Mechanism behind ''strange'' earthquakes discovered

Washington

Scientists have discovered the mechanism that generates earthquakes that occur away from tectonic plate boundaries.

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, researchers have found.

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 modelling to probe the links between seismicity and mantle convection," said Thorsten Becker, lead author of the study and professor of Earth sciences at the University of Southern California (USC) Dornsife College of Letters, Arts and Sciences.

Becker and his team used an updated mantle flow model to study the motion beneath the mountain belt that cuts north to south through the interior of the Western US.

The area is seismically active — the reason Yellowstone has geysers is that it sits atop a volcanic hotspot.

Previously, scientists had suggested that the varying density of the plates was the main cause.

Instead, the team found that the small-scale convective currents beneath the plate correlated with seismic events above in a predictable way.

They also tried using the varying plate density or "gravitational potential energy variations" to predict seismic events and found a much poorer correlation.

"This study shows a direct link between deep convection and shallow earthquakes that we didn't anticipate, and it charts a course for improved seismic hazard mapping in plate interiors," said Tony Lowry, co-author of the paper and associate professor of geophysics and geodynamics at Utah State University.

The study was published in the journal Nature. — PTI