Deep-focus earthquake

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subduction zone
. Many deep earthquakes have occurred.

A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere. They occur along a dipping tabular zone beneath the subduction zone known as the Wadati–Benioff zone.[1]

Discovery

Preliminary evidence for the existence of deep-focus earthquakes was first brought to the attention of the scientific community in 1922 by Herbert Hall Turner.[2] In 1928, Kiyoo Wadati proved the existence of earthquakes occurring well beneath the lithosphere, dispelling the notion that earthquakes occur only with shallow focal depths.[3]

Seismic characteristics

Deep-focus earthquakes give rise to minimal

upper mantle and highly variable crust only once.[3] Therefore, the body waves undergo less attenuation and reverberation
than seismic waves from shallow earthquakes, resulting in sharp body wave peaks.

Focal mechanisms

The pattern of energy radiation of an earthquake is represented by the

moment tensor solution, which is graphically represented by beachball diagrams. An explosive or implosive mechanism produces an isotropic seismic source. Slip on a planar fault surface results in a double-couple source. Uniform outward motion in a single plane due to normal shortening is known as a compensated linear vector dipole source.[3] Deep-focus earthquakes have been shown to contain a combination of these sources. The focal mechanisms of deep-focus earthquakes depend on their positions in subducting tectonic plates. At depths greater than 400 km, down-dip compression dominates, while at depths of 250-300 km (also corresponding to a minimum in earthquake numbers vs. depth), the stress regime is more ambiguous but closer to down-dip tension.[4][5]

Physical process

Shallow-focus earthquakes are the result of the sudden release of

plastic deformation.[3]
Several physical mechanisms have been proposed for the nucleation and propagation of deep-focus earthquakes; however, the exact process remains an outstanding problem in the field of deep-earth seismology.

The following four subsections outline proposals which could explain the physical mechanism allowing deep focus earthquakes to occur. With the exception of solid-solid

phase transitions
, the proposed theories for the focal mechanism of deep earthquakes hold equal footing in current scientific literature.

Solid-solid phase transitions

The earliest proposed mechanism for the generation of deep-focus earthquakes is an

isotropic signature in the moment tensor solution of deep-focus earthquakes.[1]

Dehydration embrittlement

Dehydration reactions of mineral phases with high water content would increase the

pore pressure in a subducted oceanic lithosphere slab. This effect reduces the effective normal stress in the slab and allows slip to occur on pre-existing fault planes at significantly greater depths than would normally be possible.[1] Several workers[who?] suggest that this mechanism does not play a significant role in seismic activity beyond 350 km depth due to the fact that most dehydration reactions will have reached completion by a pressure corresponding to depths of 150-300 km (5-10 GPa).[1]

Transformational faulting or anticrack faulting

Transformational faulting, also known as anticrack faulting, is the result of the phase transition of a mineral to a higher-density phase occurring in response to shear stress in a fine-grained shear zone. The transformation occurs along the plane of maximal shear stress. Rapid shearing can then occur along these planes of weakness, giving rise to an earthquake in a mechanism similar to a shallow-focus earthquake.

Metastable olivine subducted past the olivine-wadsleyite transition at 320-410 km depth (depending on temperature) is a potential candidate for such instabilities.[3]
Arguments against this hypothesis include the requirements that the faulting region should be very cold, and contain very little mineral-bound hydroxyl. Higher temperatures or higher hydroxyl contents preclude the metastable preservation of olivine to the depths of the deepest earthquakes.

Shear instability / thermal runaway

A shear instability arises when heat is produced by plastic deformation faster than it can be conducted away. The result is thermal runaway, a positive feedback loop of heating, material weakening, and strain localisation within the shear zone.[3] Continued weakening may result in partial melting along zones of maximal shear stress. Plastic shear instabilities leading to earthquakes have not been documented in nature, nor have they been observed in natural materials in the laboratory. Their relevance to deep earthquakes therefore lies in mathematical models which use simplified material properties and rheologies to simulate natural conditions.

Deep-focus earthquake zones

Major zones

Eastern Asia / Western Pacific

On the border of the

Okhotsk and Philippine Sea Plates is one of the most active deep-focus earthquake regions in the world, creating many large earthquakes including the Mw  8.3 2013 Okhotsk Sea earthquake
. As with many places, earthquakes in this region are caused by internal stresses on the subducted Pacific Plate as it is pushed deeper into the mantle.

Philippines

A subduction zone makes up most of the border of Philippine Sea Plate and Sunda Plate, the fault being partially responsible for the uplift of the Philippines. The deepest sections of the Philippine Sea Plate cause earthquakes as deep as 675 kilometres (419 mi) below the surface.[7] Notable deep-focus earthquakes in this region include a Mw  7.7 earthquake in 1972 and the Mw  7.6, 7.5, and 7.3 2010 Mindanao earthquakes.

Indonesia

The Australian Plate subducts under the Sunda Plate, creating uplift over much of southern Indonesia, as well as earthquakes at depths of up to 675 kilometres (419 mi).[8] Notable deep-focus earthquakes in this region include a Mw  7.9 earthquake in 1996 and a Mw  7.5 earthquake in 2007.

Papua New Guinea / Fiji / New Zealand

By far the most active deep focus faulting zone in the world is that caused by the

8.2 and 7.9 earthquake in 2018
, and a Mw  7.8 earthquake in 1919.

Andes

The subduction of the Nazca Plate under the South American Plate, in addition to creating the Andes mountain range, has also created a number of deep faults under the surfaces of Colombia, Peru, Brazil, Bolivia, Argentina, and even as far east as Paraguay.[11] Earthquakes frequently occur in the region at depths of up to 670 kilometres (420 mi) beneath the surface.[12] Several large earthquakes have taken place here, including the Mw  8.2 1994 Bolivia earthquake (631 km deep), the Mw  8.0 1970 Colombia earthquake (645 km deep), and Mw  7.9 1922 Peru earthquake (475 km deep).

Minor zones

Granada, Spain

Roughly 600–630 kilometres (370–390 mi) under the city Granada in southern Spain, several large earthquakes have been recorded in modern history, notably including a Mw  7.8 earthquake in 1954,[13] and a Mw  6.3 earthquake in 2010. The exact cause for the earthquakes remains unknown.[14]

Tyrrhenian Sea

The

Anatolian
microplates.

Afghanistan

In northeastern Afghanistan, a number of medium-intensity deep focus earthquakes of depths of up to 400 kilometres (250 mi) occasionally occur.[17] They are caused by the collision and subduction of the Indian Plate under the Eurasian Plate, the deepest earthquakes centered on the furthest-subducted sections of the plate.[18]

South Sandwich Islands

The South Sandwich Islands between South America and Antarctica are host to a number of earthquakes up to 320 kilometres (200 mi) in depth.[19] They are caused by the subduction of the South American Plate under the South Sandwich Plate.[20]

Notable deep-focus earthquakes

The strongest deep-focus earthquake in seismic record was the magnitude 8.3 Okhotsk Sea earthquake that occurred at a depth of 609 km (378 mi) in 2013.[21] The deepest earthquake ever recorded was a small 4.2 earthquake in Vanuatu at a depth of 735.8 km (457.2 mi) in 2004.[22] However, although unconfirmed, an aftershock of the 2015 Ogasawara earthquake was found to have occurred at a depth of 751 km (467 mi).[23]

References

  1. ^
    doi:10.1146/annurev.ea.17.050189.001303
    .
  2. doi:10.1146/annurev.earth.23.1.169
    .
  3. ^
    ISBN 978-0-521-82869-7.[page needed
    ]
  4. S2CID 4206932
    .
  5. doi:10.1016/0012-821X(84)90083-9
    .
  6. ISBN 978-1-118-68808-3.[page needed
    ]
  7. ^ "M 4.8 - Celebes Sea". earthquake.usgs.gov. Retrieved 26 December 2019.
  8. ^ "M 4.6 - Banda Sea". earthquake.usgs.gov. Retrieved 26 December 2019.
  9. ^ "M 4.2 - Vanuatu region". earthquake.usgs.gov. Retrieved 26 December 2019.
  10. ^ "Latest Earthquakes". earthquake.usgs.gov. Retrieved 26 December 2019.
  11. doi:10.3133/ofr20151031E
    .
  12. ^ "M 3.7 - Acre, Brazil". earthquake.usgs.gov. Retrieved 26 December 2019.
  13. ^ "M 7.8 - Strait of Gibraltar". earthquake.usgs.gov. Retrieved 26 December 2019.
  14. ^ "An Enigma Deep Beneath Spain". seismo.berkeley.edu. Retrieved 26 December 2019.
  15. ^ "M 3.7 - Tyrrhenian Sea". earthquake.usgs.gov. Retrieved 26 December 2019.
  16. doi:10.1111/j.1365-246X.1987.tb01661.x
    .
  17. ^ "M 5.0 - 4km SSE of Ashkasham, Afghanistan". earthquake.usgs.gov. Retrieved 26 December 2019.
  18. ^ "Cause of Afghan Quake Is a Deep Mystery". National Geographic News. 26 October 2015. Retrieved 26 December 2019.
  19. ^ "M 4.3 - 132km NNW of Bristol Island, South Sandwich Islands". earthquake.usgs.gov. Retrieved 26 December 2019.
  20. doi:10.1029/2001JB000396
    .
  21. ^ "M8.3 - Sea of Okhotsk". USGS. 2013-05-25. Retrieved 2013-05-25.
  22. ^ "M 4.2 - Vanuatu region". earthquake.usgs.gov. Retrieved 2018-01-22.
  23. ^ "Deepest earthquake ever detected struck 467 miles beneath Japan". National Geographic. October 26, 2021. Retrieved January 13, 2022.
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