Sometimes, earthquakes occur as a result of human actions such as when atomic explosives are detonated underground or when new reservoirs are filled. These artificial earthquakes are rare and relatively harmless when compared with the earthquakes caused by nature One such series of earthquakes occurred in the early 1960s near Denver, Colorado, when waste fluids were pumped into deep wells just east of the city. When the pumping stopped, so did the earthquakes. The most common damage and the greater amount of injuries caused by earthquakes is a result of falling objects. Much of the world's population live in densely populated urban areas which make them particularly vulnerable to building collapse and being struck by falling objects.
Earthquakes can also topple bridges, towers and dams as well as trigger large landslides.
Each of these present unique problems and can result in floods and fires which may be more devastating than the direct effects of the earthquake. Besides the trembling and shaking of the earth caused by earthquakes, other consequences can result in tremendous destruction. When shock waves from an earthquake strike some types of soil, they result in a condition called liquefaction . Liquefaction occurs when these soils lose their ability to bear weight and behave much like quicksand. Buildings which have been constructed on these soils quickly topple and may even be swallowed up.