Most earthquakes begin within the Earth’s crust and occur along familiar fault lines that move and grind beneath our feet. But scientists have discovered that some earthquakes start much deeper, far below the Earth’s crust, inside the mantle beneath the continents.
A new global study has created the world’s first map of these rare ‘mantle earthquakes’, showing that even hard rocks deep within continents can be cracked by stress.
This finding challenges long-held assumptions such as: Rather than breaking apart, the rocks almost curve and flow. It also opens new windows into how stress accumulates and moves within the Earth.
Mapping deep mantle earthquakes
The newly compiled global record records earthquakes that occur beneath the Earth’s crust, inside the upper mantle beneath continental landmasses.
Using these records, Shiqi (Axel) Wang Stanford Doerr School of Sustainability We built a global filter designed to identify earthquakes that originate in the mantle.
Through careful comparisons with nearby crustal earthquakes, Wang and geophysics professor Simon Klemperer were able to avoid mixing shallow and deep sources.
These findings are surprising because far below the Earth’s crust, increased heat and pressure typically causes rocks to flow rather than break down.
Once we pass the transition from brittle to ductile (rocks stop breaking and start creeping), earthquakes are expected to become rarer. But the new catalog shows that some destruction is actually occurring in the upper part of Earth’s mantle, a layer about 1,800 miles thick.
Although the resulting surface shaking is usually small, the unusual depth of these earthquakes indicates hidden internal stresses. continent Routine fault maps cannot be detected.
These signals provide new clues about how the crust and mantle share and redistribute stress.
Deep earthquakes swarm around the world
Patterns on the global map are not random. Earthquakes in the deepest continents tended to be concentrated rather than spread out evenly. Smaller pockets also appeared beneath areas with active faults spanning several continents.
Because no single crustal environment is dominant, the heterogeneous distribution limits simple explanations for why mantle rocks fracture under some land masses but not others.
Understanding these phenomena also requires a closer look at the boundary known as the Mohorovicić discontinuity, or Moho, where the crust gives way to the mantle and the speed of seismic waves changes rapidly.
Most continental earthquakes began 6 to 18 miles (10 to 30 km) below the surface, but sensors could detect rupture as far as 50 miles (80 km) below that boundary.
outside ocean trench Such deep signals have stimulated decades of debate, as mantle rocks at these depths have long been expected to deform gradually rather than break suddenly.
Tracking waves caused by mantle earthquakes
Earthquake observatories record every earthquake as a series of vibrations, and the research team focused on the waves that travel the furthest around Earth.
Rather than relying solely on estimates of the depth at which the earthquake began, they compared the strength of two different types of waves. This comparison allowed us to identify earthquakes that were likely to have occurred beneath the Earth’s crust, within the mantle.
To increase the reliability of the results, the scientists compared each suspected deep quake to nearby shallow earthquakes. This step helped remove distortions caused by the rocks the waves passed through.
“Our approach is a complete game changer because it allows us to actually identify mantle earthquakes purely based on mantle earthquakes.” waveform “This is the effect of the earthquake,” Wang said.
Deep seismic signal filtering
Earthquake monitoring systems around the world have recorded tens of thousands of earthquakes over the past few decades, but only a few showed clear signs of originating deep in the mantle.
After studying nearly 46,000 earthquakes, the researchers identified 459 earthquakes in records dating back to 1990 that likely originated beneath the continent.
Researchers believe that many more deep earthquakes may have yet to be detected because there are fewer seismic observation stations in some regions, such as the Tibetan Plateau in western China.
Adding more sensors and improving maps of underground rock layers could help scientists detect more deep earthquakes.
These improvements could also reveal whether the same deep seismic hotspots appear repeatedly over time.
What causes deep earthquakes
Mapping these deep earthquakes revealed where they occur, but left open the question of what causes them. In some cases, the timing coincides with nearby crustal earthquakes, suggesting that stress may pass between the crust and mantle.
May be linked to other events mantle convectiona slow cycle that moves heat and rock as the Earth recycles old slabs.
“Continental mantle earthquakes may be part of a series of earthquakes that are inherently interconnected. earthquake cycleBoth from the crust and the upper mantle,” Wang said.
Reading the Earth’s Deep Interior
Each time the mantle quakes, it sends waves through rocks that rarely break, giving scientists the opportunity to test how solid continents hold up at depth.
Because these ruptures occurred beneath the Moho, their signals sampled the crust-mantle boundary in a way that shallow earthquakes cannot.
Changes in wave strength suggested local temperature and composition differences, including in the upper mantle zone that supplies magma.
A better model of these deep properties could strengthen our idea of where they are. stress Destructive tectonic earthquakes are concentrated before they begin.
Problems with detecting mantle earthquakes
Even with a cleaner catalog, teams still face core challenges. That is, the epicenter of a deep earthquake is often hidden behind similar seismic signals.
Their wave ratio test works best when nearby crustal earthquakes provide a reliable local baseline, allowing fair comparisons.
In remote areas with sparse seismic histories, researchers may need portable monitoring arrays and longer observation periods before confidently identifying mantle phenomena.
As the catalog expands, scientists will be able to test whether deep earthquakes regularly follow large-scale tectonic movements or occur independently.
This growing record will provide a clearer global picture of earthquakes that occur deep underground on continents. This may reveal how often crustal earthquakes and deep mantle ruptures share the same stress cycles, and when they do not.
The research will be published in a journal science.
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