Earthquakes that shake the Earth’s middle layers may be more widespread than scientists thought.
A new map of these mysterious deep earthquakes shows they occur all over the world and may have a variety of causes. This is interesting, said study lead author Simon Klemperer, a geophysicist at Stanford University. That’s because mantle earthquakes were once thought to be impossible, or at least rare. These earthquakes occur beneath a boundary known as the Mohorovicić discontinuity, or “Moho,” the boundary between the brittle crust and the hotter, more viscous mantle.
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Most earthquakes begin in the Earth’s crust. The crust is like a layer of burnt sugar on top of a mantle of creamy filling that is soft and easily deformed. This brittle crust cannot deform, so when stress is applied it cracks, causing the ground to shift and shake. For a long time, geoscientists thought this could not happen in the mantle. The mantle has a toffee-like texture and tends to ooze instead of folding. But over the years, seismologists who study earthquakes have found evidence that the epicenter lies more than 22 miles (35 kilometers) deep, below the Moho Mountains.
However, pinpointing the location of these earthquakes is difficult, especially when they are not large. Generally, these earthquakes are so deep that they are not felt at the surface, regardless of their magnitude. The depth of the Moho also varies from place to place, so very deep earthquakes can still remain in the Earth’s crust.
Traditional methods of locating mantle earthquakes require a specific understanding of the thickness of the Earth’s crust at that particular location. But Klemperer and his co-author, Stanford doctoral student Shiqi Wang, have developed a method that uses a specific type of seismic shear wave that tends to become trapped in either the Earth’s crust or mantle. The pattern of these waves in an individual earthquake determines whether it started above or below the Moho.
First, they tested the method in Tibet in 2021. Now, in a new paper published in the journal Science on February 5th, they’ve taken their research to the world. The researchers ruled out subduction zones. Deep earthquakes occur frequently in subduction zones as rocks from the Earth’s crust are forced into the mantle. Instead, the research team focused on the elusive phenomenon of subcontinental mantle earthquakes.
They found mantle earthquakes everywhere. Their dense belt extends from the Alps to the Himalayas and is probably related to the collision of mountain-forming continents in these regions. Another cluster is emerging in East Africa, where the continent’s crust is cracking. Researchers have found that mantle earthquakes also occur in the western United States and in Baffin Bay, Canada.
Some of the cluster locations were amazing. “There were some areas, like the Bering Sea, that no one had ever discovered before,” Vera Schulte Perkum, a geologist at the University of Colorado Boulder who was not involved in the study, told Live Science. “I would love to get an interactive version of these and zoom in and take a look.”
Klemperer said the global overview should allow other scientists to conduct more specific studies of individual mantle earthquakes, perhaps pinpointing their depth and the mechanisms that cause them.
“It’s very interesting that we have this tool that can be applied on a very daily basis,” he said.
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