1. Seismic Waves in the Earth’s Crust
Earthquakes
Seismic Waves in the Earth’s Crust

2. Earthquakes
An earthquake is a series of seismic waves or tremors in the earth’s crust.
They are caused when pressure builds up at the edge of the plates as they collide or slide past one another.
When this pressure is released a huge amount of energy is released causing the plates to shake. This shaking movement is the earthquake.

3. Earthquakes – Important Words
Tremors – These are the waves of seismic energy which spread out from the Focus.
Aftershocks – These are the smaller tremors which may occur hours or days after the original earthquakes. They can also be extremely dangerous because they may cause already damaged buildings to collapse.

4. The Focus and the Epicentre
Focus – This is the place deep in the earth’s crust where the earthquake begins.
Epicentre – This is the place on the surface of the earth, directly above the Focus. The tremors are strongest at this point.

6. Measuring Earthquakes
The strength or force of an earthquake is measured on a Seismograph.
The scale used to describe this force is called the Richter Scale. 1 is the weakest and 10 is the strongest.

7. Each point on the Richter Scale means that an earthquake is 10 times stronger. For e.g. and earthquakes measuring 6 on the scale is 10 x stronger than an earthquake measuring 5 on the scale.

8. Earthquakes – The San Andreas Fault

9. What happens along the San Andreas Fault?