Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.
Different Types of Earthquakes are:-
- A tectonic earthquake is one that occurs when the earth’s crust breaks due to geological forces on rocks and adjoining plates that cause physical and chemical changes.
- A volcanic earthquake is any earthquake that results from tectonic forces which occur in conjunction with volcanic activity.
- A collapse earthquake are small earthquakes in underground caverns and mines that are caused by seismic waves produced from the explosion of rock on the surface.
- An explosion earthquake is an earthquake that is the result of the detonation of a nuclear and/or chemical device.
Distribution of earthquakes:-
The distribution and frequency of earthquakes is referred to as seismicity. Most earthquakes occur along relatively narrow belts that coincide with plate boundaries .
This makes sense, since plate boundaries are zones along which lithospheric plates mover relative to one another. Earthquakes along these zones can be divided into shallow focus earthquakes that have focal depths less than about 70 km and deep focus earthquakes that have focal depths between 75 and 700 km.
Major effects /Hazards of Earthquakes are:-
- Collapsing buildings, walls, bridges, falling furniture or objects, shattering glass windows and mirrors. Debris from collapsing structures is one of the principal dangers during an earthquake since the impact of large, heavy objects can be fatal to human beings. 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.
Falling electricity lines. 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 can cause widespread 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.
- Rock slides and/or landslides on mountains and hillsides. During an earthquake, large rocks and portions of earth high up in the hills can become dislopdged and rapidly roll or slide down into the valleys.
Floods caused by the collapse of dam walls. Earthquakes can cause dam walls to crack and eventually collapse, sending raging waters into surrounding areas and causing severe flooding.
- Tsunamis. A tsunami is a large sea wave or series of waves that can be generated by an earthquake. Large tsunamis can completely devastate low-lying coastal areas.
- Liquefaction. 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 development.