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Studying “Old” Water Trapped Inside Earth

10 Oct 2016

Not only is the majority of Earth’s surface covered by water, but it is also found in abundance within Earth’s interior. There, water helps in processes like mantle convection, plate tectonics, and arc volcanism. However, in spite of these important roles, the depth distribution of water in Earth’s mantle is still poorly known.

Scientists from the Earth Observatory of Singapore (EOS) and Yale University have come up with an idea of using GPS data following a large earthquake to study the water content of the asthenosphere (the mechanically weak layer of the upper mantle that sits below the strong lithosphere) and the rheology of olivine (the weakest and most abundant mineral in the upper mantle).

Water is found in abundance within Earth's interior, but its stratification is poorly known (image not to scale).

Because water reduces the strength of olivine significantly, this effect can be used to estimate the water content of olivine by studying the mechanical response of the asthenosphere to stress perturbations, like those produced by large earthquakes. 

In this new approach, published today in Nature, the sensitivity to water is used to ascertain the strength of olivine. With data (seismic and GPS) collected from the 2012 Mw 8.6 Indian Ocean earthquake, the largest strike-slip earthquake to date to break a tectonic plate apart, the team was able to constrain the stratification of water content in the upper mantle.

Mr Sagar Masuti, a second-year PhD student at EOS and lead author of the study, said, “We estimated the water content of the asthenosphere using the GPS data following the 2012 Mw 8.6 Indian Ocean earthquake. In fact, both our approach and tools can be used to estimate water content in the asthenosphere of other regions on Earth too. Till now, the water content estimation was limited to laboratory studies of rock samples. Our new approach of using GPS data now gives us an alternative method, which will greatly benefit those who are studying water content and other parameters of the asthenosphere.”

Estimates of primordial water in Earth's asthenosphere using GPS data. The exact content depends on the temperature at these great depths, which is poorly known, but regardless of the range of realistic values used, the water content is relatively high, at least 0.01wt%.

According to co-author Assistant Professor Sylvain Barbot, “EOS monitors seismic activity nearby Sumatra, which contains a large segment of the seismically and volcanically active ring of fire. Using displacement time series from the GPS monitoring, we can infer the stratification of water content in Earth's interior down to a 200 km depth. Since the formation of our planet, vast volumes of water have been trapped inside Earth never to reach the surface again. This primordial water connects Earth's history to the formation of the solar system and the apparition of life on Earth.”

If you are a subscriber to Nature, please click here to read more about how EOS scientists had inferred the upper-mantle water stratification from observations of the 2012 Indian Ocean earthquake.