Depth of focus (tectonics)

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In seismology, the depth of focus or focal depth refers to 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 descends beneath 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]


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.

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  1. ^ D. Armstrong, F. Mugglestone, R. Richards and F. Stratton, OCR AS and A2 Geology, Pearson Education Limited, 2008, p. 32 ISBN 978-0-435-69211-7
  2. ^ "M7.5 Northern Peru Earthquake of 26 September 2005" (PDF). National Earthquake Information Center. 17 October 2005. Retrieved 2008-08-01. 
  3. ^ USGS. "M7.5 Northern Peru Earthquake of 26 September 2005" (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. Bibcode:1989Natur.341..733G. doi:10.1038/341733a0. 
  5. ^ Marius Vassiliou, Bradford Hager, and Arthur Raefsky (1984): "The Distribution of Earthquakes with Depth and Stresses in Subducting Slabs", J. of Geodynamics 1, 11–28.

 This article incorporates public domain material from the United States Geological Survey document: William Spence. "Measuring the Size of an Earthquake".