An earthquake in itself, is not dangerous, however, the effects of that earthquake on the surrounding environment such as buildings and other manmade structures can be catastrophic. Some of the earthquake effects that can be harmful to people are:
Collapsing buildings, walls, bridges, falling furniture or objects and shattering glass. Debris from collapsing structures is one of the primary dangers during an earthquake since the impact of large, heavy objects can be fatal. Earthquakes sometimes cause glass windows and mirrors to shatter and this is also quite dangerous. Earthquake aftershocks can result in the complete collapse of buildings that were damaged during an earthquake.
During an earthquake, large rocks and portions of earth high up in the hills can become dislodged and rapidly roll or slide down into the valleys as a rock or land slide. Slopes which are already unstable may suddenly collapse leading to injury and death.
Earthquakes can cause electricity poles to fall and live wires to become exposed or to start fires. Ruptured gas lines and spillage of flammable substances. Earthquake-generated fires are one of the most dangerous secondary hazards. These can cause further destruction after a major earthquake. Escaping gas from broken gas lines and the toppling of containers with flammable substances (e.g. kerosene, household chemicals, etc.) present a significant threat of explosions and fires, which can cause death and destruction of property. Additionally, water pipes are sometimes ruptured during an earthquake and this compounds the problem of controlling such fires.
These may be caused by the collapse of dam walls. Earthquakes can cause dam walls and other crucial hydrological infrastructure such as levees to crack and eventually collapse, sending raging waters into surrounding areas and causing severe flooding.
When sediments with a high water content are subjected to prolonged shaking, the pressure of the water held in pores in the sediment graudally increases eventually, the sediments lose all cohesive strength and begin to behave as if they were liquids. Building and other structures sink into the ground or overturn and buried tanks and other cavities rise to the surface. This is known as liquefaction. Liquefaction occurred during the earthquake of 1692 in Jamaica and was responsible for the destruction of the town of Port Royal. Over the past few decades, many parts of the Eastern Caribbean have become increasingly vulnerable to liquefaction because of the increased use of reclaimed land for urban and coastal development.
A tsunami is a large ocean wave or series of waves that can be generated by a large underwater earthquake. Large tsunamis can completely devastate low-lying coastal areas.