Probing Earth's weak spots using electromagnetic fields


Weaknesses in our planet's outermost layers can be estimated – and with them, potential volcanic or earthquake risks. Belinda Smith reports.


Chris Clor / Getty Images

Predicting when and where earthquakes will strike and volcanoes erupt is notoriously difficult. But what if you could identify weak patches in the Earth’s crust that might be more likely to shake or explode?

A pair of geologists – Lijun Liu from the University of Illinois at Urbana-Champaign in the US and Derrick Hasterok at the University of Adelaide in Australia – used electrical conductivity measurements to infer the fluidity and solidness of Earth’s outer layers.

Their modelling, reported in Science, could one day help forecast volcanic eruptions and help us understand why earthquakes occur in some regions and not others.

Tracking what’s happening beneath our feet is tricky at the best of times. Earth’s lithosphere – the rigid outer layer comprising the crust and upper part of the mantle – is subjected to high stresses, pressures and temperatures, what with tectonic plate movement and magma plume churning from deep inside the planet.

One way to identify potential quake or volcanic hotspots is to see where the crust is thinner and the underlying material more liquid (or less viscous). A number of factors can decrease viscosity, such as more water and high temperatures.

But it's not possible to dip a ladle into Earth’s mantle and see how viscous it is down there. Was there a proxy?

Liu and Hasterok thought so. Electrically conductivity is affected by many of the same factors.

And the conductivity of Earth’s layers has been measurable for decades, Hasterok says, but only in the past 15 years or so have advances in laboratory studies that emulate those high temperature and high pressure environments made it possible to link observations with actual conditions below.

The technique, called “magnetotellurics”, uses electrodes and magnetometers on the surface. They measure reflected waves from different layers below.

A map of viscosity of the crust and upper mantle produced by the new modelling from data collected in the US. Warm colours are low viscosity (weak) and cool colours are high viscosity (strong).
Jiashun Hu / University of Illinois

High frequency reflections give geologists an idea of the state of shallow layers.

But probing beyond 300 kilometres requires very low frequencies and lots of time. To attain those deeper measurements, the equipment needs to be left out for months on end.

When the data returns, the ratio of electric and magnetic fields in an area gives geologists an idea of the conductivity of its layers. This can be mapped.

“Generally,” Hasterok says, “more conductive means less viscous”, which points to a weak patch. And depending on its context – whether it’s on a fault line, for instance – it could be an earthquake or volcano risk.

To see if their modelling lined up with real life observations, Liu and Hasterok used magnetotelluric data from Utah and surrounding states.

They showed the Eastern Great Basin and Transition Zone are marked by low-viscosity and numerous weak patches compared to the Colorado Plateau, which has a much thicker and more viscous crust.

And while there’s not a 100% link between viscosity and conductivity – some parameters affect one more than the other – the method could be included in future research to understand the physics of our lithosphere.

  1. http://science.sciencemag.org/cgi/doi/10.1126/science.aaf6542
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