Saturday, August 29, 2015

Scientists discover mechanism behind 'strange' earthquakes

Public Release: 26-Aug-2015
Scientists discover mechanism behind 'strange' earthquakes
Currents of semi-liquid rock key to seismicity away from tectonic plate boundaries
University of Southern California

t's not a huge mystery why Los Angeles experiences earthquakes. The city sits near a boundary between two tectonic plates -- they shift, we shake. But what about places that aren't along tectonic plate boundaries?

For example, seismicity on the North American plate occurs as far afield as southern Missouri, where earthquakes between 1811 and 1812 estimated at around magnitude 7 caused the Mississippi River to flow backward for hours.

Until now, the cause of that seismicity has remained unclear.

While earthquakes along tectonic plate boundaries are caused by motion between the plates, earthquakes away from fault lines are primarily driven by motion beneath the plates, according to a new study published by USC scientist Thorsten Becker in Nature on Aug. 27.

Just beneath the Earth's crust is a layer of hot, semi-liquid rock that is continually flowing -- heating up and rising, then cooling and sinking. That convective process, interacting with the ever-changing motion of the plates at the surface, is driving intraplate seismicity and determining in large part where those earthquakes occur. To a lesser extent, the structure of the crust above also influences the location, according to their models.

"This will not be the last word on the origin of strange earthquakes. However, our work shows how imaging advances in seismology can be combined with mantle flow modeling to probe the links between seismicity and mantle convection," said Becker, lead author of the study and professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences.


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