrieved rock and fluid samples, took geophysical and hydraulic measurements, and set up a long-term monitoring observatory inside the fault zone.

Will the Alpine Fault produce another Darfield?

The Darfield quake on 4 September 2020 was a M7.1 (M = magnitude).

If the Alpine Fault produced a M8.1 quake, it would release about 30 times more energy than Darfield. Here’s the science: for every one unit increase in magnitude – say, from M4 to M5 – there is about a 30-fold increase in energy release.

A rupture in the Alpine Fault will likely:

• happen over several hundreds of kilometres rather than several tens of kilometres
• last hundreds of seconds rather than tens of second
• affect a much larger area than the Darfield earthquake
• produce an aftershock sequence of earthquakes with at least two M7

Remember: the size of an earthquake is not the only factor determining its severity. We saw this with the 2010–2012 Canterbury earthquake sequence: the M6.3 Christchurch earthquake had a much greater effect than the M7.1 Darfield event five months earlier.