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Occur because of a slow build up of pressure in the earth's rocks, which is quickly released. This then travels to the earth's surface where its impact is dependent on several factors such as:
- The focus of the quake.
- The strength of the shock.
- The time of day.
- Building quality.
- Ground type.
- Level of development.
- Population density of area affected.
- Distance of epicentre from population.
- Land use.
These are hazards due directly to the earthquake itself, and consist of ground movement and shaking. Earthquakes emit body waves (travel through the earth) and surface waves. It is surface waves that cause most damage as they impact on what is on the surface of the earth. They can cause buildings to collapse and underground pipelines to be broken. Damage caused by surface waves varies. For example, the Mexico City quake of 1985. Its impact was increased four fold as the city is built on ancient lake sediments.
Soil liquefaction: Solid material changed into a liquid state. Damages building foundations, resulting in them sinking.
Landslides: Often as a result of the ground shaking, even if a slope is gentle. Cause burial of people and overrun buildings.
Tsunami (tidal waves): If the focus of the quake is beneath the sea, tsunami can occur. Ninety percent occur in the Pacific basin. The more movement of the sea floor and the shallower the focus the larger the wave that is created.