The crust is divided into several plates, which are set in motion by convention currents within the mantle moving in different directions. Two plates are unable to move against each other smoothly therefore friction mounts. After a while the rocks are incapable of containing the strain so the gradual build up of pressure between these crustal rocks causes the ground to rumble. The outcome is normally a sudden jerking movement on the surface that results from the sudden release of seismic energy along a fault boundary. The point at which the release in pressure occurs within the crust is known as the focus, with the epicentre being its counterpart on the surface. Alas, since the shaking rarely lasts more than a minute, it is not the immediate effects that spawn the tragic stories of destruction we hear about on the news. Instead it is the secondary effects of fires, transport disruption, lack of resources and disease that inflict such horror.
The strength of an earthquake is measured on the Richter scale. In order to cover a huge range of earthquake strengths, the magnitude of the scale is logarithmic with each unit representing a tenfold increase in severity and a 30-fold increase in energy.
Despite a recent flurry of interest in earthquake monitoring and prediction, very earth civilisations were aware that the earth’s crust was not a rigid and immobile layer. The Minoan civilisation – the first major European settlers – constructed buildings that withstood a series of major tremors at their base in Crete. Amongst these was the Royal Palace at Knossos, which experienced several earthquake strikes. It is assumed that the civilisation was ultimately wiped out by the effects of a volcanic eruption on the nearby island of Thero (Santorini). Since then, the study of earthquake science has enabled geologists to determine the structure of the earth. Having discovered the Mohoroviic or ‘Moho’ discontinuity (the junction between the crust and the mantle) they were able to notice how shock waves travelled at different speeds relating a change in state. Essentially, waves slowed down when they reached the liquid mantle, but began to travel faster through rigid crustal rock.
Below is an interactive video that depicts how earthquakes occur:
