Block-2 Resources

2. UNIT-5 EARTHQUAKES

An earthquake is the sudden shaking or vibration of the earth caused by the rapid release of energy from within the earth. This release of energy produces seismic waves that travel through the interior and along the surface of the earth. Earthquakes are closely related to movements in the earth’s crust and are generally associated with faulting and tectonic disturbances. They are more common in geologically active regions of the world.

Objectives

After studying this unit, you should be able to:

Define earthquakes and their causes

Understand seismic waves and their types

Explain the effects of earthquakes

Learn basic methods of earthquake measurement

1. Introduction

Various physical forces continuously operate inside the Earth and on its surface. These forces constantly modify the shape and size of landforms. Movements within the Earth may be slow and gradual or sudden and violent. Sudden movements inside the Earth produce vibrations in rocks. These vibrations are called earthquakes. Earthquakes occur suddenly, affect large areas, and cause severe destruction to life and property. Unlike mountain building, which occurs very slowly, earthquakes take place within a few seconds. Even with modern scientific progress, earthquakes cannot be predicted accurately in advance. Historical records show that earthquakes were mentioned by early historians like Herodotus and Pliny, and records exist from about 2500 years ago.

2. Recording of Earthquakes

The earliest instrument used to detect earthquakes was called a seismoscope, invented in 136 A.D. in China. This instrument could only indicate that an earthquake had occurred but could not measure its strength. Later, the seismograph was developed to record earthquake vibrations. A seismograph consists of a heavy mass suspended like a pendulum and attached to a firm support fixed to the ground. When the ground shakes, the support moves while the suspended mass tends to remain stationary due to inertia. The relative movement between them is recorded on a rotating drum with photographic paper. The recorded trace of earthquake vibrations is called a seismogram. The study of earthquakes is known as seismology.

3. Earthquake Waves (Seismic Waves)

During an earthquake, energy is released in the form of seismic waves. These waves travel through the Earth in different forms. There are four main types of seismic waves:

1. P waves,

2. S waves,

3. Rayleigh waves, and

4. Love waves.

1. P waves (Primary waves) are longitudinal waves consisting of alternating compressions and expansions similar to sound waves. They travel fastest and can move through both solids and liquids.
2. S waves (Secondary waves) are shear waves that cause particles to move at right angles to the direction of wave travel. These waves travel slower than P waves and can move only through solids.
3. Rayleigh waves move particles in circular motions in vertical planes.
4. Love waves move particles horizontally at right angles to the direction of wave movement.
Rayleigh and Love waves together are called surface waves or L waves. Among all waves, P waves are the fastest, followed by S waves, while L waves travel the slowest.

(Source: AI-generated diagram (OpenAI DALL·E)

4. Intensity of Earthquakes

The strength of an earthquake is measured using different intensity scales. The intensity depends on the effects of the earthquake on people, buildings, and instruments. One of the earliest scales was the Rossi–Forel scale, which has 10 divisions. Later, the Mercalli scale was introduced with 12 divisions, based on the damage caused by seismic waves. The most widely used modern scale is the Richter scale, which measures earthquake magnitude. Places experiencing equal earthquake intensity are connected by lines called isoseismal lines.

5. Classification of Earthquakes

Earthquakes are classified based on the depth of their focus. The focus or hypocenter is the exact point inside the Earth where the earthquake originates. The point directly above the focus on the Earth’s surface is called the epicenter. Earthquakes with focus deeper than 300 km are called deep-focus earthquakes. Earthquakes with focus between 70 km and 300 km are known as intermediate-focus earthquakes. Earthquakes with focus less than 70 km deep are called shallow-focus earthquakes, and most earthquakes belong to this category.

6. Causes of Earthquakes

Earthquakes occur due to several causes such as faulting, volcanic activity, landslides, and collapse of underground caverns. The most common cause is movement along faults in the Earth’s crust. Stress gradually accumulates in rocks due to crustal movements. When the rocks can no longer withstand this stress, they suddenly break or slip along faults, producing earthquake waves. According to Elastic Rebound Theory proposed by H. F. Reid, earthquakes occur when accumulated elastic strain in rocks is suddenly released during fault movement. Most earthquakes caused by crustal movements are known as tectonic earthquakes. Earthquakes may also occur due to volcanic activity, when magma and gases move within the Earth’s crust.

(Source: AI-generated diagram (OpenAI DALL·E)

7. Submarine Earthquakes and Tsunamis

Earthquakes that occur beneath the sea are called submarine earthquakes. These earthquakes can cause sudden vertical displacement of the sea floor. This disturbance may generate huge sea waves known as tsunamis. Tsunamis travel long distances and cause severe damage to coastal regions.

8. Distribution of Earthquakes

Earthquakes are not evenly distributed over the Earth. Most destructive earthquakes occur around the Pacific Ocean, forming the Circum-Pacific Belt or Ring of Fire. Another earthquake belt extends through Indonesia, Andaman–Nicobar Islands, and Myanmar. A third belt stretches from the Pamir Knot through Afghanistan, Iran, Turkey, and Greece towards Europe. Another belt runs through the Great Rift Valley of East Africa. These regions are tectonically unstable and experience frequent earthquakes.