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Page No 240:

Question 1:

The place where the earthquake occurs is the
(a) centre.
(b) epicentre.
(c) focus.
(d) origin.

Answer:

(c) focus

The focus is the point inside the earth where earthquake waves are generated.
Note: Epicentre is the geographical location lying on the surface of Earth, vertically above the focus. 

Page No 240:

Question 2:

An earthquake is caused by
(a) shaking of the earth's surface.
(b) strain building up in the tectonic plates.
(c) plates slipping along the fault lines.
(d) all of the above.

Answer:

(d) all of the above
 
Earthquake can be caused by any of the reasons mentioned above.
     

Page No 240:

Question 3:

Which of these things would you do if you live in an earthquake-prone area?
A. You would keep an emergency kit ready always.
B. You would have only open book shelves and open cupboards in your house.
C. You would fix the cupboards to the walls.
D. You would not keep breakable items on the upper shelves.

(a) Only C is correct.
(b) Both A and C are correct.
(c) All are correct.
(d) A, C and D are correct.

Answer:

(d) A,C and D are correct.
 
Option B is not correct, because items stored in open bookshelves and open cupboards could fall on people during an earthquake, causing injuries to them.

Page No 240:

Question 4:

Mark the following statements as true or false and correct the false statements.
(a) Earthquakes and tsunamis are difficult to predict.
(b) Tsunamis always follow earthquakes.
(c) Tall buildings should be built on solid ground and not on loose surface materials.
(d) People living in coastal areas should always be prepared to move if earthquakes happen nearby.
(e) The epicentre is the point inside the earth where the earthquake originates.
(f) A nuclear disaster is a natural disaster.
(g) A seismograph is a scale used for measuring the magnitude of earthquakes.
(h) Hot molten rock that flows out of volcanoes is called magma.

Answer:

(a)  True
(b)  False. A tsunami follows an earthquake when tectonic plates slip in the ocean and a huge amount of water is displaced.
(c)  True
(d)  True
(e)  False. The point inside the earth where an earthquake originates is called the focus or hypocentre of the earthquake.
(f)   False. A nuclear disaster is man-made, and is caused mainly because of an accident in a nuclear power plant.
(g)  False. The Richter scale is used for measuring the magnitude of earthquakes.
(h)  False. The hot molten rock that flows out of a volcano is called lava, and the hot molten rock inside the earth's crust is called magma.

Page No 240:

Question 5:

(a) Explain how earthquakes are caused.
(b) List some human activities that can result in earthquakes.

Answer:

(a) Earthquakes are caused by the movement of tectonic plates. These plates are part of the earth's surface and float on the molten magma present in the mantle. When two plates converge, they push against each other with enormous strength, and one might slide over the other. This causes disturbances on the earth's crust, because of which earthquake tremors are felt.
     
 (b) Some activities that can result in earthquakes are given below.

  1. Construction of large dams. The weight of the water that collects in dams exerts a huge amount of force on the land below, and may cause the land to shift. 
  2. Implementation of more geothermal energy projects
  3. Extraction of substances from the ground. Example: Mining.
  4. Construction of huge skyscrapers, which can trigger a movement of fault because of the enormous pressure exerted by thousands of tonnes of building material.

Page No 240:

Question 6:

Explain the following terms:
epicentre, fault lines, seismic waves, volcanoes

Answer:

(1) Epicentre: The epicentre of an earthquake is the point on earth's surface that is directly above the point inside the earth where the earthquake has originated.

(2) Fault lines: Fault lines are the fractures on the earth's crust along which tectonic plates move.

(3) Seismic waves: Seismic waves are the low-frequency energy waves produced by earthquakes, volcanic eruptions or explosions.

(4) Volcanoes: A volcano is an opening on the earth's surface or crust that allows hot lava, volcanic ash and gases to escape from the magma below the surface.

Page No 240:

Question 7:

Clearly differentiate between the following:
(a) earthquakes and tsunamis
(b) epicentre and focus

Answer:

(a)     

Earthquake Tsunamis
Earthquakes are caused by the movement of tectonic plates beneath the crust of the earth. Tsunamis are caused by earthquakes and tectonic activities in the ocean that result in the movement of an enormous amount of water towards the shore.
Earthquakes are mainly caused by tectonic activities and  volcanic eruptions. Tsunamis are mainly caused by earthquakes near or in the ocean bed.
   

(b) Focus: The point inside the earth where an earthquake has originated is called the focus of the earthquake.   
           
Epicentre: The point on the earth's surface that is directly above the focus of an earthquake is called the epicentre of the earthquake. 
 
Epicentre Focus
Earthquakes are caused by the movement of tectonic plates beneath the crust of the earth. Tsunamis are caused by earthquakes and tectonic activities in the ocean that result in the movement of an enormous amount of water towards the shore.
Earthquakes are mainly caused by tectonic activities and  volcanic eruptions. Tsunamis are mainly caused by earthquakes near or in the ocean bed.



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