{{distinguish|depth of field|focal length}} 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 {{convert|70|km|mi|abbr=on}} are classified as shallow-focus earthquakes, while those with a focal depth between {{convert|70|km|mi|abbr=on}} and {{convert|300|km|mi|abbr=on}} are commonly termed mid-focus or intermediate-depth earthquakes.<ref>Spence, William, Stuart A. Sipkin, and George L. Choy (1989). [https://earthquake.usgs.gov/learn/topics/measure.php "Measuring the Size of an Earthquake."] ''Earthquake Information Bulletin (USGS).'' '''21''' (1), 58–63.</ref> 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 {{convert|300|km|mi|abbr=on}} up to {{convert|700|km|mi|abbr=on}}.<ref>{{cite web|publisher=National Earthquake Information Center|title=M7.5 Northern Peru Earthquake of 26 September 2005|date=17 October 2005|url=ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf|archive-url=https://web.archive.org/web/20170525100314/ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf|archive-date=2017-05-25|url-status=dead|accessdate=2008-08-01}}</ref><ref>{{cite web|publisher=United States Geological Survey|title=M7.5 Northern Peru Earthquake of 26 September 2005|url=ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf|archive-url=https://web.archive.org/web/20170525100314/ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf|archive-date=2017-05-25|url-status=dead|accessdate=2008-08-01}}</ref>
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,<ref name="olivine">{{cite journal|last1=Greene II|first1=H. W.|last2=Burnley|first2=P. C.|title=A new self-organizing mechanism for deep-focus earthquakes|journal=Nature|volume=341|issue=6244|pages=733–737|date=October 26, 1989|doi=10.1038/341733a0|bibcode=1989Natur.341..733G|s2cid=4287597 }}</ref> 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.<ref>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.</ref>
==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.<ref>Spence, William, Stuart A. Sipkin, and George L. Choy (1989). {{USGS|title=Measuring the Size of an Earthquake|url=https://earthquake.usgs.gov/learn/topics/measure.php|name-list-style=amp}} ''Earthquake Information Bulletin (USGS).'' '''21''' (1), 58–63.</ref>
==Fixed depth== When seismic data is too scarce to calculate a focal depth, a "fixed depth" is assigned. For example, many earthquakes occurring in oceans are assigned fixed depth because of limited local seismic records.<ref>{{cite journal |last1=Hongjian |first1=Fang |last2=Abercrombie |first2=Rachel E. |title=A new method to determine the depth of earthquakes on oceanic transform faults using teleseismic arrays: application to the Chain transform, equatorial Atlantic |journal=AGU Fall Meeting 2022, held in Chicago, IL, 12-16, id. T44A-04 |date=December 2022 |url=https://ui.adsabs.harvard.edu/abs/2022AGUFM.T44A..04F/abstract}}</ref> The United States Geological Survey presently determines {{cvt|10|km}} to be the fixed depth for most shallow earthquakes as their actual focal depth is usually close to that value. Many earthquakes in their catalogue are assigned {{cvt|10|km}} depth. The previous fixed depth used by the agency was {{cvt|33|km}}.<ref>{{cite web |title=Why do so many earthquakes occur at a depth of 10 km? |url=https://www.usgs.gov/faqs/why-do-so-many-earthquakes-occur-a-depth-10km |publisher=United States Geological Survey |access-date=7 May 2024}}</ref>
==See also== *Asthenosphere *Focus (earthquake) *Lithosphere
==References== {{Reflist}}
Category:Plate tectonics Category:Types of earthquake
{{seismology-stub}} {{tectonics-stub}}