Earthquakes are a shifting of rock beneath the earth's crust. Earthquakes can be horrific natural disasters that kill hundreds of thousands of people. The primary earthquake hazards cause secondary hazards that kill people. These are aftershocks, buildings and bridges collapsing, fires, flooding, rupturing dams and tsunamis. Earthquakes cause three primary hazards.
Shifting of the Land Surface
Earthquakes occur along fault zones in the Earth that may experience relative motion. Rocks that slide against each other in a fault accumulate energy for centuries and release it almost instantaneously. This energy radiates out as a series of waves, causing an earthquake. This ground breaking and shifting of the land's surface is a primary hazard of earthquakes.
Release of Seismic Energy
Ground shaking is caused by a release of seismic energy, which are waves of energy caused by the sudden breaking of rock within the Earth in the earthquake's epicentre. Ground shaking is a primary hazard of earthquakes. The damage the shaking causes is measured according to its intensity and magnitude on the Richter scale. The largest earthquake recorded had a magnitude of 8.9. Grand shaking causes a multitude of secondary earthquake hazards.
Liquefaction occurs in water-saturated, unconsolidated sediment. The ground shaking causes soil grains to lose grain-to-grain contact and the soil tends to flow. Liquefaction is a primary hazard of earthquakes. It produces secondary hazards since it can be damaging to bridges and buildings that lose the ground's support.