Squishy rocks and earthquakes

Earthquakes are a fact of life in New Zealand, but we still don’t understand exactly what triggers them. Microscopes are some of the many tools that scientists are using to get a better understanding of what happens in an earthquake.

Rights: Dave Prior

Alpine Fault hanging wall

The reverse fault motion of the Alpine Fault means that one side is raised up over time to form a hanging wall (as shown by this cliff face near Gaunt Creek, Westland). The hanging wall contains rocks that were once underground. Over millions of years, the uplift of the hanging wall has formed the Southern Alps.

Professor Dave Prior and his team at the University of Otago are looking at ‘squished’ rocks from deep in the Alpine Fault (an active fault on the west coast of the South Island). They use microscopes of several kinds to study the features of each rock and work out its life story – the things that have happened to it over time. By doing this, they hope to build up a picture of the forces at work deep within the Alpine Fault.

Hard as rock?

We tend to think of rocks as being very hard objects that shatter when they’re placed under stress (such as during an earthquake). Rocks do shatter at the Earth’s surface, but deeper underground, they get squished under pressure instead of breaking. This is because they are quite cold and brittle at the surface, but hotter and softer underground. Dave describes rocks as being like chocolate – on a cold day, your chocolate bar will break in two when it’s forced, but on a warmer day, it’ll bend without breaking.

Many geologists believe that movement of rock within the ‘squishing zone’ plays a big role in triggering earthquakes. No one knows, though, exactly how the squishing and earthquakes are linked. Dave thinks the Alpine Fault might hold the answers – he says it’s the best place in the world to study squishy deformation of rocks in a fault zone.

Rights: The University of Waikato Te Whare Wānanga o Waikato

The ‘squishing zone’

Deep below the Earth’s surface, rocks tend to get ‘squished’ under pressure instead of shattering. In the Alpine Fault zone in New Zealand, the ‘squishing’ zone starts at about 10 km below the surface.

The Alpine Fault: bringing up rocks from the deep

The Alpine Fault is special because it has a reverse fault motion, as well as moving in other ways, which means that one side of the fault is being raised up with respect to the other. Rocks that were once deep underground are brought to the surface by the action of the fault. They can be studied to give information about what happened deep within the fault in the distant past.

There are other big reverse faults in the world, but the Alpine Fault is unusual because it has been moving in the same way for approximately the last 5 million years. As Dave puts it, “It’s probably the only major fault in the world where you can see at the surface the squished rocks that were formed less than a million years ago at 10–20 km depth.”

We’re using the Earth as a laboratory to create material that tells us about how rocks move around and change shape.

Professor Dave Prior, Department of Geology, University of Otago

Reading the rocks with microscopes

By observing rocks from the Alpine Fault under the microscope, Dave and his colleagues have learnt a lot about conditions deep in the fault. They focus on two techniques:

• Polarising optical microscopy: The optical microscopes used by geologists have a polarising filter so that different minerals show up as different colours under polarised light. Dave and his team use the polarising optical microscope to see which minerals are in the squished rocks and to learn about their shapes, sizes and orientations.
• Electron backscatter diffraction: When rocks get squished within the fault zone, the crystals within them tend to line up in preferred directions. The direction of the crystals depends on the direction the rock was squished in. Electron backscatter diffraction, which is done using a scanning electron microscope (SEM), measures the orientation of the crystals (lattice planes) in very small volumes of the rock. This means Dave’s team can tell the direction in which the rock was squished and the conditions (the temperature, for example) deep in the fault zone at the time.

Rights: Virginia Toy, University of Otago

Micrograph of rock microstructure with polarising filters

Alpine Fault mylonitic rock sample with polarising filters. Different minerals show up as different colours under polarised light. The quartz grains (seen in red) would appear as a continuous clear band without the filter.

Drilling into the fault

Dave and his colleagues are part of the Deep Fault Drilling Project – a multinational collaboration that aims to drill down several kilometres in the Alpine Fault zone. Rock cores will be collected through the depth of the drilling holes, and Dave and his team will study these using microscopy in the same way they study their samples from the surface. The drilling project won’t go as deep as the squishing zone, but it will allow the team to study rocks from shallower parts of the fault where earthquakes are generated.

From mountains to microscopes

In this interactive follow a core sample as it makes its journey from the Alpine Fault to microscopic examination.