Tremors emanating from Earth’s middle layer may be more frequent than scientific estimates have suggested.
A novel map depicting these enigmatic deep-focus earthquakes reveals their global distribution and suggests a multitude of potential origins. This is significant, notes lead author Simon Klemperer, a geophysicist at Stanford University, because mantle earthquakes were once deemed improbable, or at least uncommon. Such seismic events originate beneath the boundary known as the Mohorovičić discontinuity, or “Moho”—the demarcation between the brittle crust and the hotter, more viscous mantle below.
“We believe this provides compelling evidence for earthquakes occurring below the Moho boundary across numerous parts of the world,” stated Klemperer. “It’s not just isolated spots. It might be ubiquitous.”
The majority of earthquakes initiate within the Earth’s crust, which can be likened to a brittle layer of toasted sugar sitting atop the softer, more deformable, creamy filling that is the mantle. This rigid crust cannot sustain deformation and thus fractures under stress, causing ground displacement and shaking. For a long period, geophysicists doubted that such fracturing could occur in the mantle, given its taffy-like consistency, which suggests flowing rather than cracking. However, over the years, seismologists studying earthquakes have gathered evidence of deep-seated quakes originating below a depth of 35 kilometers, crossing the Moho boundary.
Pinpointing the exact location of these earthquakes, especially weaker ones, remains challenging. Typically, these tremors occur at such profound depths that even significant magnitudes go unfelt at the surface. Furthermore, the Moho boundary’s depth varies geographically, meaning some very deep earthquakes might still be situated within the crust in certain locales.
Conventional methods for precisely locating mantle earthquakes historically required a detailed understanding of the crust’s thickness at a given site. However, Klemperer and co-author Shiqi Wang of Stanford developed a technique employing specific types of seismic shear waves that tend to be delayed either within the crust or the mantle. The signature of these waves during a particular earthquake allows researchers to infer whether it likely originated above or below the Moho boundary.
Initially, the method was piloted in Tibet in 2021. Now, in a fresh paper published in the journal Science, they have expanded this investigation globally. The researchers deliberately excluded subduction zones—areas prone to deep earthquakes where crustal material is forced into the mantle. Instead, the team focused on the more elusive phenomenon: mantle earthquakes beneath continental landmasses.
The researchers detected mantle earthquakes across the board. A dense band of such activity stretches from the Alps to the Himalayas, likely associated with the mountain-building processes from continental collisions in those regions. Another cluster of seismic events was observed in East Africa, a site currently undergoing continental rift splitting. Additionally, the team documented mantle earthquakes beneath the western United States and in Canada’s Baffin Bay.
Some identified clusters were unexpected. “There were areas where nobody had found them before, like the Bering Sea,” commented Vera Schulte-Pelkum, a geologist at the University of Colorado Boulder who was not involved in the research. “I would love to have an interactive version of this data where I could zoom in and navigate around.”
According to Klemperer, this global survey provides a foundation enabling other scientists to conduct more focused studies on individual mantle earthquakes and potentially better constrain their depths and the mechanisms driving them.
“It’s tremendously exciting that we now have a tool that can be applied routinely,” he concluded.
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