Hidden "Mantle Scars" Can Come To Life And Cause Earthquakes

White lines show active plate boundaries; yellow show the locations of potential mantle scars. Russell Pysklywec, Philip Heron, Randell Stephenson

Earthquakes are complicated, but there’s a few things seismologists definitely do know. Firstly, they occur along fault lines, which can range from the small and winding to the truly gargantuan. Secondly, they happen when stress, which has built up over a considerably long period of time, is suddenly released; the longer the stress accumulation, the more powerful the resulting earthquake.

This is a huge generalization, and there are many nuances to this, but these are the basics. However, a new study in the journal Nature Communications may force a rewrite of geology textbooks. A series of computer simulations suggests that there are plenty of earthquake-prone zones hiding across the world that are often far from any major fault lines.

“This is a potentially major revision to the fundamental idea of plate tectonics,” the study’s lead author Philip Heron, a postdoctoral fellow in Russell Pysklywec's research group in University of Toronto’s (UT) Department of Earth Sciences, said in a statement.

The San Andreas Fault forms the bed of San Andreas Lake, pictured here. As this new research shows, these boundaries aren't the only places where seismic activity can occur. James P. Blair/National Geographic/Getty Images

Using a super-computer called Sci-Net to model the movement of the Earth’s crust and its malleable upper mantle, a team of geoscientists have discovered the presence of “mantle scars,” huge gashes in the crust that once formed the boundaries between two incredibly ancient plates that no longer exist. As plate tectonics are nowadays still moving around, there’s a chance that their propagation could reactivate one of these scars, if only temporarily.

Their simulations suggest that these scars are everywhere, crisscrossing Australia and streaking through the UK, two almost completely aseismic parts of the world. As a result, the team have proposed an atlas showing where these mantle scars are, and have fashioned a preliminary one in the meantime.

“It's based on the familiar global tectonic map that is taught starting in elementary school,” said Russell Pysklywec, chair of UT's Department of Earth Sciences and co-author of the paper. “What our models redefine and show on the map are dormant, hidden, ancient plate boundaries that could also be enduring or "perennial" sites of past and active plate tectonic activity.”

Billions of years ago, the Earth was internally hotter than it is today. This meant that plate tectonic movements were comparatively more violent and energetic, volcanism was more powerful, and the collisions of plates were far more catastrophic.

Masses of crust rapidly compacted together managed to survive later continental break-ups, and these so-called “cratons” still exist today. The scars of their ancient tectonic activity may still remain, however, and this team of researchers wanted to know where they could be found. By using one of the most powerful super-computers available, they simulated a virtual Earth, inputting a range of geodynamic conditions in order to see how the crust and mantle interacted.

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The cycle of supercontinent formation and destruction – also known as the Wilson Cycle – cannot explain everything we see at the surface. In particular, intraplate deformation and LLSVPs (mantle plumes) cannot be effortlessly linked to the movement of tectonic plates alone. Russell Pysklywec, Philip Heron, Randell Stephenson

These models revealed that although the major tectonic activity, including earthquakes and mountain-building, happens on the known tectonic lines, long-dead tectonic activity would have left chasms within the deep crust at the center of tectonic plates. Although there is no evidence that these scars pose an immediate reactivation threat, they could be resurrected if a mantle plume emerges beneath them or the movement of the modern plates tear them open again.

Remarkably, the idea of earthquakes happening far from tectonic plate boundaries isn’t actually new. In one study, scientists discovered that parts of the upper mantle are falling off into the lower, more molten section; the upwelling of hotter, buoyant material designed to fill in these gaps are causing earthquakes in regions far from tectonic faults, such as in Washington D.C just a few years ago.

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