Earthquakes

The cause of Earthquakes

Earthquakes are sudden ground movements which result from the sudden release of built up energy. This energy is released in the form of seismic waves. Earthquakes are caused due to tectonic motions in the earth's crust. Earthquakes are found at all four of the major plate boundaries (constructive, destructive, collision and conservative boundaries), due to the forces of collision between plates as well as the irregular movement and build up of friction as plates move past each other. Earthquakes also occur away from plate boundaries at weaknesses in the earth's crust known as faults.

As plates move past each other, friction between them results in the build up of pressure. As the plates continue to move and the pressure builds up, eventually the pressure is great enough to overcome friction and the plate jolts forward releasing the pent up energy in the form of seismic waves. The point at the rocks break apart and shock waves start is known as the focus of the earthquake. The point on the surface directly above the focus is known as the epicentre of the earthquake. For further explanation of how earthquakes occur, see this excellent animation from the BBC.

Measuring Earthquakes

We measure the magnitude (strength) of an earthquake using a seisometer, the results of which are recorded on a seismograph. To see how a seismograph works, watch this excellent animation. The magnitude of the earthquake, reflecting the energy released, is measured on the Richter Scale (from 1-10). This is a logarithmic scale and therefore each point on the scale is 10x greater than the previous one. Therefore an earthquake measuring 8 on the richter scale is 10 times more powerful than an earthquake measuring 7 on the richter scale.

The Effects of Earthquakes

The effects of earthquakes are far ranging and often involve death and destruction. In 1906, a particularly large earthquake, measuring 8.2 on the Richter scale hit San Francisco (California, USA). Whilst at the time around 500 deaths were actually reported, the actual figure is believed to be around 3,000, the largest death toll from a natural disaster to be recorded in California. Although the actual shaking of the ground during the earthquake was incredibly damaging, it is estimated (wikipedia source) that 90% of the destruction was caused by the severe fires (see photograph) which raged following the earthquake, many starting as a consequence of ruptured gas mains.

We can divide the effects of an earthquake into those known as the primary effects and those known as the secondary effects. Primary effects of an earthquake are those resulting directly from the earthquake itself. These include; buildings collapsing; roads cracking; bridges giving way; shattering of glass and injuries / deaths resulting from these. Secondary effects are those that result from the primary effects. For example ground shaking may result in the cracking of gas and water pipes (primary effects) this can result in severe fires due to explosion from escaping gas and difficulties in putting out fires due to lack of water from burst mains (secondary effects). Other secondary effects include, homelessness, business going bankrupt and closing etc.

You need to learn a case study of the causes and consequence of a major earthquake - see post on the 1989 San Francisco earthquake.

Follow up links:
USGS - The Richter Magnitude Scale
National Earthquake Information Centre
Earthquakes - General interest publication from the USGS
Wikipedia - 1906 San Francisco Earthquake
Discover our Earth - Earthquakes
How earthquakes work
Understanding Earthquakes
Earthquakes do occur in Britain - see the British Geological Survey site Earthquakes in the British Isles
Map of recent earthquakes in the UK (BGS)
Earthquake Animation

Key Term Check:
Earthquake - a sudden ground movement
Epicentre - this is the point on the surface directly above the focus of the earthquake
Fault - a weakness in the earth's crust where an earthquake may occur
Focus - this is the point underground where the earthquake starts
Richter Scale - a logarithmic scale used to measure the magnitude of an earthquake
Seismic Waves - waves of energy released in the event of an earthquake
Seismograph - used to measure seismic waves released during an earthquake