Editing Hypocenter (section)

==Earthquakes==
{{Earthquakes}}
[[File:Epicenter Diagram.svg|thumb|240px|Hypocenter (Focus) and epicenter of an earthquake]]
An earthquake's hypocenter or focus is the position where the [[Strain (materials science)|strain]] energy stored in the rock is first released, marking the point where the [[fault (geology)|fault]] begins to rupture.<ref name="USGS">''The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus.'' {{Cite web|title=Earthquake Glossary – hypocenter|publisher=United States Geological Survey|url=https://earthquake.usgs.gov/learn/glossary/?term=hypocenter|archive-url=https://web.archive.org/web/20100315110912/http://earthquake.usgs.gov/learn/glossary/?term=hypocenter|archive-date=15 March 2010|url-status=live}}</ref> This occurs directly beneath the [[epicenter]], at a distance known as the ''hypocentral depth'' or ''[[focal depth]]''.<ref name="USGS" />

The focal depth can be calculated from measurements based on [[seismic wave]] phenomena. As with all [[wave]] phenomena in [[physics]], there is uncertainty in such measurements that grows with the [[wavelength]] so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.

Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring.<ref>{{Cite web|title=Using GIS for Visualizing Earthquake Epicenters, Hypocenters, Faults and Lineaments in Montana|author1=Kennelly, Patrick J.  |author2=Stickney, Michael C.|year=2000|url=http://pubs.usgs.gov/of/2000/of00-325/kennelly.html|publisher=United States Geological Survey|id=USGS Open-File Report 00-325|work=Digital Mapping Techniques '00 -- Workshop Proceedings|archive-url=https://web.archive.org/web/20040323205317/http://pubs.usgs.gov/of/2000/of00-325/kennelly.html|archive-date=23 March 2004|url-status=live}}</ref> The expanding wavefront from the earthquake's rupture propagates at a speed of several kilometers per second; this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through.<ref name="How">{{Cite web|title=FAQs – Measuring Earthquakes: Q: How do seismologists locate an earthquake? |publisher=United States Geological Survey |department=Earthquake Hazrads Program |url=https://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118/index.html |url-status=dead |archive-url=https://web.archive.org/web/20120311190548/http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118%2Findex.html |archive-date=2012-03-11 }}</ref> With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter.  These equations are solved by the method of [[least squares]] which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed.  The system iterates until the location is pinpointed within the margin of error for the velocity computations.<ref name="How" />