Editing Epicenter (section)

==Epicentral distance==
{{Main|Epicentral distance}}
During an earthquake, [[seismic wave]]s propagates in all directions from the hypocenter. [[Seismic shadowing]] occurs on the opposite side of the Earth from the earthquake epicenter because the planet's [[Earth's outer core|liquid outer core]] [[refraction|refracts]] the [[longitudinal waves|longitudinal]] or compressional ([[P wave]]s) while it absorbs the [[transverse wave|transverse]] or shear waves ([[S wave]]s). Outside the seismic shadow zone, both types of wave can be detected, but because of their different velocities and paths through the Earth, they arrive at different times. By measuring the time difference on any seismograph and the distance on a travel-time graph on which the P wave and S wave have the same separation, geologists can calculate the distance to the quake's epicenter. This distance is called the ''epicentral distance'', commonly measured in [[Degree (angle)|°]] (degrees) and denoted as Δ (delta) in seismology. The [[Václav Láska (mathematician)|Láska's empirical rule]] provides an approximation of epicentral distance in the range of 2,000−10,000&nbsp;km.

Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using [[trilateration]].

Epicentral distance is also used in calculating [[Seismic scale#Local magnitude scale and related scales|seismic magnitudes as developed by Richter and Gutenberg]].<ref>{{cite web|publisher=United States Geological Survey|author=Tyler M. Schau|url=http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm|title=The Richter Scale (ML)|year=1991|access-date=2008-09-14|archive-date=2016-04-25| archive-url=https://web.archive.org/web/20160425121745/http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm|url-status=dead}}</ref><ref>{{cite web|publisher=United States Geological Survey|author=William L. Ellsworth|url=http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm|title=Surface-Wave Magnitude (M<sub>s</sub>) and Body-Wave Magnitude (mb)|year=1991|access-date=2008-09-14|archive-date=2009-02-02| archive-url=https://web.archive.org/web/20090202043713/http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm|url-status=dead}}</ref>