An earthquake also known as a quake or tremor is the shaking of the surface of the earth resulting from an immediate release of the energy in the earth's lithosphere that creates seismic waves.

Earthquakes can range in size from those that some shakes are so weak that they cannot create enough damage to propel objects and wreak destruction across entire cities.

This NCERT notes on Earthquake- body waves, causes, and types will help you prepare for your IAS Exams, UPSC Civil Services Exams, State Civil Service Exams, PO, and others.

Characteristics of an Earthquake

• All-natural earthquakes occur only in the lithosphere.
• An earthquake is simply the shaking of the earth’s crust.
• It is caused due to the sudden energy release, which triggers the waves that travel in all directions.
• The effusion of energy occurs along a fault.
• A fault is a fracture or crack between two blocks of rock.
• Rocks along a fault generally move in opposite directions.

Types of Earthquakes

• Tectonic earthquake: The most common form of an earthquake is caused by the sudden movement of loose fragmented pieces of land on the earth’s crust which is known as tectonic plates.
• Volcanic earthquake: The less widespread compared to the tectonic type, these earthquakes happen before or after the eruption of a volcano. It is caused when magma released from the volcano is filled by rocks being pushed to the surface.
• Collapse earthquake: This earthquake occurs at the underground mine level. The main cause of this earthquake is due to pressure generated within the rocks.
• Explosion earthquakes: The occurrence of this type of earthquake is an artificial form. High-density explosions such as nuclear explosions are the primary cause of this earthquake.

Causes of Earthquakes

• It is caused due to the tectonic movements of the earth’s surface.
• It is caused due to the energy release produced by waves that travel in all directions.
• The point where energy is released is called the focus or hypocenter. It is generally located at a depth of 60 km on the earth’s surface.

• This causes a release of energy and the energy waves that travels in all directions.
• The focus of an earthquake is the point where the energy is released.
• The epicentre is the first place where the waves are created, the surface of the earth which is vertically above the focus point is called the epicentre.

Effects of Earthquake

• Shaking and ground eruption.
• Soil liquidation. That is, the soil turns into a fluid-like mass during earthquakes.
• An earthquake may cause injury and loss of life to society, road and bridge damage.
• Landslides in the hilly areas.
• Earthquakes can cause fires by damaging electrical power or gas lines.
• Tsunami.
• Floods may be one of the effects of earthquakes if dams are damaged. Earthquakes may cause damage to dam rivers, which collapse and cause floods.

Body Waves

A body wave is also called a seismic wave that gets into the interior surface of the earth, as opposed to surface waves that travel on the earth's surface. P and S waves are the two different body waves.

Each type of wave shakes the earth’s surface in different ways. Body waves travel through the interior part of the Earth along paths controlled by the material properties in terms of density and modulus.

The density and modulus vary according to the temperature, composition, and material phase. This effect parallels the refraction of light waves. Two different types of particle motion result in two types of body waves: Primary and Secondary waves.

Body Waves: Primary Waves

Primary waves (P-waves) are constricting waves that are longitudinal in nature. P-waves are a high-pressure wave which travels faster than the S- waves through the earth surface to land at seismograph stations first, hence this wave is named as "Primary".

These waves can travel through any type of material on the earth’s surface, including fluids, and can travel nearly 1.7 times faster than the S-waves. In the air, they take the speed and form of sound waves; hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water, and about 5000 m/s in granite.

Body Waves: Secondary Waves

Secondary waves (S-waves) are shortening waves that are crossway in nature. Following an earthquake, S-waves arrive at seismograph stations after the faster-moving of P-waves and displace the ground vertical to the direction of propagation.

Depending on the transmitting direction, the wave can go through on different surfaces; for example, in the case of S waves that travel horizontally, the ground moves differently to one side and then the other.

S-waves can travel only through solids, liquids, and gases that do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves on any surface. Shorten waves will not travel through liquid substances, so the absence of the S-wave in the earth's outer core surface suggests a liquid state.

Shadow Zone

• The shadow zone is that portion of the earth in the earthquake that does not receive any direct P-waves. It is from the angular distances of 104 to 140 degrees from the given earthquake.
• Shadow zones are formed when the liquid core stops the S waves and refracts the P waves.
• A zone between the epicentre’s 105-145 degrees is found to be a shadow zone for both the waves. It appears as a band away from the epicentre,
• Beyond 105 degrees of the epicentre, the entire zone is free of the S-waves.
• S-waves have larger shadow zones than P-waves.

Measurement

All earthquakes are different from each other in their intensity and magnitude. The instrument used for the measurement of vibrations is known as Seismograph.

Magnitude scale

• The magnitude of the earthquake is measured by the equipment called the Richter scale.
• The energy released during an earthquake is expressed in numbers out of 0 to 10.

Intensity scale

• The intensity of an earthquake is measured by the equipment called the Mercalli scale.
• It measures the tangible damage caused by the earthquake.
• It is expressed in the range from 1 to 12.

FAQs

MCQs

1. What are the two main types of Earthquake Body Waves?

A) Longitudinal and Transverse
B) P-waves and S-waves
C) Surface and Body Waves
D) Primary and Secondary Waves

2. Which type of seismic wave travels the fastest?

A) Surface Waves
B) S-waves
C) P-waves
D) Both P-waves and S-waves travel at the same speed

3. Which of the following materials can P-waves travel through?

A) Only Solids
B) Solids and Liquids
C) Only Liquids
D) Only Gases

4. What is the primary cause of Earthquake Body Waves?

A) Volcanic eruptions
B) Sudden release of energy due to tectonic plate movement
C) Meteorite impacts
D) Underground nuclear tests

5. Which type of seismic wave cannot travel through liquids?

A) P-waves
B) S-waves
C) Surface waves
D) Both P-waves and S-waves

GS Mains Questions and Model Answers

Q1: Analyze the role of Earthquake Body Waves in the study of the Earth's interior. How do they help in understanding the Earth's structure?

Answer: Earthquake Body Waves, particularly P-waves and S-waves, play a crucial role in understanding the Earth's internal structure. The different velocities and behavior of these waves as they travel through various layers of the Earth provide valuable insights into the composition and state of these layers. For instance, the fact that S-waves do not travel through the Earth's liquid outer core helped scientists confirm its liquid state. Additionally, by analyzing how these waves are refracted, reflected, or absorbed at various boundaries between the Earth's layers, geologists can infer the nature of materials inside the Earth, including the mantle and core. This seismic data is fundamental in shaping our understanding of the Earth's interior composition and structure.

Q2: Explain the difference between body waves and surface waves in terms of their behavior and impact during an earthquake.

Answer: Body waves and surface waves are two major types of seismic waves generated during an earthquake. Body waves, including P-waves and S-waves, travel through the interior of the Earth. P-waves are compressional waves and are the fastest seismic waves, moving through solids, liquids, and gases. S-waves are shear waves that move slower and can only travel through solids. Surface waves, on the other hand, travel along the Earth's surface and are typically slower than body waves but are more destructive due to their higher amplitude and longer duration. The impact of surface waves is more noticeable in terms of structural damage during earthquakes, while body waves provide critical information about the Earth's interior.

Q3: Discuss the importance of studying Earthquake Body Waves for earthquake prediction and preparedness. What advancements have been made in this field?

Answer: Studying Earthquake Body Waves is vital for earthquake prediction and preparedness as they provide essential information about the location, depth, and magnitude of an earthquake. By analyzing the arrival times of P-waves and S-waves at seismograph stations, scientists can quickly assess the intensity and potential impact of an earthquake. Advances in seismic technology, such as real-time data transmission and more sensitive seismometers, have significantly improved our ability to detect and analyze these waves. However, while Earthquake Body Waves offer insights into the structure of the Earth and help in estimating the impact of earthquakes, predicting the exact time and location of an earthquake remains challenging due to the complex and unpredictable nature of tectonic processes.

Previous Year Questions on Earthquake Body Waves

1. UPSC CSE Prelims 2019:

Question: Which of the following seismic waves can travel through both solids and liquids?

A) S-waves
B) P-waves
C) Surface waves
D) Both S-waves and Surface waves

Answer: (B)

Explanation: P-waves (Primary waves) can travel through both solids and liquids, unlike S-waves, which can only travel through solids.

2. UPSC CSE Mains 2020 (GS Paper 1):

Question: Discuss the seismic waves responsible for the propagation of earthquake energy through the Earth. How do they provide insights into the Earth's internal structure?

Answer: Earthquake body waves, including P-waves and S-waves, are responsible for the propagation of seismic energy through the Earth. P-waves are compressional waves that move faster and can travel through both solids and liquids, while S-waves are shear waves that can only travel through solids. By studying the arrival times and behavior of these waves at different stations, scientists have been able to infer the composition and state of different layers of the Earth, such as the solid inner core and liquid outer core. This study has been pivotal in advancing our understanding of the Earth's interior and its tectonic processes.