There is a network of seismometers (earth-quake detectors) setup around the world. When an earthquake occurs it releases a set of powerful waves which travel through the Earth and are detected by the seismometers. By comparing the time taken to reach each seismometer the epicentre of the earthquake can be triangulated.
the point within earth where the earthquake starts
It starts at the focus or hypocentre of the earthquake.
tertiary
it starts in the focus
Typically, at least three seismic stations are needed to accurately locate an earthquake by using triangulation. The more stations that record the earthquake, the more accurate the determination of its location can be.
Locate is a verb. For example: Engineers worked to locate the fault responsible for the earthquake. Locatable, a derivative of locate, is an adjective. Location is a noun.
3
By using seismographs and scales
There will, no doubt, be an earthquake in 2012 but no one can know when or where it will happen, or how severe it will be.
The point at which the earthquake starts - is the Epicentre.
Quiver
AWSOME