Depth of focus (tectonics)

In seismology, the depth of focus or focal depth is the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than 70 km (43 mi) are classified as shallow-focus earthquakes, while those with a focal depth between 70 km (43 mi) and 300 km (190 mi) are commonly termed mid-focus or intermediate-depth earthquakes.[1] In subduction zones, where older and colder oceanic crust sinks under another tectonic plate, deep-focus earthquakes may occur at much greater depths in the mantle, ranging from 300 km (190 mi) up to 700 km (430 mi).[2][3]

The cause of deep-focus earthquakes is still not entirely understood since subducted lithosphere at that pressure and temperature regime should not exhibit brittle behavior. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure,[4] with which they are believed to be associated. Earthquakes at this depth of focus typically occur at oceanic-continental convergent boundaries, along Wadati–Benioff zones.[5]

## Discovery

The evidence for deep-focus earthquakes was discovered in 1922 by H.H. Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismograms of several earthquakes, which in turn led to the construction of travel-time curves for intermediate and deep earthquakes.[6]

## See also

* Asthenosphere
* Focus (earthquake)
* Lithosphere

## References

1. ↑ Spence, William, Stuart A. Sipkin, and George L. Choy (1989). "Measuring the Size of an Earthquake." Earthquake Information Bulletin (USGS). 21 (1), 58–63.
2. ↑ "M7.5 Northern Peru Earthquake of 26 September 2005". National Earthquake Information Center. 17 October 2005. ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf. Retrieved 2008-08-01.
3. ↑ USGS. "M7.5 Northern Peru Earthquake of 26 September 2005". ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf. Retrieved 2008-08-01.
4. ↑ Greene II, H. W.; Burnley, P. C. (October 26, 1989). "A new self-organizing mechanism for deep-focus earthquakes". Nature 341 (6244): 733–737. doi:10.1038/341733a0. Bibcode: 1989Natur.341..733G.
5. ↑ Marius Vassiliou, Bradford Hager, and Arthur Raefsky (1984): "The Distribution of Earthquakes with Depth and Stresses in Subducting Slabs", Journal of Geodynamics 1, 11–28.
6. ↑ Spence, William, Stuart A. Sipkin, and George L. Choy (1989).  This article incorporates public domain material from the United States Geological Survey website https://earthquake.usgs.gov/learn/topics/measure.php. Earthquake Information Bulletin (USGS). 21 (1), 58–63.

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