Fill in the blanks
Most earthquakes occur on account of tectonic plate movements.
P (Primary) waves are the first earthquake waves to be recorded on a seismograph of an earthquake.
The last of the earthquake waves to arrive on the surface of the earth are surface waves.
The giant sea waves caused by earthquakes in the oceans are called Tsunamis.
About 70 per cent of the earthquakes occur in the Circum Pacific Mountain Belt.
Short Answer Questions
What is an earthquake? Give one recent example of an earthquake.
An earthquake is defined as a tremor below the surface of the earth which causes shaking of the crust. For example, an earthquake of 7.5 magnitude struck Sulawesi Province in Indonesia in 2018.
State two natural causes of an earthquake.
Two natural causes of an earthquake are-
- Plate Tectonics
Name two man-made causes of an earthquake. Give one example.
Two man-made causes of an earthquake are-
- Construction of huge dams
- Nuclear explosions
For example, Koyna earthquake in Maharashtra was caused by construction of Koyna dam in a seismically active region.
What are known as seismic focus and epicentre with respect to an earthquake?
The point of origin of seismic waves is called seismic focus. The point on the earth's surface directly above the seismic focus is called epicentre.
What is known as Richter scale? State its uses.
Richter scale is an instrument that is used to measure the absolute intensity of an earthquake with mathematical precision. It measures the power of an earthquake on a scale of 1 to 9.
How are earthquakes useful?
Earthquakes are useful for the following reasons-
1. Energy release — They help the earth to release its stored up energy which helps to keep the earth in good shape.
2. Landforms — Earthquakes help in the formation of various landforms.
Give any two destructive effects of earthquakes.
Two destructive effects of earthquakes are-
1. Collapse of structures — Due to earthquakes, houses and other structures often collapse causing great loss of life and property.
2. Submergence — The forces of uplift and subsidence cause submergence of coastal parts.
What is a Tsunami? Give one recent example of a place struck by a Tsunami.
A Tsunami is a large destructive ocean wave that can be generated by any disturbance that rapidly displaces a large mass of water.
For example, a Tsunami struck Java and Sumatra in Indonesia on December 22, 2018.
How do Japanese predict earthquakes?
Japanese use the methods of measuring changes in sea level and variations in Earth's magnetic field to predict earthquakes.
Name the two belts where most of the earthquakes occur.
Two belts where most of the earthquakes occur are-
- The Circum Pacific Mountain Belt
- The Midworld Mountain Belt
Match the following:
|Column A||Column B|
|Tsunami||The waves that make the inhabitants feel the ground motion|
|Seismograph||The instrument for measuring the intensity of an earthquake.|
|S-waves||Harbour waves generated by oceanic earthquakes|
|Richter scale||The point on the earth surface directly above the seismic focus.|
|Epicentre||An instrument for recording the movement of earthquake waves.|
|Column A||Column B|
|Tsunami||Harbour waves generated by oceanic earthquakes|
|Seismograph||An instrument for recording the movement of earthquake waves.|
|S-waves||The waves that make the inhabitants feel the ground motion|
|Richter scale||The instrument for measuring the intensity of an earthquake.|
|Epicentre||The point on the earth surface directly above the seismic focus.|
Describe how earthquakes are caused on the surface of the earth.
Earthquakes are caused by the strain in the earth's crust which in turn is a result of a number of factors. When the earth’s crust is unable to accommodate itself to the strain, it results in a sudden release of energy in the form of a sudden violent shock or an earthquake.
Earthquakes are caused by severe tectonic movement of plates, volcanic eruption forces, folding and faulting and man-made causes.
What is the difference between seismic focus and epicentre?
|The point of origin of seismic waves is called seismic focus.||The point on the earth's surface directly above the seismic focus is called epicentre.|
Give a geographical reason for each of the following:
(i) Most earthquakes occur on account of plate movements.
(ii) Primary waves are the first earthquake waves to be recorded on a seismogram.
(iii) Volcanic activity and earthquakes occur in the same belt.
(i) Most earthquakes occur on account of plate movements because when two plates slip past each other or collide against each other, their edges produce faults along the lines of weakness.
(ii) Primary waves are the first earthquake waves to be recorded on a seismogram because they travel at the greatest speed and can pass through solids, liquids and gases.
(iii) Volcanic activity and earthquakes occur in the same belt because earthquakes and volcanoes are formed along faults between tectonic plates, where the crust is weakest.
Explain the three types of earthquake waves.
The three types of earthquake waves are-
1. P or Primary wave — These are the first earthquake waves to be recorded on a seismogram of an earthquake. They are compressional,i.e., the particles vibrate in the direction of movement of the wave. They can pass through solids, liquids and gases.
2. S or Secondary wave — These waves are recorded on a seismogram after the P waves. They are distortional waves in which the particles vibrate at right angles to the direction of movement of the wave. S waves travel through the earth's interior but cannot be transmitted by liquids.
3. L or Surface waves — These waves travel along the surface of the earth and are recorded after the P and S waves. Two types of L waves are identified- Love waves and Rayleigh waves.
The destructive effects of an earthquake are far more than its constructive effects. Give reasons to support your answer.
Constructive effects of earthquakes are-
- Release of stored up energy of the earth
- Vertical and lateral displacement of the earth
- Formation of lakes
Destructive forces of earthquakes are-
- Collapse of structures causing great loss of life and property
- Submergence of coastal areas
- Changed course of rivers, rendering many areas unsuitable for irrigation and agriculture
- Resulting landslides disrupt the transport system
- Accidental fires cause damage to life and property
In geographical terminology, earthquakes are a part of endogenic forces and are regarded as constructive movements of the earth's crust but from human point of view, earthquakes are regarded as destructive causing loss of life and property.
How is the intensity of an earthquake measured?
The intensity of an earthquake is measured by using instruments like Richter scale and Mercalli scale.
Is it possible to predict an earthquake? Give reasons to support your answer.
No, it is not possible to predict an earthquake.
The Russians were the first ones to discover the P and S waves on the basis of which predictions were made. The theory of Plate Tectonics offers another means of prediction on scientific lines. Japanese use the methods of measuring the changes in sea level and variations in Earth's magnetic field to predict earthquakes.
However, no method is foolproof as has been proved by recent earthquakes in Japan, India and USA.
How are earthquakes distributed over the globe?
The main earthquake belts in the world are-
1. The Circum Pacific Mountain Belt — This belt has 70% of all earthquakes. A part of San Andreas Fault in the USA lies in this belt. The area of Japan, the Philippines and Indonesia all form parts of this belt.
2. The Midworld Mountain Belt — It stretches from Eastern Europe covering Alpine-Himalayan ranges in Europe ans Asia. About 20% of all earthquakes occur in this zone.
3. The Mid-Atlantic Ridge — This belt comprises areas along the mid-oceanic ridges as well as many island near the ridges of the Atlantic Ocean and the remaining 10% of earthquakes occur in this belt. The Rift Valley of East Africa is considered an extension of the belt.