Earthquakes are rare in Earth’s mantle because the rock there is too hot to crack. At the temperatures and pressures found below about 30 to 70 kilometers deep, rock stops behaving like a brittle solid and instead flows slowly, like extremely thick putty. This means stress doesn’t build up and release in sudden snaps the way it does near the surface. The few earthquakes that do occur in the mantle require special circumstances to trigger them.
How Heat Changes Rock Behavior
Near Earth’s surface, rock is cold and rigid. When stress builds, it eventually fractures, releasing energy as an earthquake. But deeper in the Earth, rising temperatures change the rules. Mantle rock, primarily a mineral-rich material called peridotite, transitions from brittle to ductile (flowing) behavior at roughly 700 to 750°C. That temperature is reached within the uppermost mantle in most regions.
Once rock crosses that threshold, it no longer snaps under pressure. Instead, it deforms plastically, creeping and flowing over time without storing the elastic energy needed to produce a quake. Think of the difference between snapping a cold chocolate bar versus bending a warm one. The warm bar deforms without breaking. Mantle rock behaves the same way at depth.
The Mantle Flows Instead of Breaking
The mantle’s viscosity, essentially its resistance to flow, tells us a lot about why earthquakes can’t easily happen there. The upper mantle has a viscosity around 0.5 × 10²¹ Pascal-seconds. That’s extraordinarily thick compared to anything in everyday life (honey is about 10 Pascal-seconds), but over geological timescales it flows readily. The lower mantle, starting around 670 kilometers deep, is stiffer still, with viscosity estimates ranging from about 1.6 to 13 × 10²¹ Pascal-seconds depending on the model and depth.
This flowing behavior means the mantle continuously redistributes stress. Rather than locking up and storing strain energy the way crustal rocks do along fault lines, mantle rock slowly oozes in response to pressure differences. No locked stress means no sudden release, and no sudden release means no earthquake.
Where Mantle Earthquakes Actually Happen
The mantle isn’t completely earthquake-free. Seismologists detect earthquakes at depths between about 70 and 700 kilometers, and they occur almost exclusively inside subducting slabs, the cold slices of ocean floor that dive beneath other tectonic plates at convergent boundaries. These slabs are the key exception to the rule because they carry relatively cold, rigid rock deep into the hot mantle, maintaining lower temperatures than the surrounding material for millions of years.
Earthquakes at these depths are classified as intermediate (70 to 300 km) or deep-focus (300 to 700 km). They’re far less common than shallow earthquakes and are concentrated in specific subduction zones around the Pacific Rim and parts of the Mediterranean and Indonesian archipelago. Below about 700 kilometers, earthquakes essentially vanish entirely.
Three Mechanisms Behind Deep Earthquakes
Normal brittle fracture can’t explain how rock breaks at mantle depths where pressures are enormous and temperatures favor flow. Scientists have identified three main mechanisms that can trigger these unusual events.
Dehydration Embrittlement
As a subducting slab descends, water-bearing minerals inside it break down and release water. This released water dramatically weakens the surrounding rock, reducing friction along potential fault surfaces and allowing sudden slip. This mechanism is well established for intermediate-depth earthquakes and may even operate at much greater depths. Researchers analyzing the massive 2013 magnitude 8.3 earthquake beneath the Sea of Okhotsk, which struck at around 609 kilometers deep, argued that dehydration embrittlement was the simplest explanation for that event.
Transformational Faulting
The dominant mineral in the upper mantle, olivine, undergoes a structural transformation into denser crystal forms as pressure increases. At around 410 kilometers, olivine transforms into a higher-pressure structure called wadsleyite. In cold subducting slabs, however, this transformation can be delayed because the low temperatures slow the reaction. The result is a wedge of “metastable” olivine, mineral that should have transformed but hasn’t yet, persisting to depths where it’s increasingly unstable.
When this metastable olivine finally does transform, it can do so suddenly and violently, triggering mechanical failure that mimics brittle fracture. Laboratory experiments using an analog mineral that undergoes the same type of transformation showed that the shift to the higher-pressure phase triggers brittle failure at pressures of 4 to 5 gigapascals and temperatures of 800 to 1100 K. The pattern of failure closely matched patterns seen in natural deep-focus earthquakes in cold subduction zones. Seismological studies have also detected evidence of these metastable olivine wedges inside cold subducting slabs, supporting the idea that this mechanism drives many of the deepest earthquakes on Earth.
Thermal Shear Instabilities
When rock in a subducting slab deforms rapidly enough, the friction generates heat faster than it can dissipate. This localized heating softens a thin zone of rock, which then deforms even faster, generating more heat in a runaway feedback loop. The result can be a narrow band of intense shearing, possibly even localized melting, that mimics an earthquake rupture. This mechanism doesn’t require any mineral transformation or water release, just sufficiently rapid deformation in a concentrated zone.
Why Earthquakes Stop Below 700 Kilometers
The deepest recorded earthquakes bottom out near 700 kilometers, close to the boundary between the upper and lower mantle. Below this depth, several factors conspire to shut down seismicity. The mineral transformations that enable transformational faulting are largely complete by this point, so there’s no more metastable olivine left to suddenly convert. The surrounding mantle is hot enough that even subducting slab material has warmed significantly, making ductile flow increasingly dominant. And the enormous pressures at these depths make brittle failure progressively harder to initiate through any mechanism.
The 660-kilometer discontinuity, where another major mineral phase change occurs as the spinel structure of ringwoodite breaks down into denser phases, also plays a structural role. Some slabs stall or flatten at this boundary rather than penetrating deeper, removing the cold, rigid material that serves as the host for deep earthquakes. Slabs that do penetrate into the lower mantle gradually lose their thermal distinctiveness, warming toward the ambient mantle temperature and losing their capacity to generate seismic events.
The Mantle’s Role in Surface Earthquakes
While the mantle itself rarely produces earthquakes, its slow flow is the ultimate driver of nearly all earthquakes at the surface. Convection currents in the mantle drag tectonic plates, pushing them apart at mid-ocean ridges and pulling them together at subduction zones. The stress that builds along crustal faults, from the San Andreas to the Himalayan thrust, originates in mantle convection operating over millions of years. The mantle is too soft to quake on its own, but it’s the engine behind virtually every earthquake you feel at the surface.