Deep-focus earthquake

A deep-focus earthquake, sometimes called a plutonic earthquake, is a seismic event with a hypocenter depth exceeding 300 km. These earthquakes are significant because they occur in the deep interior of our planet. Most earthquakes happen in the shallow crust, but these occur much further down. They happen almost exclusively at convergent boundaries. This is where one tectonic plate slides beneath another. These events occur along a dipping, tabular zone known as the Wadati–Benioff zone. This zone follows the subducted oceanic lithosphere as it sinks into the mantle.

The seismic characteristics of these events differ greatly from shallow earthquakes. Deep-focus earthquakes produce minimal surface waves. Because of their extreme depth, the energy is less likely to concentrate at the Earth's surface. When seismic waves travel from the focus to a recording station, they pass through the heterogeneous upper mantle and variable crust only once. This path results in less attenuation, which is the loss of energy, and less reverberation. Consequently, the body waves arrive with very sharp peaks, making them distinct to seismologists.

Scientists have worked for decades to understand the focal mechanisms of these quakes. A focal mechanism describes the pattern of energy radiation. This is often represented by a moment tensor solution, which can be shown using beachball diagrams. These mechanisms vary depending on the depth within the subducting plate. At depths greater than 400 km, down-dip compression is the dominant force. At depths between 250 and 300 km, the stress regime is more ambiguous. However, at that specific depth, it often appears closer to down-dip tension.

Determining how these earthquakes actually happen is a major challenge in seismology. In shallow areas, earthquakes result from brittle fracture and frictional slip. However, at depths greater than 300 km, the high pressure and temperature should prevent brittle behavior. Instead, the rock should undergo plastic deformation, meaning it flows like warm wax. Several theories attempt to explain this mystery. One proposal is solid-solid phase transitions. This suggests an implosion occurs when material changes to a higher-density, lower-volume phase. For example, metastable olivine might suddenly transition to a spinel structure at 410 km depth. This theory is largely discredited because it lacks a significant isotropic signature in seismic data.

Another theory is dehydration embrittlement. This process involves the dehydration of mineral phases that contain high water content. As these minerals release fluids in-situ, the pore pressure within the rock increases. This rise in pressure reduces the effective normal stress in the slab. This allows slip to occur on pre-existing fault planes at much greater depths than usual. However, many scientists believe this mechanism is less significant beyond 350 km. This is because most dehydration reactions likely reach completion between 150 and 300 km. A third idea is transformational faulting, or anticrack faulting. This occurs when a phase transition happens in response to shear stress in a fine-grained zone. Rapid shearing along these planes of weakness could then trigger an earthquake.

Finally, some researchers study shear instability, also known as thermal runaway. This occurs when heat produced by plastic deformation is generated faster than it can be conducted away. This creates a positive feedback loop of heating and material weakening. This process is called strain localization. While mathematical models use this to simulate deep earthquakes, it has not been documented in nature or the laboratory. The exact process of deep-focus earthquakes remains an outstanding problem in the field of deep-earth seismology.

Deep-focus earthquakes occur in several major zones around the world. The most active is the region from Papua New Guinea to New Zealand. Here, the Pacific plate subducts under the Australian, Tonga, and Kermadec plates. This area is so active that earthquakes of magnitude 4.0 or higher occur almost daily. The deepest earthquake ever recorded was a small 4.2 magnitude event in Vanuatu in 2004, reaching a depth of 693 km. The strongest deep-focus earthquake on record was the 8.3 magnitude Okhotsk Sea earthquake in 2013. Other notable regions include the Andes, where the Nazca plate subducts under the South American plate. This has caused massive events, such as the 8.2 magnitude earthquake in Bolivia in 1994 at a depth of 631 km.