Ncert class 11 geography chapter 3 || Earthquake

 

How does an earthquake happen?

In simple terms, an earthquake is the shaking of the earth.

 When crustal rock breaks by a sharp cut. It is called fault. The tendency of this crustal rock move opposite direction along the fault. But the overlaying rock strata press them, and friction also locks them together. At some point, the tendency to move opposite direction overcame the overlaying pressure and friction lock. And finally, they move apart. This time energy is released and travels in all directions.

 Where the energy is release is called the focus of the earthquake. Also called as Hypocenter.

This energy travels in all directions and reaches in surface. The point of surface, where the energy first reaches called the Epicenter. It is directly above of Hypocenter.

 

                        

Different types of earthquake waves

All-natural earthquake occurs in the Lithosphere.

A seismograph is an instrument, records earthquake waves that reach the surface.

Earthquake waves are broadly 2 types. Body waves and surface waves. Body weaves occur from the focus of the earthquake. It travels in all directions of the body of the earth. When body weaves reach the surface, a new type of weave is created that travels along the surface called surface weaves.

When waves travel, the velocity of the wave changes according to the different types of density of the material. A higher density of material leads higher velocity. Direction also changes according to density as it reflects and refracts (Here refract means, weaves move different direction).

There are 2 types of body waves: 1) P Waves, 2) S waves.

P Waves

    These waves first reach the surface.

Ø         Also called Primary waves.

Ø         It is similar to sound waves.

Ø         It travels through solid, liquid, and gaseous.

Ø         P waves vibrate parallel to the direction. It creates a stretching and squeezing effect. This led to a density difference in the material.

S Waves

Ø       These waves reach some time after P waves.

Ø       Also called secondary waves.

Ø       S waves travel only through solid material.

Ø      This characteristic helps scientists to understand of interior structure of the earth.

Ø      S waves vibrate perpendicular to the direction. By which they create troughs and crests in the material.

Surface waves

Ø      It is more destructive waves. It is the main cause of structure collapse and, displacement of rocks.

                           

What is the Shadow Zone

Earthquake waves recorded by seismographs. But some specific areas where the waves were not reported. These types of zones are called shadow zones. Within 150° from the epicenter, P and S waves are reported. But from 150° to 145°, P and S waves are not reported, and beyond 145° again P waves are reported but S waves are not reported.

The shadow zone for P waves is between 150° to 145° from the epicenter. And for S waves, the shadow zone is beyond 150° from the epicenter. The shadow zone of S waves is larger than P waves.

 

Source - Fundamentals of Physical Geography, chapter 3 

   

          

Source - Fundamentals of Physical Geography, chapter 3 

    

                                  

Different types of earthquakes

·        Tectonic earthquake – Rocks sliding along a fault.

·        Volcanic earthquake – By volcanic activity.

·        Collapse earthquake – In underground mining areas, sometimes roofs collapse and create minor tremors.

·        Explosion earthquake – When nuclear or chemical devices explode.

·        Reservoir-induced earthquake – Occurs due to large reservoirs.

 

Earthquake measurement

Earthquakes are scaled based on magnitude or intensity.

 Magnitude measures the amount of energy released during earthquakes. The name of the magnitude measurement scale is Richter scale. The absolute number on this scale is 0- 10.

Intensity is the degree of shaking and damage in a specific location. The name of the intensity measurement scale is Mercalli (named after Mercalli, an Italian seismologist). The absolute number on this scale is from 1 – 12.

 

Effects of an earthquake

1.      Ground shaking

2.      Differential ground settlement

3.      Land and mud slides

4.      Soil liquefaction

5.      Ground lurching

6.      Avalanches

7.      Ground displacement

8.      Floods from dam and levee failures

9.      Fires

10.   Structural collapse

11.   Falling objects

12.   Tsunami

 Special Notes - when the epicenter is below the oceanic water and the magnitude is so high, then a Tsunami occurs. Earthquake is a natural hazard and are also created by manmade. High magnitude earthquakes like 8+ are quite rare events, whereas tiny earthquakes occur almost every minute.   

 

   Go for Google search as "earthquake zone of India". you will see different earthquake-prone areas.