few meters. This description of how earthquakes occur is calledelastic rebound theory
(Figure7.25).
Figure 7.25: Elastic rebound theory. Stresses build on both sides of a fault, causing the
rocks to deform plastically (Time 2). When the stresses become too great, the rocks return
to their original shape but they move (Time 3). This motion releases the energy that creates
an earthquake. ( 46 )
The point where the rock ruptures is usually below the Earth’s surface. The point of rupture
is called the earthquake’sfocus. The focus of an earthquake can be shallow - less than 70
kilometers (45 miles), intermediate - 70 to 300 kilometers (45 to 200 miles), or deep - greater
than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15
kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because
the focus is near the Earth’s surface where people live.
Just above the focus on the land surface, is the earthquake’sepicenter(Figure7.26). It is
the epicenter of an earthquake that is reported by scientists and the media. The epicenter