Other measurement techniques...

The simplest method of locating an earthquake on a globe is to find the time interval between the P- and S-wave arrivals at several seismograph stations. The distance to the earthquake from each station is then determined from standard travel-time tables and travel-time curves. Great-circle arcs are drawn on the globe using the distance of the earthquake to the station as a radius. All the arcs should intersect at a common point - the epicentre.

Another method of locating an earthquake is to use the P-wave arrival-time minus origin-time (P - O) interval instead of distance. This method is more common because the time can be taken directly from surface focus travel-time tables assuming an origin of 00 hours. This method, however, requires that travel-time tables be available for various depths of focus. For locating a deep shock, one 700 kilometers deep, for example, travel-time tables and travel-time curves for that depth have to be used to calculate the origin time and distances.