Earthquakes - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
 
BASICS - WHAT CAUSES EARTHQUAKES? 
 
Earthquakes are usually caused when rock underground suddenly breaks 
along a fault. This sudden release of energy causes the seismic waves that 
make the ground shake.  
When two blocks of rock or two plates are rubbing against each other, 
they stick a little. They don't just slide smoothly; the rocks catch on each 
other. The rocks are still pushing against each other, but not moving. After 
a while, the rocks break because of all the pressure that's built up. When 
the rocks break, the earthquake occurs.  
 
TECHNICAL TERMS: 
Where Do Earthquakes Happen? 
Earthquakes occur all the time all over the world, both along plate edges 
and along faults. 
Page 2


 
 
BASICS - WHAT CAUSES EARTHQUAKES? 
 
Earthquakes are usually caused when rock underground suddenly breaks 
along a fault. This sudden release of energy causes the seismic waves that 
make the ground shake.  
When two blocks of rock or two plates are rubbing against each other, 
they stick a little. They don't just slide smoothly; the rocks catch on each 
other. The rocks are still pushing against each other, but not moving. After 
a while, the rocks break because of all the pressure that's built up. When 
the rocks break, the earthquake occurs.  
 
TECHNICAL TERMS: 
Where Do Earthquakes Happen? 
Earthquakes occur all the time all over the world, both along plate edges 
and along faults. 
 
Along Plate Edges 
Most earthquakes occur along the edge of the oceanic and continental 
plates. The earth's crust (the outer layer of the planet) is made up of 
several pieces, called plates. The plates under the oceans are called 
oceanic plates and the rest are continental plates. The plates are moved 
around by the motion of a deeper part of the earth (the mantle) that lies 
underneath the crust. These plates are always bumping into each other, 
pulling away from each other, or past each other. Earthquakes 
usually occur where two plates are running into each other or sliding 
past each other. 
Along Faults 
Earthquakes can also occur far from the edges of plates, along 
faults. Faults are cracks in the earth where sections of a plate (or two 
plates) are moving in different directions. Faults are caused by all 
that bumping and sliding the plates do. They are more common near 
the edges of the plates. 
WHAT IS AN EARTHQUAKE ? 
Earthquake is a result of a sudden release of energy in the earth's crust 
that creates seismic waves that travel in all directions.  
 
NOTE: 
What are Seismic Waves: 
Seismic waves are waves of energy that travel through the Earth's 
layers, and are a result of earthquakes, volcanic eruptions, magma 
movement, large landslides and large man-made explosions that give 
out low-frequency acoustic energy. 
The seismic waves shake the earth as they move through it, and when 
the waves reach the earth’s surface, they shake the ground and 
anything on it, like our houses and us. 
Page 3


 
 
BASICS - WHAT CAUSES EARTHQUAKES? 
 
Earthquakes are usually caused when rock underground suddenly breaks 
along a fault. This sudden release of energy causes the seismic waves that 
make the ground shake.  
When two blocks of rock or two plates are rubbing against each other, 
they stick a little. They don't just slide smoothly; the rocks catch on each 
other. The rocks are still pushing against each other, but not moving. After 
a while, the rocks break because of all the pressure that's built up. When 
the rocks break, the earthquake occurs.  
 
TECHNICAL TERMS: 
Where Do Earthquakes Happen? 
Earthquakes occur all the time all over the world, both along plate edges 
and along faults. 
 
Along Plate Edges 
Most earthquakes occur along the edge of the oceanic and continental 
plates. The earth's crust (the outer layer of the planet) is made up of 
several pieces, called plates. The plates under the oceans are called 
oceanic plates and the rest are continental plates. The plates are moved 
around by the motion of a deeper part of the earth (the mantle) that lies 
underneath the crust. These plates are always bumping into each other, 
pulling away from each other, or past each other. Earthquakes 
usually occur where two plates are running into each other or sliding 
past each other. 
Along Faults 
Earthquakes can also occur far from the edges of plates, along 
faults. Faults are cracks in the earth where sections of a plate (or two 
plates) are moving in different directions. Faults are caused by all 
that bumping and sliding the plates do. They are more common near 
the edges of the plates. 
WHAT IS AN EARTHQUAKE ? 
Earthquake is a result of a sudden release of energy in the earth's crust 
that creates seismic waves that travel in all directions.  
 
NOTE: 
What are Seismic Waves: 
Seismic waves are waves of energy that travel through the Earth's 
layers, and are a result of earthquakes, volcanic eruptions, magma 
movement, large landslides and large man-made explosions that give 
out low-frequency acoustic energy. 
The seismic waves shake the earth as they move through it, and when 
the waves reach the earth’s surface, they shake the ground and 
anything on it, like our houses and us. 
 
PLEASE NOTE: 
The spot underground where the rock breaks is called the focus of the 
earthquake. It is the origin of the earthquake. 
The place right above the focus (on top of the ground) is called the epicenter 
of the earthquake. 
 
ADDITIONAL TERMS: 
AFORSHOCKS 
Foreshocks are earthquakes that precede larger earthquakes in the same 
location.  
MAINSHOCKS  
An earthquake cannot be identified as a foreshock until after a larger 
earthquake in the same area occurs. 
 AFTERSHOCKS 
Aftershocks are earthquakes that follow the largest shock of an earthquake 
sequence. They are smaller than the mainshock and within 1-2 fault 
lengths distance from the mainshock fault.   
Aftershocks can continue over a period of weeks, months, or years. 
 
 
Page 4


 
 
BASICS - WHAT CAUSES EARTHQUAKES? 
 
Earthquakes are usually caused when rock underground suddenly breaks 
along a fault. This sudden release of energy causes the seismic waves that 
make the ground shake.  
When two blocks of rock or two plates are rubbing against each other, 
they stick a little. They don't just slide smoothly; the rocks catch on each 
other. The rocks are still pushing against each other, but not moving. After 
a while, the rocks break because of all the pressure that's built up. When 
the rocks break, the earthquake occurs.  
 
TECHNICAL TERMS: 
Where Do Earthquakes Happen? 
Earthquakes occur all the time all over the world, both along plate edges 
and along faults. 
 
Along Plate Edges 
Most earthquakes occur along the edge of the oceanic and continental 
plates. The earth's crust (the outer layer of the planet) is made up of 
several pieces, called plates. The plates under the oceans are called 
oceanic plates and the rest are continental plates. The plates are moved 
around by the motion of a deeper part of the earth (the mantle) that lies 
underneath the crust. These plates are always bumping into each other, 
pulling away from each other, or past each other. Earthquakes 
usually occur where two plates are running into each other or sliding 
past each other. 
Along Faults 
Earthquakes can also occur far from the edges of plates, along 
faults. Faults are cracks in the earth where sections of a plate (or two 
plates) are moving in different directions. Faults are caused by all 
that bumping and sliding the plates do. They are more common near 
the edges of the plates. 
WHAT IS AN EARTHQUAKE ? 
Earthquake is a result of a sudden release of energy in the earth's crust 
that creates seismic waves that travel in all directions.  
 
NOTE: 
What are Seismic Waves: 
Seismic waves are waves of energy that travel through the Earth's 
layers, and are a result of earthquakes, volcanic eruptions, magma 
movement, large landslides and large man-made explosions that give 
out low-frequency acoustic energy. 
The seismic waves shake the earth as they move through it, and when 
the waves reach the earth’s surface, they shake the ground and 
anything on it, like our houses and us. 
 
PLEASE NOTE: 
The spot underground where the rock breaks is called the focus of the 
earthquake. It is the origin of the earthquake. 
The place right above the focus (on top of the ground) is called the epicenter 
of the earthquake. 
 
ADDITIONAL TERMS: 
AFORSHOCKS 
Foreshocks are earthquakes that precede larger earthquakes in the same 
location.  
MAINSHOCKS  
An earthquake cannot be identified as a foreshock until after a larger 
earthquake in the same area occurs. 
 AFTERSHOCKS 
Aftershocks are earthquakes that follow the largest shock of an earthquake 
sequence. They are smaller than the mainshock and within 1-2 fault 
lengths distance from the mainshock fault.   
Aftershocks can continue over a period of weeks, months, or years. 
 
 
 
 
HOW IS AN EARTHQUAKE MEASURED? 
1) RICHTER SCALE 
 
The Richter scale is used to rate the magnitude of an earthquake -- the 
amount of energy it released. It describes the earthquake's magnitude by 
measuring the seismic waves that cause the earthquake.  
 
 
NOTE: Seismic Waves are read by Seismograph.  
 
2)  MERCALLI SCALE 
 Mercalli scale describes the intensity of an earthquake based on its 
observed effects. 
NOTE: 
The two scales have different applications and measurement techniques. 
 
 
 
Page 5


 
 
BASICS - WHAT CAUSES EARTHQUAKES? 
 
Earthquakes are usually caused when rock underground suddenly breaks 
along a fault. This sudden release of energy causes the seismic waves that 
make the ground shake.  
When two blocks of rock or two plates are rubbing against each other, 
they stick a little. They don't just slide smoothly; the rocks catch on each 
other. The rocks are still pushing against each other, but not moving. After 
a while, the rocks break because of all the pressure that's built up. When 
the rocks break, the earthquake occurs.  
 
TECHNICAL TERMS: 
Where Do Earthquakes Happen? 
Earthquakes occur all the time all over the world, both along plate edges 
and along faults. 
 
Along Plate Edges 
Most earthquakes occur along the edge of the oceanic and continental 
plates. The earth's crust (the outer layer of the planet) is made up of 
several pieces, called plates. The plates under the oceans are called 
oceanic plates and the rest are continental plates. The plates are moved 
around by the motion of a deeper part of the earth (the mantle) that lies 
underneath the crust. These plates are always bumping into each other, 
pulling away from each other, or past each other. Earthquakes 
usually occur where two plates are running into each other or sliding 
past each other. 
Along Faults 
Earthquakes can also occur far from the edges of plates, along 
faults. Faults are cracks in the earth where sections of a plate (or two 
plates) are moving in different directions. Faults are caused by all 
that bumping and sliding the plates do. They are more common near 
the edges of the plates. 
WHAT IS AN EARTHQUAKE ? 
Earthquake is a result of a sudden release of energy in the earth's crust 
that creates seismic waves that travel in all directions.  
 
NOTE: 
What are Seismic Waves: 
Seismic waves are waves of energy that travel through the Earth's 
layers, and are a result of earthquakes, volcanic eruptions, magma 
movement, large landslides and large man-made explosions that give 
out low-frequency acoustic energy. 
The seismic waves shake the earth as they move through it, and when 
the waves reach the earth’s surface, they shake the ground and 
anything on it, like our houses and us. 
 
PLEASE NOTE: 
The spot underground where the rock breaks is called the focus of the 
earthquake. It is the origin of the earthquake. 
The place right above the focus (on top of the ground) is called the epicenter 
of the earthquake. 
 
ADDITIONAL TERMS: 
AFORSHOCKS 
Foreshocks are earthquakes that precede larger earthquakes in the same 
location.  
MAINSHOCKS  
An earthquake cannot be identified as a foreshock until after a larger 
earthquake in the same area occurs. 
 AFTERSHOCKS 
Aftershocks are earthquakes that follow the largest shock of an earthquake 
sequence. They are smaller than the mainshock and within 1-2 fault 
lengths distance from the mainshock fault.   
Aftershocks can continue over a period of weeks, months, or years. 
 
 
 
 
HOW IS AN EARTHQUAKE MEASURED? 
1) RICHTER SCALE 
 
The Richter scale is used to rate the magnitude of an earthquake -- the 
amount of energy it released. It describes the earthquake's magnitude by 
measuring the seismic waves that cause the earthquake.  
 
 
NOTE: Seismic Waves are read by Seismograph.  
 
2)  MERCALLI SCALE 
 Mercalli scale describes the intensity of an earthquake based on its 
observed effects. 
NOTE: 
The two scales have different applications and measurement techniques. 
 
 
 
 
 
EARTHQUAKE WAVES 
All natural earthquakes take place in the lithosphere. 
Earthquake waves are basically of two types :  
1) Body Waves  
2) Surface Waves 
Body waves are generated due to the release of energy at the focus and 
move in all directions travelling through the body of the earth. Hence, 
the name body waves. 
The body waves interact with the surface rocks and generate new set of 
waves called surface waves. These waves move along the surface. 
NOTE: 
The velocity of waves changes as they travel through materials with 
different elasticity. The more elastic the material is, the higher is the 
velocity.  Their direction also changes as they reflect or refract when 
coming across materials with different densities. Hence ; the velocity and 
direction of the earthquake waves undergo changes when the medium 
through which they are travelling changes. 
TYPES OF BODY WAVES: 
There are two types of body waves :  
1) P  Waves 
2) S-waves 
The earthquake waves are measured with the help of a seismograph and are of 
three types —