A study reveals the extent of rare earthquakes in the deep layer beneath the Earth’s crust

Stanford University researchers have created the first-ever global map of a rare type of earthquake that occurs not in the Earth’s crust but in our planet’s mantle, the layer sandwiched between the Earth’s thin crust and molten core. The new map will help scientists learn more about the mechanisms of mantle earthquakes, which in turn opens a window into the complexities and triggers of all earthquakes.

As reported in a study published February 5 in the journal Science, continental mantle earthquakes occur worldwide but are clustered regionally, especially in the Himalayas in southern Asia and the Bering Strait between Asia and North America, south of the Arctic Circle. By analyzing these deep earthquakes, scientists expect to gain unique insights into crust-mantle boundaries and the behavior of the upper mantle — the source of volcanic magma that partly drives tectonic plate movements.

“Until this study, we did not have a clear global perspective on how many continental mantle earthquakes actually occur and where,” said lead study author Shiqi (Axel) Wang, a former doctoral student in the lab of geophysics professor Simon Klemperer at the Stanford Doerr School of Sustainability. “With this new dataset, we can begin to explore the different ways rare mantle earthquakes initiate.”

Continental mantle earthquakes are too deep to cause much shaking or danger at the Earth’s surface. But its unique origins will advance multiple fields in Earth sciences, which in turn could improve understanding of risks from common, shallower earthquakes.

“Although we know the broad strokes along which earthquakes generally occur when stress is released at fault lines, why a particular earthquake occurs where it does and the key mechanisms behind it are not well understood,” added Klemperer, the study’s senior author. “Mantle earthquakes offer a new way to explore the origins of earthquakes and the Earth’s internal structure beyond ordinary crustal earthquakes.”

Above and below the moho

Unlike Earth’s cold, fragile crust, the mantle is a warm, semi-solid region of dense rock about 1,800 miles thick that makes up the bulk of our planet’s interior. The boundary between the crust and mantle is known as the Mohorovich discontinuity, also called the “Moho”.

For decades, seismologists and geophysicists have debated whether a viscous mantle could support significant seismic activity. The points of origin of most continental earthquakes reach depths of about 6 to 18 miles, just above the Moho and thus in the Earth’s crust. Notable exceptions are subduction zones, where denser oceanic plates sink beneath lighter crustal plates, sometimes triggering earthquakes hundreds of miles down. However, sensor measurements have sometimes indicated the presence of sub-Moho cores much deeper beneath continental land masses, far from subduction zones, even as far as 50 miles below the Moho.

Based on accumulating evidence, most researchers over the past 10 years have accepted that rare earthquakes originate in the mantle, perhaps about 100 times fewer than crustal earthquakes. But defining them unambiguously has proven challenging due to a lack of data.

To distinguish between mantle earthquakes and crustal earthquakes, Wang and Klemperer developed a method to compare two types of seismic waves. These vibrations, caused by earthquakes and other phenomena, resonate throughout the Earth as if our planet were a ringing bell.

These two types of waves are called Sn or “mantle” waves, which are a type of shear wave that travels across the top of the mantle known as the “mantle,” and Lg waves, which are high-frequency ripples that easily bounce through the crust. The ratio of wave sizes determines their origins.

“Our approach is completely game-changing because you can now identify a mantle earthquake based on the earthquake waveforms,” ​​Wang said.

Anecdotes galore

By studying data from seismic monitoring stations around the world and incorporating other important information such as crustal thickness, the researchers narrowed down an initial set of more than 46,000 earthquakes to 459 identified in the continental mantle since 1990.

Map showing continental earthquakes around the world. (Image credit: Axel Wang)

The total number is conservative, researchers said. More mantle earthquakes are likely to be picked up by expanding sensor networks, especially in remote areas such as the Tibetan Plateau that extends northward from the great height of the Himalayas. Klemperer spent most of his career researching earthquakes in this geographically isolated region. His initial exposure to the idea of ​​continental mantle earthquakes there eventually led his student Wang to pursue the topic further.

With a wealth of mantle tremors now on the books, as well as their reliable method for identifying future earthquakes, Wang and Klemperer plan to delve deeper into the details of these rare events. Some of them appear to be aftershocks caused by the propagation of seismic waves from crustal earthquakes. Others may arise from convection of the mantle itself, generated by heat, as it recycles subducting slabs of the Earth’s crust.

Looking to the future, Stanford researchers expect a much fuller picture of the Earth’s hidden workings to emerge.

“Continental mantle earthquakes may be part of an inherently interconnected seismic cycle, both from the crust and also from the upper mantle,” Wang said. “We want to understand how these layers of our world function as a whole system.”

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