Depth of focus (tectonics)

Depth of focus (tectonics)

In seismology, the depth of focus refers to the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than 60 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 60 and 300 km 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 up to 700 kilometers).[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.

See also

References

  1. ^ D. Armstrong, F. Mugglestone, R. Richards and F. Stratton, OCR AS and A2 Geology, Pearson Education Limited, 2008, p. 32 ISBN 978-0435692117
  2. ^ "M7.5 Northern Peru Earthquake of 26 September 2005" (PDF). 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" (pdf). 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. Bibcode 1989Natur.341..733G. doi:10.1038/341733a0.