Mapping A Volatile Earth With Magnetotellurics
By Jof Enriquez,
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Geologists are using electromagnetic fields that penetrate deep into the Earth's lithosphere to identify which locations are more prone to earthquakes and volcanic eruptions.
This passive geophysical technique called magnetotellurics uses electrodes and magnetometers buried at ground level to measure time variations of the Earth's magnetic field, as well as the electrical resistivity and conductivity of rocks and minerals in the lithosphere – the rigid outer layer comprising the crust and upper part of the mantle.
The ratio of electric and magnetic fields in an area gives geologists an idea of the state of these layers – without having to physically drop an instrument hundreds of kilometers below the surface – in order to understand these geologic processes. To study depths beyond 300 kilometers, however, equipment needs to be left out for months.
While the correlation between conductivity and viscosity is not that straightforward (due to other factors involved), the scientists claim their model could help map geophysical hazards.
"Generally, 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," explains Derrick Hasterok at the University of Adelaide in Australia, who developed the model to understand lithosphere dynamics based on a high-resolution magnetotelluric (MT) survey across the western United States, according to an article published in Cosmos Magazine.
Working with fellow geologist Lijun Liu from the University of Illinois at Urbana-Champaign in the United States, Hasterok tested their model by mapping Utah and surrounding states. Their data showed that 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.
"The high sensitivity of MT fields to the presence of electrically conductive fluids makes it a promising proxy for determining mechanical strength variations throughout the lithosphere," the two scientists wrote in a study abstract published in the journal Science.
"We demonstrate how a viscosity structure, approximated from electrical resistivity, results in a geodynamic model that successfully predicts short-wavelength surface topography, lithospheric deformation, and mantle upwelling beneath recent volcanism. We further show that this viscosity is physically consistent with and better constrained than that derived from laboratory-based rheology (the study of the deformation of flow of matter).”