Researchers at Stanford University have developed the first worldwide map of rare earthquakes occurring deep in Earth's mantle, rather than the crust. These elusive events cluster in regions like the Himalayas and near the Bering Strait. The study, published on February 5 in Science, identifies hundreds of such quakes and introduces a new method to detect them using seismic waves.
Earth's mantle, a thick layer of warm, dense rock between the thin outer crust and the molten core, has long puzzled scientists regarding its potential to generate earthquakes. Most continental earthquakes originate 6 to 18 miles below the surface within the brittle crust, above the Mohorovičić discontinuity, or Moho. However, evidence has mounted over the past decade that rare mantle quakes do occur beneath continents, away from subduction zones, sometimes up to 50 miles below the Moho. These events are estimated to happen about 100 times less frequently than crustal quakes.
To address the challenge of confirming these deep tremors, Shiqi (Axel) Wang, a former PhD student in geophysics professor Simon Klemperer's lab at the Stanford Doerr School of Sustainability, and Klemperer developed a technique comparing two types of seismic waves. Sn waves, or "lid" waves, travel along the top of the mantle, while Lg waves propagate efficiently through the crust. By measuring the ratio of these waves, along with crustal thickness data, the researchers distinguished mantle origins from crustal ones.
"Our approach is a complete game-changer because now you can actually identify a mantle earthquake purely based on the waveforms of earthquakes," said Wang.
Analyzing data from global seismic stations, the team examined over 46,000 earthquakes since 1990 and confirmed 459 continental mantle earthquakes. These cluster notably beneath the Himalayas in southern Asia and near the Bering Strait, south of the Arctic Circle. The researchers note this number likely underestimates the total, particularly in remote areas like the Tibetan Plateau, and expanding networks could reveal more.
Although too deep to cause surface damage, these quakes provide insights into earthquake mechanisms and Earth's internal structure. "Until this study, we haven't had a clear global perspective on how many continental mantle earthquakes are really happening and where," Wang explained. Klemperer added, "Mantle earthquakes offer a novel way to explore earthquake origins and the internal structure of Earth beyond ordinary crustal earthquakes."
Future work will probe triggers, such as aftershocks from crustal events or mantle convection recycling subducted crust. "Continental mantle earthquakes might be part of an inherently interconnected earthquake cycle, both from the crust and also the upper mantle," Wang said. The research was supported by the National Science Foundation.
