ATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT 2.1 Earthquakes Epicentre (most damage here)
Earthquakes occur at all plate boundaries (see Chapter 1). They are vibrations or tremors in the earth’s crust, and are caused by the movement of plates over the mantle which makes the crust stretch and tear. This increase in stress and pressure is greater than the strength of the rocks which suddenly give way along a fault line in the crust. Shock waves or tremors travel out from the origin or focus of the earthquake just as ripples travel across the surface of water when a stone is thrown. The site directly above the focus on the surface of the earth is known as the epicentre. Most damage is done here. There are thousands of earthquakes that are strong enough to be felt by people each year but there are over one million other earthquakes that are detected only by instruments. As the plates settle back to their normal
Fault line Fault scarp
Fault Tremors Focus Fig. 1 Exam Diagram: Focus and epicentre of an earthquake
positions after the earthquake (elastic rebound) more earthquakes, or aftershocks, can occur. These can do more damage to already weakened buildings and send panic through affected populations.
2.2 Location and depth of earthquakes
Earthquakes occur at all types of plate boundaries and at different depths in the crust.
Shallow earthquakes: Less than 70 km below the surface; associated with mid-ocean ridges, continental–continental plate collision and conservative plate boundaries
Intermediate earthquakes: 70 – 300 km below the surface; associated with oceanic–continental plate collision
Fig. 2 Map to show the distribution of earthquakes
Deep earthquakes: Greater than 300 km below the surface; associated with oceanic–oceanic plate collision