GCSE Revision - Earthquakes

What is an Earthquake
 Earthquakes are vibrations in the earth's crust They occur along all plate boundaries but are more destructive usually at destructive plate boundaries

How are they measured
The magnitude or size of an earthquake can be measured  by an instrument called a seismometer and are shown on a seismograph. The earthquakes are measured on the Richter scale from a value of 1 to 10. Each level of magnitude is 10 times more powerful than the previous. Therefore a level 7 is 100 time more powerful than a level 5.

The Distribution of Eartquakes

Why do earthquakes occur?
Most earthquakes occur when there is a movement along the plate boundaries and over 90% of all earthquakes take place at boundaries where there are converging plates. At a plate boundary the plates move as a result of the convection currents but any movement is not steady or smooth. At times the plates become stuck just like teeth in a zip. As the pressure builds up one plate will jerk past the other creating a sudden movement or earthquake.
    The point where the earthquake originates is called the focus and the area of land surface directly above it, the epicentre.


The effects of Earthquakes
   An earthquake causes both primary and secondary effects

Primary effects (the immediate damage)

  • Collapsing bridges and buildings

  • Cracked and twisted roads & other transport links

  • Death and injuries to individuals

  • Panic and shock of the people affected

Secondary effects (the after affects of an earthquake)

  • Fires caused by broken gas mains and electrical cables. Fires develop due to the lack of water from broken pipes

  • Tidal waves or Tsunamis often result from an earthquake such as the boxing day Tsunami in 2004.

  • Landslides in steep sided valleys where the rocks are often weak

  • Disease and famine due to lack of clean water and medical facilities

  • Death caused by the cold of winter such as in the Kashmir quake of 2005

  • Economic impacts - ie Many tourists were put off from visiting areas that had suffered due to the Boxing Day Tsunami

Why do some earthquakes cause more damage than others?

  • Some earthquakes take place in rural areas where few people live and little is damaged

  • If they occur near large urban areas with many people, transport services, large buildings and services such as gas and electricity - there can be a great amount of deaths and damage

  • In Poor countries - LEDC's they have poorly built buildings, few emergency facilities, and the equipment to help rescue people - a good term to use is that they have a poor infrastructure.

  • In rich countries - MEDC's such as the USA they prepare for an earthquake with drills and have a great deal of trained emergency personnel and equipment to go to help the injured. They also try to build to withstand earthquake damage - ie The Transamerica building

  • The time of day can be important - If the earthquake hits when it is the rush hour or when there is a large number of people located in a certain area - this can cause a great loss of life

  • Obviously if they occur near to coasts then Tsunamis can cause a great deal of damage and deaths such as the 2004 Boxing Day Tsunami

Links - Case Study on Kobe from GeoResources
          - Revision DVD on Kobe
          - Your own Case study on Kobe

Common Questions -

  • Describe how an earthquake is caused?

  • Using the map provided describe the location of Earthquakes

  • Why do some earthquakes produce more damage and deaths than others?

  • Using an example describe the primary and secondary effects of an earthquake