Plate Tectonics- Seismicity

Causes of earthquakes

The movement of the earth’s plates due to the rising and falling convection currents in the asthenosphere, means there is a slow build up of pressure in the rocks. When this pressure is released, parts of the surface experience intense shaking for a few seconds, this is called an earthquake.

The point at which the pressure is released is called the focus, and the point immediately above the focus on the surface is called the epicentre. The seismic shock waves have their highest level of energy at the focus; energy decreases as the waves spread outwards.

The depth of the earthquake is significant:

• Shallow 0-70kmkm
• Intermediate 70-300km
• Deep 300-700km

Seismic waves

Seismic waves travel out from the focus; there are three types of waves:

P Waves- (primary)

• Are the fastest and shake the Earth backwards and forwards.
• These travel the fastest and move through solids and liquids.
• These travel in all directions from the focus, like a ripple

S- Waves- (Secondary)

• Slower 
• Move with a sideways motion
• Shaking the Earth at right angles to the direction of travel. 
• They cannot move through liquids but are more damaging.

L & R Waves- (Surface Waves)

• These travel near to the surface 
• Slower than the two mentioned already & they’re more destructive than either.
• They include L waves (long waves) 
• L waves cause the ground to move sideways
• Raleigh waves which make it move up and down. 

Distribution

Earthquakes usually happen around plate boundaries, the most destructive occur at destructive faults especially if there is a subduction zone, also at transform faults such as the San Andreas Fault.

Earthquakes can also happen away from plate boundaries and are associated with the reactivation of old fault lines, such as the 2002 earthquake in Dudley, 4.8 on the Richer scale, along the old fault line known as the Malvern lineament.

Human activity is also thought to trigger earthquakes examples include subsidence of deep mine workings and extreme building.

Measuring earthquakes

There is two ways of measuring earthquakes:

Richter Scale

• Logarithmic scale
• Each point represents a ten-fold increase in the amount of energy involved
• Measured using a seismograph 
• Scale of 1-10

Mercalli Scale

• Measures the intensity of the event
• Impacts
• Scale 1-12

The Richter Scale and the Mercalli Scale don’t match up exactly, and event 12 on the Mercalli Scale, is only a 8.5 on the Richter Scale.