Geo-graphy notes for grade 12 on Earthquakes

 

Earth’s internal processes

Earth Quakes

Earth quake is defined as a violent tremor in the earth’s crust which sends out series of vibrating waves in all directions.

Explanation of terms

i.                 Origin: point at which begins. It is also called the focus.

ii.               Tremor:  this is a vibration within the earth’s crust produced by gentle waves. When this vibration occurs on ocean floor, they produce tsunami (gigantic water wave up to 10m high).

iii.             Epicenter: the is the region immediately above the focus where the shock wave first hits the earth surface.

iv.              shock wave: there are two types:

a.     body wave: it travels through the earth crust and are two types:

-        Primary wave: it causes crustal rocks to move backward and forward in the direction of the wave movement.

-        Secondary wave causes crustal rocks to move from side to side at right angles in to the direction of the wave.

b.     Surface wave: this travel through surface rocks. They are of two types:

-        Long wave: it causes surface rocks to move from side to side at right angle to the wave movement

-        Rayleigh wave: this causes surface rocks to have a vertical circular movement  

 

Causes and regions of earth quakes

Causes:

a.      The development of faults (cracks) in the crust which results from the collision between tectonic plates

b.     The sudden release of stress which has slowly built up along the plane.

Regions of Earthquake

This is often concentrated in belts marking boundaries of plates which includes the ocean ridge, the ocean deeps and volcanic island and regions of crystal depression.

Some earthquakes in the past

-        The great Lisbon Earthquake 1755 so disastrous

-        Yokohoma and Tokyo Japan September,1^st 1923

-        San Francisco 1906

-        Acra Ghana Earth tremor 1939 etc.  

Effects of earth’s quakes

a.     displacement of earth’s crust,

c.     rising and lowering of coastal rocks, rising and lowering

d.     of ocean’s floor, landslides and cracks.

e.      Destruction of lives and properties

f.      Destruction of roads, building, telecommunication system etc.