The release of stress that has built up within the earth's crust caused by tension, compression or the shearing of rocks
Earthquake focus
The location where the stress is suddenly released
Epicentre
The point on the earth's surface immediately above the earthquake focus
Seismicity
The earth shaking, strongly linked to tectonic theory
5% of earthquakes do not happen near plate boundaries
Most earthquakes are usually unfelt
Seismic shockwaves
1. Primary/pressure (P) waves
2. Secondary (S) waves
3. Surface (L) waves
4. Rayleigh (R) waves
Primary/pressure (P) waves
The fastest waves, reach the surfacefirst, very loud, travel through both the mantle and the core, compressional waves that vibrate in the direction in which they travel (longitudinal)
Secondary (S) waves
Half the speed of P waves, travel through the mantle but not the core, responsible for the shaking of the ground in an earthquake, vibrate at right angles to the direction in which they travel (transverse), cannot pass through liquid
Surface (L) waves
The slowest waves, cause the least damage as they are not very strong, travel through the outercrust only
Rayleigh (R) waves
Move in a rolling motion, very low frequency, rotate from the epicentre
Depth of focus
Shallow focus (surface down to about 70km)
Deep focus (70-700km deep)
Shallow focus earthquakes
Occur in cold, brittle rock resulting from the fracturing of rock due to stress within the crust
Very common, with many releasing only low levels of energy, although other high-energy shallow quakes are capable of causing severe impacts – less distance over which energy can dissipate
Deep focus earthquakes
Poorly understood
With increasing depth pressure and temperatures increase to very high levels
Minerals change type and volume, which may contribute to a release of energy