Earthquake Location

• The earthquake hypocentre or focus is the point on the fault plane where the rupture starts.
• The fault may rupture in one direction from the hypocentre, or in both directions. That is, the hypocentre may be at the edge of the rupture or within the rupture.
• The earthquake epicentre is the point on the earth's surface vertically above the hypocentre.
• The earthquake depth is usually taken as that of the hypocentre, even if the fault ruptures to the surface.
• The centroid of an earthquake is located at the centre of energy release, usually near the centre of the rupture area.

"Locating" an earthquake normally means determining the origin time, coordinates of the hypocentre, and the earthquake magnitude or size.

• The origin time of an earthquake was the precise time that the rupture commenced. The rupture duration then varies from a fraction of a second for a small earthquake to a few tens of seconds for a very large earthquake.
• The longitude and latitude of the hypocentre are, by definition, the same as those of the epicentre. They are usually given in degrees, although metric grid zone coordinates are sometimes used.
• The depth of an earthquake is usually given in kilometres below mean sea level.
• The magnitude is computed after the earthquake has been located, using the hypocentral distance from earthquake to seismograph.