Body waves are seismic waves that travel through the earth's interior, as opposed to surface waves, which travel near the earth's surface. Body waves and surface waves are the two types of earthquake waves. Body waves are formed when energy is released at the focus and travels in all directions through the earth's body. This article will explain to you about Body Waves - Seismic Activity which will be helpful in Geography preparation for the UPSC Civil service exam.

Seismic Waves

• Seismic waves are the vibrations or oscillations that travel through the Earth's interior and across its surface in response to various sources of energy release, such as earthquakes, volcanic activity, or human-induced events.
• These waves carry energy and information about the Earth's subsurface and are crucial for understanding the planet's interior structure, seismic activity, and geological processes.
• These seismic waves travel as vibrations through the earth. The vibrations are recorded and measured by a device known as a seismometer.
• The seismometer generates a graph called a Seismograph that depicts these vibrations. These waves travel at different speeds through various materials.
• Earthquake waves are caused by the rapid release of energy along a fault (a sharp rupture in the crustal layer).
• The force produced on the rock layers along a fault causes them to move in opposing directions, but they are kept in place by the frictional force exerted by the above rock strata.
• Over time, the pressure on the rock strata builds up and exceeds the frictional barrier, causing an abrupt movement that generates shockwaves (seismic waves) in all directions.
• Seismic waves are categorized into two main types: Body waves and Surface waves.

Body Waves

• Body waves are generated due to the release of energy at the focus and move in all directions traveling through the body of the earth.
• Body waves are seismic waves that travel through the Earth's interior.
• They are generated by the sudden release of energy from events such as earthquakes, volcanic eruptions, or even man-made explosions.
• Since they move through the Earth, they are instrumental in providing information about the Earth's internal structure, including layer densities, thicknesses, and states of matter.
• By analyzing the travel times and paths of these waves, seismologists can infer a lot about the Earth's interior structure and the properties of different layers.
• The variations in the speed and direction of body waves as they interact with different materials inside the Earth help scientists construct models of the Earth's interior, including the identification of the core, mantle, and crust layers.
• There are two primary types of body waves: Primary Waves (P-waves) and Secondary Waves (S-waves).

Body waves

P-waves

• They are the fastest among seismic waves and thus reach the recording instruments first.
• P-waves are compressional or longitudinal waves, which means the ground particles move in the direction of the wave propagation (similar to sound waves).
• P-waves are also known as longitudinal waves because the displacement of the medium occurs in the same or opposite direction (parallel to) the wave's propagation direction.
• Compressional waves because they cause compression and rarefaction when travelling through a medium; and pressure waves because they cause pressure changes in the medium.
• P-waves cause density variations in the material, causing it to expand (rarefaction) and compress (compression).
• They can travel through solid, liquid, and gaseous materials, which means they can pass through the Earth's crust, mantle, outer core, and inner core.
• When they pass through the Earth's surface, they can shake the ground in the direction of their movement, often feeling like a sudden jolt during an earthquake.

P-waves - Significance

• These waves have a low frequency and are the least damaging of the earthquake waves.
• The earth's surface trembles in an up-down motion as a result of these waves (vertical).
• They may pass through any medium, and their speed is determined by the medium's shear strength (elasticity).
• As a result, the P-wave velocity is of the order: Solids > Liquids > Gases.
• When these waves penetrate the atmosphere, they take the shape of sound waves.
• In earthquakes, the P-wave velocity ranges from 5 to 8 km/s.
• The precise speed varies depending on the region of the Earth's interior, ranging from less than 6 kilometres per second in the crust to 13.5 kilometres per second in the lower mantle and 11 kilometres per second in the inner core.

S-waves

• They are slower than P-waves and arrive at the recording instruments after P-waves.
• S-waves are shear or transverse waves, which means the ground particles move perpendicular to the direction of wave propagation.
• As a result, they leave troughs and crests in the material they move through (they distort the medium).
• They resemble water ripples or light waves in appearance.
• They can only travel through solid materials. This characteristic of S-waves has provided critical evidence that the Earth's outer core is liquid because S-waves do not pass through it.
• They cause more intense shaking than P-waves, moving the ground up and down and side-to-side and are generally responsible for damage and destruction during earthquakes.

S-waves - Significance

• When compared to P-waves, these waves have a greater frequency and a somewhat larger destructive force.
• The earth's surface trembles from side to side as a result of these waves (horizontal).
• Fluids (liquids and gases) do not tolerate shear stresses, hence S-waves cannot flow through them.
• They move through the solid component of the Earth at variable speeds (proportional to shear strength).

Difference Between S-Waves and P-Waves

Shadow Zone

• The shadow zone is the area of the earth between 105° and 145° angular distance from an earthquake that does not receive any direct P waves.
• P waves are refracted by the liquid core, while S waves are entirely halted by the liquid core, resulting in the shadow zone.
• The shadow zone for both wave types was identified as a zone between 105° and 145° from the epicenter.
• S-waves are not received in the zone beyond 105°.
• S-wave shadow zones are bigger than P-wave shadow zones.
• The P-wave shadow zone appears as a ring around the globe, ranging from 105° to 145° out from the epicenter.

P-wave Shadow zone

S-wave Shadow zone

Conclusion

Body waves provide crucial insights into the Earth's internal layers. Their interaction with the Earth's internal materials helps create accurate models of the Earth's structure.