Causes of earthquakes
As the crust of the Earth is mobile, there tneds to be a slow build up of stress within the rocks. When this pressure is suddenly released parts of the surface experience an intense shaking motion that lasts for just a few seconds, This is an earthquake. The point at which this pressure is release occurs within the crust os knwon as the forcus, and the point immediately above that on the Earth’s surface is called the epicentre. The depth of the focs is significant and three broad categories of earthquake are recognised:
Shallow-focus – (0-70km deep): these tend to cause the greatest damage and account for 75% of all the earthquake energy released.
Intermediate focus – (70-300km deep)
Deep focus – (300-700km deep)
•Seismic waves radiate from the focus rather like the ripples in water when a rock is thrown into a pond
Three types of seismic waves
•Primary (P) waves:
–Vibrate in the direction in which they travelling
•Secondary (S) waves:
–Half the speed of P waves
–Shear rock by vibrating at right angles to the direction of travel
•Surface (L) waves:
–Near to the ground surface
–Some surface waves shake the ground at right angles to the direction of wave movement
–Some have a rolling motion that produces vertical ground movement
–P (All)&S (just mantle) waves travel through the interior of the Earth and are recorded on a seismograph
The vast majority of earthquakes occur along plate bounadries, the most powerful being associated with destructive margins. At conservative margins, the boundary is marked by a fault, movement along which produces the earthquake. Perhaps the most famous of these is the San Andreasfauly in California which represents the boundary between the North American and Pacific plates. In reality, the San Andreas system consists of a broad complex zone in which there are a number of fractures of the crust.
Earthquakes occur at all four plate margins:
· destructive margins – when one plate tries to sink below the other. The pressure resulting from the sinking of the subducting plate and its subsequent melting can trigger strong earthquakes as this pressure is periodically released.
· collision zones – where the plates are pushing together. Strong earthquakes
· constructive margins – when both plates are moving apart. These earthquakes tend to be less severe than those at destructive plate margins. The friction and pressure
caused by the plates moving apart is less intense than at destructive plate margins.
· conservative margins – when plates moving alongside each other get stuck. These are very strong earthquakes. Here, where the plates slide past each other, the plates
tend to stick for periods of time. This causes stresses and pressure to build. The release of the pressure occurs in a sudden, quick release of the plates and the result is an earthquake
Magnitude and Frequency