Earthquakes - Questions & Answers

1. A tremor below the surface of the earth which causes shaking of the crust:
(b) Earthquake

2. 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.
(a) Both A and R are true and R is the correct explanation of A.

3. A series of shocks that result from sudden earth movement:
(c) Earthquake waves

4. Which amongst the following is NOT a factor that causes an earthquake?
(c) Lava

5. Which waves are recorded on a seismograph after the P waves?
(c) Shear waves

6. Which of the following measures the power of an earthquake on a scale of 1 to 9?
(b) Richter scale

7. The point on the earth's surface directly above the seismic focus is called:
(a) Epicentre

8. The instrument that grades the earthquakes on the basis of observed effects on a 12-point scale.
(d) Mercalli scale

9. Which, amongst the following, is NOT a destructive effect of earthquakes?
(c) Energy release

10. A large destructive ocean wave that can be generated by any disturbance (earthquakes, volcanoes etc.) that rapidly displaces a large mass of water:
(b) Tsunami

11. Which of the following is not correct?
(d) Pacific Ring of Fire : 15% of world earthquakes

1. What is called 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. A recent example is the 5.1 magnitude earthquake that struck central Assam on January 5, 2026.

2. State two natural causes of an earthquake.
Two natural causes of an earthquake are Volcanic eruptions and Plate Tectonics (movements of the earth's crustal plates).

3. With the help of an example explain how human activities cause earthquakes.
Human activities like the construction of huge dams, nuclear explosions, and deep mining can alter stresses in the crust and induce earthquakes. For example, the Koyna earthquake in Maharashtra was caused by the construction of the Koyna dam in a seismically active region.

4. What are known as seismic focus and epicentre with respect to an earthquake?
The point of origin of the seismic waves deep inside the earth is known as the seismic focus. The point on the earth's surface directly above the seismic focus is known as the epicentre.

5. What is known as Richter scale? State its uses.
The Richter scale is a measurement scale used to quantify the magnitude of an earthquake from 1 to 9. It is used to measure the absolute intensity and the total amount of energy released by an earthquake with mathematical precision.

6. How are earthquakes useful?
Earthquakes are useful because they help the earth release its stored-up pent-up energy, keeping the earth in good shape. They can also create new landforms, uplift seashores, and form lakes through vertical and lateral displacement.

7. Give any two destructive effects of earthquakes.
Two destructive effects are the collapse of structures (like buildings and bridges) which causes immense loss of life and property, and the triggering of secondary disasters like massive landslides or devastating tsunamis.

8. 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, most commonly an undersea earthquake. A recent example is the tsunami that struck Java and Sumatra in Indonesia on December 22, 2018.

9. How do Japanese predict earthquakes?
The Japanese use methods of measuring changes in sea level and observing variations in the Earth's magnetic field to predict earthquakes.

10. Name the two belts where most of the earthquakes occur.
Most earthquakes occur in the Circum Pacific Mountain Belt (also known as the Pacific Ring of Fire) and the Mid-world Mountain Belt.

1. (a) A 5.1 magnitude earthquake struck central Assam at 4:17 a.m. on January 5, 2026. The epicentre of the tremor was near Assam's Morigaon district at a depth of 50 km. — Akashvani News. Explain the following terms: (i) Magnitude; (ii) Epicentre.
(i) Magnitude refers to the total amount of energy released during an earthquake, usually measured on the Richter scale. (ii) Epicentre is the point on the earth's surface that is directly above the seismic focus (the underground origin point of the earthquake).

1. (b) What is the difference between seismic focus and epicentre?
The seismic focus is the actual point of origin of the seismic waves deep inside the earth's interior, whereas the epicentre is the corresponding point on the surface of the earth located exactly vertically above the seismic focus.

1. (c) Give a geographical reason for each of the following:
(i) Most earthquakes occur on account of plate movements.
The earth's crust consists of a series of movable plates. When these tectonic plates slip past or violently collide with each other, the resulting friction and stress produce faults along the lines of weakness, releasing tremendous energy in the form of earthquakes.
(ii) Primary waves are the first earthquake waves to be recorded on a seismogram.
Primary (P) waves are the fastest traveling seismic compressional waves. Because of their high velocity, they reach the recording station and the seismogram first before the slower Secondary or Surface waves.
(iii) Volcanic activity and earthquakes occur in the same belt.
They are deeply interrelated phenomena. During a volcanic eruption, the upward pushing of hot gases and magma shakes the earth's crust. Furthermore, the unstable plate boundaries where oceanic and continental plates converge create conditions ideal for both submarine ridges (volcanoes) and faulting (earthquakes).

1. (d) Explain the three types of earthquake waves.
1. P-Waves (Primary or push waves): Compressional waves where particles vibrate in the direction of wave movement. They arrive first and pass through solids, liquids, and gases.
2. S-Waves (Secondary or shear waves): Distortional waves arriving after P-waves. Particles vibrate at right angles to the direction of wave movement. They can only travel through solid parts of the earth.
3. L-Waves (Surface or long waves): These waves travel along the surface of the earth. They are the last to arrive and are responsible for the ground motion felt by inhabitants and the resulting damage.

2. (a) The destructive effects of an earthquake are far more than its constructive effects. Give reasons to support your answer.
Constructive effects are limited mainly to releasing earth's internal energy and forming localized geographical features like lakes. Conversely, the destructive effects are catastrophic on a global scale. Earthquakes collapse buildings, submerging coastal areas, altering river courses, and triggering secondary disasters such as deadly landslides, huge fires, and massive tsunamis. This results in the loss of hundreds of thousands of human lives and staggering economic property damage, far outweighing any minor geological benefits.

2. (b) How is the intensity of an earthquake measured?
Earthquake intensity and magnitude are primarily measured using a Seismograph (to record the waves), the Richter Scale (to measure the absolute power and energy released on a scale of 1 to 9), and the Mercalli Scale (which grades earthquakes on a 12-point scale based on visually observed damage and effects on the environment).

2. (c) Is it possible to predict an earthquake? Give reasons to support your answer.
It is not entirely possible to predict an earthquake with complete foolproof accuracy. Modern scientific approaches utilize the theory of Plate Tectonics, measurement of P and S waves, variations in the Earth's magnetic field, and changes in sea levels. However, as demonstrated by sudden severe earthquakes in well-equipped regions like Japan and the USA, these methods still cannot pinpoint exactly when and where a quake will strike.

2. (d) How are earthquakes distributed over the globe?
Global earthquakes predominantly originate along tectonic plate boundaries and are largely concentrated in three distinct belts: The Circum Pacific Mountain Belt (Pacific Ring of Fire), which accounts for 70% of earthquakes; the Mid-world Mountain Belt (stretching from Eastern Europe to the Himalayas) with 20%; and the Mid-Atlantic Ridge, accounting for the remaining 10%.

1. 'Prevention is better than cure.' How does this statement apply to earthquakes? Give reasons to support your answer.
Because earthquakes cannot be accurately predicted or stopped, human populations are highly vulnerable to their catastrophic destruction. 'Curing' the aftermath involves dealing with massive loss of lives, collapsed infrastructure, and ruined economies, which is often an impossible recovery. Therefore, 'Prevention'—in the form of constructing earthquake-resistant buildings, regulating land use in fault zones, and implementing community disaster-preparedness plans—is vastly better because it proactively saves lives and minimizes the devastating impact before the quake strikes.

2. Imagine you were sitting in the balcony of a seaside resort, when you suddenly noticed waves rising high in the sky. Suddenly there was a surge in the height of the waves. You knew it was a tsunami. What could have caused this tsunami and what were its consequences.
The sudden tsunami was likely caused by a massive undersea earthquake, a violent volcanic eruption, or a large submarine landslide that rapidly displaced a huge mass of ocean water. The consequences of this event would be devastating flash floods crashing into the coast, completely destroying the seaside resort and surrounding infrastructure, causing severe coastal erosion, and leading to a tragic loss of human and animal lives as the immense wall of water sweeps inland.

1. Make a record of recent earthquakes in India. Find out the damage caused.
One recorded recent example from the text is the 5.1 magnitude earthquake in central Assam (Morigaon district) on January 5, 2026. Another historic and severely damaging example is the Bhuj earthquake in Gujarat in 2001 (7.7 magnitude), which caused extreme devastation, flattening villages, collapsing buildings, and resulting in immense loss of human life.

2. On an outline map of the world show the world distribution of earthquakes. Mark earthquake prone areas in India. Identify the reasons for earthquakes in each region.
The major global distribution belts are the Circum Pacific Mountain Belt, Mid-world Mountain Belt, and the Mid-Atlantic Ridge. In India, the highly earthquake-prone areas include the Himalayan region and North-East India. The primary reason for earthquakes in these Indian regions is their location within the unstable Mid-world Mountain Belt, where compressional forces from the ongoing collision of the Indian tectonic plate sliding below the Asiatic plate produce severe faults and tremors.