GEOGRAPHY

EARTHQUAKE

An earthquake is a sudden earth movement or vibration in the crust.The scientific study and interpretation of earthquakes is called seismology.                                                                                               

CAUSES OF EARTHQUAKES                                                                                                              

 1. Movement or sliding of tectonic plate, when tectonic plates meet they cause stress and fracture of the crustal rocks.This sudden action causes release of energy which generates shock waves which shake the earth.

2. Volcanic eruption or movement of magma below or on the earth’s surface, where high pressure in the crust pushes out magma strongly and causes vibration.                                           

3. Locally earthquakes may be caused by mass wasting, falling of meteors, from the space and human activities such as quarrying by using explosives and movement of big animals 

 

NATURE OF AN EARTHQUAKE

The point at which an earthquake originates is called focus or origin and sometimes it is several kilometers below the surface. The point on the earth’s crust immediately above focus is called epicenter.  

                                                               

SEISMIC WAVES                                             

Wave generated by earthquake is referred to as seismic waves. There are two seismic waves, these are:                                                              
1.Body wave; These are the waves that travel within the crust, they are of two types                           

  a) Primary wave (P) they are very fast and they can travel far from the interior as they are very strong. Primary waves cause the crust to move back and forth in the direction of wave

b) Secondary wave (S) they cause the crustal rock to move from side that is at a right angle to the direction of the wave movement              

 2. Surface wave;These waves travel through the surface rock and they are two types.                          

a) Rayleigh wave (R) these causes the surface rocks to have vertical circular movements very similar to that of water in the sea or ocean wave                                                                  

b) Love wave (L) These cause the surface rock to move from side to side at right angle to the direction of wave movement.

Would distribution of Earthquakes                                                                                                           
  1.Earthquakes are concentrated in area of ocean tranches, volcanic arch, young fold mountains and along the rift valleys as they are areas of crustal instability.                                                           

2. Earthquakes also are concentrated where tectonic plates collide or zones and along the zone of divergence.  

                                    

      EFFECTS OF EARTHQUAKE                                                                                                         

1. Earthquakes can cause fires by damaging electrical power or gas lines.  For examples, more death in 1906 San Francisco earthquake were caused by fire than by the earthquake itself.

2. Earthquakes, along with severe storms, volcanic activity, and coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard.

3. Shaking and ground rupture are the main effect created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. Ground rapture is a major risk for large engineering structures such as dams, bridges and nuclear power station.

   4. Earthquakes may cause flooding. Flood occur usually when the volume of water within a body of water, such as river or lake. However, floods may be secondary effects of earthquakes, if dams are damaged.Earthquakes may cause landslip to dammed rivers, which then collapse and cause flood.

5. Earthquakes may lead to loss of life due to disease, lack of basic necessities, loss of life, general property damage, road and bridge damage and collapse or destabilization of buildings.                        Earthquakes can also precede volcanic eruptions, which cause further problems. Examples, substantial crop damage.                                                                                                                           6. Earthquake can cause tsunami. These large waves or submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across Open Ocean and wreak destruction on far shores hours after the earth quick that generate them                                    

   7.Another effect is soil liquefaction is another effect which occurs because of the shaking of water saturated granular material such as sand making it temporarily lose its strength and transform from a solid to liquid.Examples In the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground eventually collapsing upon them selves