Seismicity

Causes of Earthquakes :

As the crust of the earth us mobile, there tends to be a slow build up of stress within the rock. When this pressure is suddenly released parts if the surafec expirence intense shaking motions that lasts for just a few secounds. This is known as an earthquake. The point at which this pressure release occurs is called the focus, and the point immediatley above that on the earths surface is called the epicentre. The depth of the focus is signifcant and three broad categories of earthquake are recognised.

.Shallow- focus (0-70 km deep) these tend to cause the greatest damage and account for 75% of all the earthquake energy released.

.Intermediate focus (70-300 km deep)

.Deep focus (300 700 km deep )

The seismic waves which are created radiate from the focus and there are three types of these waves P waves S waves and L waves

Primary (P) waves - waves trave fastest and are compressional vibrating in the direction in which they are travelling.

Secondary (S) waves - travel at half the speed of P waves and shear rock by vibrating a right angles to the direction of travel.

Surface (L) waves travel slowest and near to the ground surface. Some surface waves shake the gorund at right angles to the direction of wave movment and some have a rolling motion that produces vertical ground movement.

P and S waves travel through the interior of the core and are recorded on a seismograph. studying earthquakes and the seismic waves they generate has made it possible to build up a picture of the interior of the earth.

Distribution

The vast majority of earthquakes occur along plate boundaries, the most powerful being associated with destructive margins ( LEARN PLATE BOUNDARIES!!). At conservative margins the boundary is marked by a fault, movement along which produces the earthquake. The most famous of these is the San Andreas fault in California which represents the boundary between the North American and Pacific Plates. In reality, the San Andreas system consists of a borad complex zone in which there are a number of fractures of the crust. Some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines. An examole is the event that occured on 23 of September 2002 in the Uk Midland. This earthquakes measured 4.8 on the richter scale, and the epicentre was located in Dudley, west of Birmingham. It is believed that the cause was movement along and old fault line known as the Malvern lineament. It has been suggested that human activity could be a cause for some minor earthquakes. Examples are building of large reservoirs in which the water puts pressure on the surface rocks or subsidence of deep mine workings.

Magnitude and Frequency. 

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