1. Cause of Earthquakes What is an Earthquake?
An earthquake is the vibration of Earth produced by the sudden, rapid release of energy.
Seismology
When an earthquake occurs, the elastic energy is released and sends out vibrations that travel throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave within Earth is called seismology.
Reference: Pages

2. Focus and Epicenter Earthquake Focus and Epicenter? Focus: Epicenter:
The exact location within Earth were seismic waves are generated by sudden release of stored elastic energy. Most often located on a pre-existing fault.
Focus
Epicenter:
The point on the surface of Earth directly above the focus.
Epicenter
Earth’s Surface
Fault Plane

3. Cause of Earthquakes
Earthquakes occur when energy stored in elastically strained rocks is suddenly released.
This release of energy causes intense ground shaking in the area near the source of the earthquake (Focus) and sends waves of elastic energy, called seismic waves, in all directions throughout Earth.
Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults.
Earthquakes are definitely a geologic hazard for people living in earthquake regions, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

4. Cause of Earthquakes How Does Earthquakes Originate?
Movement in areas along the fault plane stops (fault sticks).
Elastic energy is stored in the rock as the rock becomes deformed and bends, much like a bent stick.
When the elastic strain built up along the fault exceeds the elastic limit, the rock will break or slip at its weakest point which we call the the focus.
This slippage along the fault allows the rock to “snap back” and the vibrations sends out waves of energy in all directions called seismic waves, or earthquake waves.
The springing back of the rock is called “elastic rebound”.

5. Elastic Rebound Theory
The mechanism that cause earthquakes was not understood until H. F. Reid proposed his idea of “Elastic rebound.”
Reid suggested that most natural earthquakes are caused by sudden slippage along a fault zone.
Original position of rocks before any strain.

6. Elastic Rebound Theory
The elastic rebound theory suggests that if movement along a fault gets stuck, elastic strain energy builds up deforming rocks on either side of the fault.
A feature across the fault bends as the rocks on both sides of the fault pushes in opposite directions and elastic strain builds up.
Fault

7. Elastic Rebound Theory
When the rocks along the fault can no longer hold the strain slippage occurs at the weakest point along a fault (focus) which causes rock to start moving on both sides of the fault.
Energy is released in all directions causing an earthquake.
The fault ruptures releasing elastic energy.
Fault
Energy Released

8. Elastic Rebound Theory
When the elastic strain is released the rocks on both sides of the fault will “snap-back” to their unstrained positions.
Elastic strain on rocks released.
Fault

9. Sample Problem
Earthquakes commonly occur at plate boundaries, with reference to elastic rebound, what causes an earthquake?
Answer:
There is a build up of stress (energy) within the tectonic plates, where it reaches the elastic limit. The plates then rupture (snap) and the stored energy is released. Plates then move (snap) back to unstressed positions releasing the stored energy as seismic waves. This is what we call an earthquake.