The plates in Earth’s crust constantly side past each other, but can get stuck. Pressures then builds up until the plates finally move, sending out shock waves. The focus of an earthquake is the point inside the ground where pressure builds up. The epicenter is the point on the surface above the focus.
The point at which this slippage occurs is called the FOCUS, whilst the point on the ground surface above the earthquake FOCUS is called the EPICENTRE. Seismic shock waves will emanate radially outwards from these points and their energy will reduce with distance. This is typical of destructive margins (which account for 90% of the World’s earthquakes) where the Oceanic plate grinds under a Continental plate (as on the East coast of Japan -see Kobe case study). They also occur at conservative margins, such as the San Andreas Fault line, where the North American plate and Pacific plate are grinding past one another.
Picture Credit : Google