The point of origin of an earthquake is called the focus or hypocenter. It may occur anywhere from the surface to a depth of about 700 km.
A jar applied to such a point generates a series of elastic waves, which spread outward in all directions from the focus, as a series of waves will pass through a stone which is struck at blow with the hammer.
If the medium through which the waves pass is uniform, these waves will spread with equal rapidity in all directions, and will gradually lose their intensity equally in all directions from the center of disturbance.
In different earthquakes the depth of the focus shows great variations. Its depth shows a wide variance ranging from about 10 to 700 km. The epicenter or epicentral line of an earthquake is the point or line on the earth’s surface that lies directly above the focus.
It is at the epicenter that the strongest shock is normally recorded. Besides, it is at the epicenter that the waves generated by an earthquake are recorded first. If the time of arrival of the earthquake is noted at a number of places, it is possible to determine the position of the epicenter.
Lines drawn through places which were affected by the earthquake at the same point of time are called homoseismal lines. These lines are generally elliptical in shape, and the middle of the ellipse is the epicenter.
The actual place where the earthquake originates is just below the epicenter. The instrument which records the earthquake waves is called seismograph or seismometer. The science which deals with earthquakes is known as seismology.
As regards the depth of the focus, Oldham examined the records of 5605 earthquake shocks in Italy and discovered that in about 90 per cent of the earthquakes the depth of the focus was only about 8 km; about 8 per cent originated at depth between 8 and 30 km, and the rest at greater depths. It is, however, easier to find the epicenter of an earthquake than to determine the depth of the focus.