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Solutions for Geography, Class 9, ICSE
Assertion (A): A tsunami is a large destructive ocean wave that can be generated by any disturbance that rapidly displaces a large mass of water.
Reason (R): The most common cause is an undersea earthquake.
Both A and R are true and R is the correct explanation of A.
Explanation — A tsunami is a large destructive ocean wave that can be generated by any disturbance that rapidly displaces a large mass of water, such as an earthquake, volcanic eruption, landslide or meteorite impact. However, the most common cause is an undersea earthquake.
Pacific Ring of Fire: 15% of world earthquakes
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.
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.
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.
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.
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.
(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.
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.
Constructive effects of earthquakes are-
Destructive forces of earthquakes are-
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.
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.
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 and 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.
The statement "Prevention is better than cure" applies very well to earthquakes. Once an earthquake occurs, the damage has already been done. Therefore, the best approach is to prevent the earthquake from happening or mitigate its effects.
We cannot prevent earthquakes caused by natural causes but we can certainly check the man made causes of earthquakes. Human activity like construction of huge dams, nuclear explosions, blasting of rocks, mining, etc., near fault zones are responsible for earthquakes.
Apart from disregarding nature, retrofitting buildings to withstand earthquakes or constructing new buildings with earthquake-resistant materials can be expensive, but it is often cheaper than repairing or rebuilding after an earthquake. Educating people on earthquake safety or implementing early warning systems can help to protect people from harm.
A tsunami can be caused by:
The consequences of a tsunami can be severe: