Depth of focus (tectonics)
Script error: No such module "Distinguish". 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]
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.[7] The United States Geological Survey presently determines Template:Cvt 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 Template:Cvt depth. The previous fixed depth used by the agency was Template:Cvt.[8]
See also
References
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- ^ Spence, William, Stuart A. Sipkin, and George L. Choy (1989). "Measuring the Size of an Earthquake." Earthquake Information Bulletin (USGS). 21 (1), 58–63.
- ^ Page Module:Citation/CS1/styles.css has no content."M7.5 Northern Peru Earthquake of 26 September 2005" (PDF). National Earthquake Information Center. 17 October 2005. Archived from the original (PDF) on 2017-05-25. Retrieved 2008-08-01.
- ^ Page Module:Citation/CS1/styles.css has no content."M7.5 Northern Peru Earthquake of 26 September 2005" (PDF). United States Geological Survey. Archived from the original (PDF) on 2017-05-25. Retrieved 2008-08-01.
- ^ Page Module:Citation/CS1/styles.css has no content.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. S2CID 4287597.
- ^ 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.
- ^ Spence, William, Stuart A. Sipkin, and George L. Choy (1989). Script error: No such module "Template wrapper". Earthquake Information Bulletin (USGS). 21 (1), 58–63.
- ^ Page Module:Citation/CS1/styles.css has no content.Hongjian, Fang; Abercrombie, Rachel E. (December 2022). "A new method to determine the depth of earthquakes on oceanic transform faults using teleseismic arrays: application to the Chain transform, equatorial Atlantic". AGU Fall Meeting 2022, held in Chicago, IL, 12-16, id. T44A-04.
- ^ Page Module:Citation/CS1/styles.css has no content."Why do so many earthquakes occur at a depth of 10 km?". United States Geological Survey. Retrieved 7 May 2024.
