Deep Focus Earthquakes (Patreon)

The above image displays a narrow cross section of the Earth's interior, which also states the layer's thickness, state of matter, and density. 

Typically, the vast majority of earthquakes which occur originate in our planet's crust as you generally need a non flexible rock to move to generate such an event, unlike what is often found in the mantle. However, if you have looked inland of certain subduction zones such as in Bolivia, Kamchatka (Russia), or Tonga, you may have occasionally noticed some unusually deep earthquakes which occurred. These earthquakes did not simply occur at under 50 kilometers depth, but instead some occurred at between 300 and 750 kilometers depth. These quakes in the mantle are referred to as "deep focus earthquakes" with some even occurring in the upper levels of the lower mantle! But, what might cause such a deep earthquake to occur? The answer, oddly enough is the gemstone known as peridot. (Peridot is the gemstone form of olivine, and I am specifically referring to olivine). Olivine is generally stable in the mantle up until a depth of 410 kilometers. At which point, it changes into a mineral which is 11.5% denser but has the exact same chemcial formula, wadsleyite. Going further down, wadsleyite can then change to ringwoodite which is 12.8% denser at 520 to 650 km depth. This change does not immediately occur once the relevant mineral drops below this level. Instead, it will remain as a prior mineral for some time if it exists within a subducted slab. Then, all at once, large sections of olivine will change into wadsleyite or on rarer occasions ringwoodite, causing the volume of the slab to suddenly decrease. This decrease causes an implosion to occur, which then triggers a fault to move, generating a deep focus earthquake.

Sources/Citations:

[1] hebrov, Victor & Chebrov, Danila & Abubakirov, Iskander & Chebrova, Anastasiya & Gusev, Alexander & Guseva, Evgeniia & Droznin, D. & Droznina, S. & Ivanova, E. & Kravchenko, N.M. & Kugaenko, Yu & Lander, Alexander & Matveenko, Evgeniy & Mitushkina, s & Ototuk, Dmitry & Pavlov, V. & Raevskaya, A. & Saltykov, V. & Skorkina, Anna & Titkov, N.. (2016). Notable events of Kamchatka in 2013. 

[2] Levitas, V.I. Resolving puzzles of the phase-transformation-based mechanism of the strong deep-focus earthquake. Nat Commun 13, 6291 (2022). https://doi.org/10.1038/s41467-022-33802-y, CC BY 4.0

[3] Figure 9.6b from "Physical Geology", 9.1 Understanding Earth through Seismology, by: Steven Earle; Earle, Steven; and Earle, Steven, Chapter link: https://opentextbc.ca/geology/chapter/9-1-understanding-earth-through-seismology/, Photo link: https://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/07/image015.png, CC BY 4.0.

[4] U.S. Geological Survey

[5] Mindat, https://www.mindat.org/

[6] Mindat, Forsterite, https://www.mindat.org/min-1584.html

[7] Mindat, Fayalite, https://www.mindat.org/min-1458.html

[8] Mindat, Wadsleyite, https://www.mindat.org/min-4228.html

[9] Mindat, Ringwoodite, https://www.mindat.org/min-3421.html

[10] O'Keeffe, M., Hyde, B. Why olivine transforms to spinel at high pressure. Nature 293, 727–728 (1981). https://doi.org/10.1038/293727a0

Creative Commons Licenses:

CC BY 4.0: CC BY 4.0 Legal Code | Attribution 4.0 International | Creative Commons