Earthquakes are common, but most seismic activity happens in the shallow crust, typically within the top 70 kilometers. These shallow events are caused by the brittle fracture of rock under immense stress. Deep earthquakes, however, are a rare and perplexing phenomenon that challenges fundamental assumptions about how rock behaves. These deep events originate in the mantle, where high temperatures and pressures should cause rock to flow plastically, rather than break suddenly. The existence of deep-focus earthquakes poses a major scientific puzzle regarding how rock can fracture so far beneath the surface.
Defining Deep Earthquakes
Seismologists categorize earthquakes by the depth of their hypocenter, the point within the Earth where the rupture originates. Shallow-focus earthquakes occur from the surface down to about 70 kilometers. Intermediate-focus earthquakes are found between 70 kilometers and 300 kilometers deep. Deep-focus earthquakes are defined as those occurring at depths greater than 300 kilometers, extending down to a hard limit of about 700 kilometers.
This depth range is unusual because the surrounding mantle rock is hot and ductile, meaning it deforms slowly under pressure. Conventional faulting, which requires cold and brittle rock, should be impossible at these mantle conditions. The maximum depth of approximately 700 kilometers is significant because it marks a physical boundary where conditions for sudden brittle failure cease to exist. This depth corresponds roughly to the transition from the upper to the lower mantle.
Global Geography of Deep Quakes
Deep-focus earthquakes are not scattered randomly across the globe; they are almost exclusively confined to specific geological structures known as subduction zones. These zones are locations where one tectonic plate, typically a dense oceanic plate, descends beneath another plate and sinks deep into the Earth’s mantle. This cold, rigid oceanic plate is referred to as the subducting slab.
The seismic signature of this descending slab is a distinct, dipping plane of earthquake hypocenters that extends from the surface down to the maximum depths. This inclined zone of seismicity is known as the Wadati-Benioff Zone. The most seismically active regions for deep earthquakes are concentrated around the Pacific Ring of Fire. The Tonga-Fiji region is particularly notable, hosting the deepest recorded earthquakes. Other major locations include:
- The Kuril-Kamchatka Trench
- The Japan Trench
- The South American subduction zone, which generates deep seismicity beneath the Andes mountains
In all these areas, the deep earthquakes occur within the core of the sinking oceanic plate, which remains relatively cold compared to the ambient mantle.
The Unusual Mechanism of Deep Earthquakes
The high pressure and temperature environment of the deep mantle makes the standard shallow-earthquake mechanism of frictional sliding unworkable, necessitating alternative explanations for rock failure. Scientists propose two primary mechanisms that allow the rock in the subducting slab to fracture suddenly, even under these extreme conditions. These theories involve changes to the mineral structure or the presence of fluids.
Transformational Faulting
One leading hypothesis is transformational faulting, driven by mineral phase transitions. Olivine, a major component of the descending oceanic slab, is only stable at lower pressures. As the slab sinks deeper than about 400 kilometers, the immense pressure causes olivine to become metastable, meaning it should change into a denser crystal structure.
The sudden, rapid transformation of olivine can create tiny faults known as “anticracks.” This rapid phase change, which involves a volume reduction, can be localized along shear zones within the slab. The transformation process generates ultra-fine-grained material that acts as a weak lubricant, allowing for a sudden slip event. This mechanism explains the concentration of deep earthquakes between 400 and 600 kilometers, where the metastable olivine wedge is expected to exist.
Dehydration Embrittlement
A second mechanism is dehydration embrittlement. Oceanic crust and mantle material carry water locked within the crystal structures of hydrous minerals like serpentine. As the subducting slab heats up and is subjected to rising pressure, these hydrous minerals break down, releasing water.
The liberated water acts as a high-pressure fluid that penetrates micro-cracks and pores within the rock. This dramatically reduces the effective normal stress and weakens the rock structure. This process allows the rock to fracture suddenly in a brittle manner, initiating an earthquake. This fluid-driven failure may operate at depths exceeding 400 kilometers, suggesting that some hydrous minerals carry water much deeper into the mantle than previously assumed.