Earthquakes

Another consequence of plate movements is earthquakes. Predominantly earthquakes occur at transform boundaries, however earthquake activities are also present at the other boundaries. An earthquake is a sudden violent vibration in the earth’s crust caused by a rapid release of pressure built up as the plates get locked or stuck together. Earthquakes usually trigger movement along a crack or fault in the earth’s crust. Here where the earthquake originates is called the focus. The shaking on the ground is felt by seismic waves that radiates from the focus outwards. The ground immediately above the focus is called the epicentre. This is where the most vibrant shaking is felt and damage on the ground occurs.

How Earthquakes Occur

Earthquakes are a result of a build up of pressure along plate margins. The movement of plates along the boundaries is not smooth, there is friction and the plates become stick or locked to each other as they collide, diverge and slide pass each other. When this occurs, the plates continue to move building up pressure and stored energy. Eventually, the pressure will release and the plates will snap free in a violent, jerking movement releasing the energy in seismic waves which cause the ground vibrations and earthquakes to occur. 

Distribution of Earthquakes

Earthquakes occur daily across the world, however most of these are not strong enough to be felt. Most of these earthquakes are registered along the plate margins in a linear belt form. Earthquakes occur in the areas along the Pacific Ring of Fire from the East Indies and Japan to the Aleutian Islands along the plate margin to Mexico and Nicaragua. Also, earthquakes occur within the West Indies around the Caribbean Plate, particularly the northern areas from the Cayman Is. to Puerto Rico and the Virgin Is. In the eastern hemisphere, earthquakes occur in the Mediterranean Sea to the east of Africa on to Turkey, Iran and Italy. 

Measuring Earthquakes

The scientific study of earthquakes is called Seismology. Seismologists use a Richter scale measures the magnitude (strength and force) of earthquakes. These are measured on a seismometer and shown on a seismograph. On the other hand, the Mercalli scale relates ground movement to effects of the shaking. Therefore, persons can provide information about the strength of the earthquake.

Effects of Earthquake

The destructive effects of an earthquake can be classified into primary and secondary effects.

Primary effects are the immediate damage caused by the quake, such as collapsing buildings, roads and bridges, which may kill many people. Those lucky enough to survive can suffer badly from shock and panic.

Secondary effects are the after-effects of the earthquake, such as fires, tsunami, landslides and disease.

•   Fire - earthquakes destroy gas pipes and electric cables, causing fires to spread. Broken water mains prevent the Fires being extinguished. Fires spread very quickly in cities, especially in poor-quality housing areas where wooden buildings are common.

•   Tsunamis - an earthquake on the sea floor or close to the coast may cause huge waves.

•   Landslides - earthquakes often cause landslides, especially in steep river valleys and areas of weak rocks.

•   Disease and famine - fresh water supplies are often cut off causing typhoid and cholera. Lack of shelter and food causes much suffering.

•   soil liquefaction when soils with a high water content are violently shaken they lose their mechanical strength and behave like a fluid and so buildings can literally sink.

•   Fukushima nuclear power station

Shaking: Seismic waves vibrate the ground causing collapse of building and other structures, as well all trees and other plants. Faulting and Ground Rupture

Fires

Fires, often associated with broken electrical and gas lines, is one of the common side effects of earthquakes. Gas is set free as gas lines are broken and a spark will start bringing "inferno". To complicate things water lines are broken and so there is no water to extinguish the fire. The San Francisco earthquake of 1906 caused 90% of damage by fire.

Tsunamis

For sure, one of the most dangerous effects of an earthquake is a Tsunami. Tsunamis are giant waves that can cause floods and in some cases may reach up to 100 feet in height. These deadly waves strike a great distance from the epicentre. Tsunamis often result from sub-sea faulting of ocean floor sending seismic shocks through the water and creating large waves of low amplitude but of long period, moving at 500-700 mph.

Figure 1 http://www.sms-tsunami-warning.com/pages/earthquake-effects#.WCYE-2ViZ7Y

Landslides and ground subsidence

Avalanches, landslides, rockslides are triggered by ground shaking. These landslides are often more destructive than the earthquakes as they plough through villages and other developments. Often unstable regions of hillsides or mountains fail. In addition landslides can cause problems when they block roads, which makes it difficult for emergency and rescue operations.

Liquefaction of water-laden sediments: A process by which water-saturated sediment temporarily loses strength and acts as a fluid.

Groundwater, sand and soil combine during seismic shaking to form liquefaction during a moderate to powerful earthquake. A quicksand like soil is the result of this process. When liquefaction takes place under buildings the foundations sink and the building collapse. After the earthquake has passed, the soil firms again and the water settles deeper in the ground. Areas with sandy soil and groundwater close to the surface are far more at risk of liquefaction.

Ground Rupture: The movement of the ground along one side of a Fault relative to the other side, caused by an earthquake. When an earthquake event occurs, ground rupture is only where the fault zone moves. Those constructions built adjacent to the fault will survive while structures built across these zones will collapse.