An earthquake is the movement of large rock bodies past each other, and the fracturing of rock as stress is released. This is often along a fault, which could be as small as a few centimetres, or as large as a tectonic plate boundary.
The site of the initial rupture and therefore the earthquake is called the FOCUS or the HYPOCENTRE. This will be at some depth beneath the surface, and is also called the HYPOCENTRE. Many earthquakes begin in an area called the BENIOFF ZONE.
The point on the surface immediately above the focus, and therefore where the power of the earthquake is felt most strongly is called the EPICENTRE ( so this is NOT where the earthquake starts...)
Waves are started off by the tension and compression of the rocks. The shock waves produced are seismic waves, and are measured on a seismograph. Three stations will allow the triangulation of the starting point. The 2 main types of waves are:
P Waves (compressional) - these move at speeds of 6-8 kilometres per second. The wave form is parallel to the direction of movement.
S Waves (shear) - these move more slowly (4-5 kilometres per second). Wave form is perpendicular to direction of movement. Don't pass through liquids.
Force of an earthquake can be measured in different ways:
1. The area of rock displaced by the earthquake
2. The extent to which the surface is displaced
3. Duration of shaking
4. Intensity scales e.g Mercalli scale - based on Human perception. Not all earthquakes can be felt by people.
5. Magnitude scales: these are based on the energy released by the earthquake.
The Richter Scale is the best known of these, and is based on the energy that is received at a distance of 100km from the epicentre of a particular quake. The largest earthquake ever recorded is 8.9, and this could be close to the actual maximum possible due to the strength of earthquake. One of its features is that it is logarithmic: each point up the scale represents a 10 times increase in the shaking (and 30 times the energy released.)
Plate Tectonics tells us that the distribution of earthquakes is not random.
It is often said that animals can sense an earthquake better than people: the natural signs that something may be about to happen are called PRECURSORS and are well documented. The KOBE earthquake apparently triggered strange behaviour in animals. There was an Equinox documentary which looked at some individuals who claimed to have the ability to foresee earth movements in advance.