Causes of Earthquakes
Earthquakes and Plate Boundaries
- Earthquakes can occur anywhere but mostly occur at or near plate boundaries
- Earthquakes happen at all plate boundaries - constructive, destructive, collision and conservative
- An earthquake is the sudden, violent shaking of the ground
- Earthquakes are the result of pressure building when tectonic plates move
- The epicentre is the point on the Earth's surface directly above the focus
- The focus is the point at which the earthquake starts below the Earth's surface
Measuring earthquakes
- Seismometers are used to record the size of the seismic waves
- The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale
The damage caused by earthquakes is measured on the Mercalli Scale
Mercalli Scale Diagram
Mercalli Scale
Earthquake focus
- The depth of the earthquake focus can affect the impact of an earthquake
- A deep focus earthquake starts at a depth greater than 70 km below the surface
- They tend to occur at destructive plate boundaries earthquakes and may be very powerful
- The seismic waves tend to move vertically so a smaller area is affected
- A shallow focus earthquake starts at a depth less than 70 km below the surface
- They can occur at all plate boundaries and tend to be less powerful
- The seismic waves tend to move horizontally so a larger area is affected
Earthquake sequence
- The sequence of an earthquake is the same regardless of the boundary at which it happens:
- As the tectonic plates move, they can get stuck
- Pressure builds as the plates continue to try to move
- Eventually the plates jolt free and the pressure is released as energy
- The point at which the earthquake starts is the focus
- The epicentre is the point directly above the focus on the earth's surface
- The energy passes through the Earth's crust as waves which is the earthquake
- Earthquakes can happen as a result of human activity such as drilling into the crust or mining
Earthquake Features Diagram
Features of an earthquake