Researchers at the University of Bristol have developed a seismological "speed gun" which takes multiple seismic readings of single events to determine how quickly the Earth's mantle is moving. While the instruments used to measure this movement aren't hand-held or new to the field, the way that the data is interpreted is ground breaking.
“The notion of a 'seismic speed gun' is a metaphor to describe the technique we have helped develop," said Andy Nowacki, PhD student at the University of Bristol School of Earth Sciences. "We use seismometers to detect the minute shaking that occurs when a seismic wave from an earthquake a thousands of kilometers away. They are incredibly sensitive instruments that essentially consist of a weight which stays still whilst the ground beneath it moves up and down and side to side, recording the motion of the ground. So the technique uses standard seismometers, but we analyze the data they produce in a novel way.”
The mantle is a layer of liquid rock and metals that lies 2,900 km (1800 miles) beneath our feet which is super heated to around 900°C (91,652°F) by pressure and the Earth's core. This liquid rock moves around in what are known as convection currents which behaves like a pot of water starting to boil – the liquid at the bottom heats and rises and as it reaches the surface it cools and falls again only to be heated again and the cycle goes on. Understanding what's going on in this region is important because convection is what drives the motion of the surface of the Earth – it controls the location of our continents and oceans, and where the tectonic plates collide to cause earthquakes and volcanic eruptions.
What the new technique looks at is distortions in the part of the Earth known as D" (pronounced ‘dee-double-prime’) which is the lowermost section of the mantle.
“Because the mantle is made of rock, it flows incredibly slowly," said Nowacki. "It probably takes on the order of a billion years for it to do a full circle from top to bottom and back again. Hence it's probably not possible in our human lifetimes to measure it moving directly. Instead we infer the direction it's moving in by the way the crystals line up in the D'' region. When you deform something, imagine Smarties in a muffin, then the crystals inside tend to line up in a certain way, depending on which way you squash it. We try to measure how the crystals have lined up, and hence how the mantle has been flowing.”
Nowacki has found that the way crystals line up can be measured because they allow seismic waves to travel through them at different speeds, depending on which way you look at them and what their orientation is. This is know as being anisotropic. The seismometers measure S waves, seismic waves that oscillate side-to-side, to detect how well certain parts of D'' allow these waves to pass through. By using multiple seismographs they have found that they can get a stereo reading of the D" giving them an indication of the alignment of the crystals which tells them which way and how fast the Earth's mantle is moving.
“The trick is, you have to do this in multiple directions if you really want to know how this anisotropic region behaves; previous work has only been in one direction, which is why our results are interesting,” said Nowacki. “We have looked at a region beneath the Caribbean and Central America using our technique. We find that it doesn't behave as we thought it did before, it's even more complicated. However, we can infer that the mantle is moving from side to side in this region, east-west, but we require other research to be certain that we can accurately measure it.”
The paper, Deformation of the lowermost mantle from seismic anisotropy was published in Nature on 28 October 2010.
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