Earthquakes

Earthquake

Reason — An earthquake is a sudden tremor or shaking of the Earth's crust caused by the release of energy below the Earth's surface. The released energy travels outward as seismic (earthquake) waves.

Both A and R are true and R is the correct explanation of A.

Explanation — A tsunami is a large, destructive ocean wave caused by the sudden displacement of a large volume of water due to events such as undersea earthquakes, volcanic eruptions, landslides, or meteorite impacts. Since undersea earthquakes are the most common cause of tsunamis, the Reason correctly explains the Assertion.

Earthquake waves

Reason — A series of shocks that result from sudden earth movement, such as along a fault, are known as earthquake (seismic) waves. These waves spread out from the seismic focus and can cause widespread destruction.

Lava

Reason — Earthquakes are caused by volcanoes, plate tectonics, folding, faulting, and human activities. Lava is a product of volcanic eruptions, not a cause of earthquakes.

Shear waves

Reason — S-waves (secondary or shear waves) arrive after P-waves and move particles at right angles to the direction of wave travel.

Richter scale

Reason — The Richter scale measures the power of an earthquake on a scale of 1 to 9. It measures the absolute intensity of an earthquake with mathematical precision.

Epicentre

Reason — The epicentre is the point on the earth's surface directly above the seismic focus, which is the point of origin of the seismic waves.

Mercalli scale

Reason — The Mercalli scale grades the earthquakes on the basis of observed effects on a 12-point scale.

Energy release

Reason — Energy release is not a destructive effect but the natural process that causes an earthquake by releasing stress accumulated within the Earth's crust. Submergence, landslides, and tsunamis are destructive effects of earthquakes.

Tsunami

Reason — A tsunami is a large, destructive ocean wave caused by the sudden displacement of a large mass of water due to earthquakes, volcanic eruptions, landslides, or meteorite impacts.

Pacific Ring of Fire: 15% of world earthquakes

Reason — The Circum-Pacific Mountain Belt (Pacific Ring of Fire) accounts for about 70% of the world's earthquakes. The Mid-world Mountain Belt accounts for about 20%, and the Mid-Atlantic Ridge for about 10%.

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.

Two natural causes of an earthquake are-

1. Volcanoes
2. Plate Tectonics

Human activities such as the construction of large dams, mining, rock blasting, and underground nuclear explosions can trigger earthquakes, especially in seismically active regions. These activities alter the stress and pressure within the Earth's crust, causing movement along existing faults.

For example, the 1967 Koyna earthquake in Maharashtra is widely associated with the construction and filling of the Koyna Dam, making it a well-known case of reservoir-induced seismicity.

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.

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.

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.

Japanese use the methods of measuring changes in sea level and variations in Earth's magnetic field to predict earthquakes.

Two belts where most of the earthquakes occur are-

1. The Circum Pacific Mountain Belt
2. The Midworld Mountain Belt

(i) Magnitude — Magnitude is a measure of the amount of energy released during an earthquake. It is measured on the Richter scale on a scale of 1 to 9, with each higher point representing a ten-fold increase in the amplitude of the shock waves. In the given news report, the magnitude of the Assam earthquake was 5.1.

(ii) Epicentre — The epicentre is the point on the earth's surface directly above the seismic focus (the point of origin of the seismic waves). In the given news report, the epicentre of the tremor was near Assam's Morigaon district.

(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-

1. Release of stored up energy of the earth
2. Vertical and lateral displacement of the earth
3. Formation of lakes

Destructive forces of earthquakes are-

1. Collapse of structures causing great loss of life and property
2. Submergence of coastal areas
3. Changed course of rivers, rendering many areas unsuitable for irrigation and agriculture
4. Resulting landslides disrupt the transport system
5. 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.

The intensity of an earthquake is measured by using instruments like Richter scale and Mercalli scale.

No, it is not possible to predict an earthquake accurately.

Scientists use methods such as monitoring seismic waves, studying plate tectonics, measuring changes in sea level, and observing variations in the Earth's magnetic field to identify earthquake-prone areas and monitor possible warning signs.

However, no method can accurately predict the exact time, location, and magnitude of an earthquake. Recent earthquakes in countries such as Japan, India, and the USA have shown that earthquake prediction is still not foolproof.

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 well to earthquakes because, although natural earthquakes cannot be prevented, their impact can be reduced through proper planning and preparedness. Human-induced earthquakes can also be minimised by controlling risky activities.

Human activities such as the construction of large dams, underground nuclear explosions, rock blasting, and mining near fault zones can trigger earthquakes.

1. The 1967 Koyna earthquake in Maharashtra is widely associated with the construction of the Koyna Dam in a seismically active region.
2. In the 1960s, the disposal of chemical waste and underground explosions triggered several earthquakes in Colorado, USA.

In addition, earthquake-resistant construction, retrofitting old buildings, public awareness, and early warning systems help reduce the loss of life and property. Thus, preparedness and mitigation are far more effective than dealing with the consequences after an earthquake.

A tsunami can be caused by:

1. Underwater earthquakes — The most common cause, where a sudden movement of the ocean floor displaces a large volume of water.
2. Underwater landslides — A landslide on the seabed can also displace water and generate tsunami waves.
3. Volcanic eruptions — An underwater volcanic eruption may trigger a tsunami by rapidly displacing seawater.

The consequences of a tsunami can be severe:

1. Severe flooding of coastal areas.
2. Destruction of buildings, roads, and infrastructure.
3. Loss of life and property, along with heavy damage to agriculture, fishing, and tourism.