Model Answer
0 min readIntroduction
Seismic waves are vibrations that travel through Earth carrying the energy released during an earthquake, volcanic eruption, or even human-induced explosions. These waves are the primary means by which we understand the Earth’s internal structure. Generated at the focus (hypocenter) of an earthquake, they radiate outwards in all directions, causing ground motion at the Earth’s surface. Understanding the nature and effects of these waves is crucial for seismic hazard assessment and mitigation, particularly in earthquake-prone regions like the Himalayan belt and coastal areas vulnerable to tsunamis.
Types of Seismic Waves
Seismic waves are broadly classified into two main categories: Body Waves and Surface Waves.
Body Waves
Body waves travel through the Earth’s interior. They are further divided into:
- P-waves (Primary Waves): These are compressional waves, meaning they cause particles to move in the same direction as the wave is traveling. They are the fastest seismic waves and can travel through solids, liquids, and gases. Their velocity varies with density and elasticity of the medium.
- S-waves (Secondary Waves): These are shear waves, causing particles to move perpendicular to the direction of wave propagation. S-waves can only travel through solids, as liquids and gases do not support shear stress. This property is crucial in determining the Earth’s internal structure, specifically the liquid outer core.
Surface Waves
Surface waves travel along the Earth’s surface. They are generally slower than body waves but cause more damage due to their larger amplitudes.
- Love Waves: These are horizontally polarized shear waves that travel along the surface. They are faster than Rayleigh waves and cause horizontal ground motion.
- Rayleigh Waves: These waves exhibit a rolling motion, similar to ocean waves. They are slower than Love waves and cause both vertical and horizontal ground motion.
The following table summarizes the key differences between these wave types:
| Wave Type | Mode of Propagation | Velocity | Medium | Damage Potential |
|---|---|---|---|---|
| P-wave | Compressional | Fastest | Solid, Liquid, Gas | Low |
| S-wave | Shear | Slower than P-wave | Solid only | Moderate |
| Love Wave | Horizontal Shear | Faster than Rayleigh wave | Surface | High |
| Rayleigh Wave | Rolling Motion | Slowest | Surface | High |
Effects of Seismic Waves
Seismic waves cause a range of effects, depending on their intensity and the geological conditions of the affected area:
- Ground Shaking: The most immediate effect, causing damage to structures. The intensity of shaking is measured using scales like the Modified Mercalli Intensity Scale.
- Landslides: Ground shaking can trigger landslides, especially in mountainous regions.
- Liquefaction: In saturated, loose soils, shaking can cause the soil to lose its strength and behave like a liquid, leading to building collapse.
- Tsunamis: Underwater earthquakes can generate tsunamis, which are large ocean waves that can cause widespread coastal flooding and destruction. The 2004 Indian Ocean tsunami is a prime example.
- Fault Rupture: Visible displacement of the ground surface along the fault line.
Conclusion
Seismic waves are fundamental to understanding Earth’s dynamics and pose significant hazards. Their diverse characteristics – velocity, propagation mode, and amplitude – dictate the extent of damage caused by earthquakes. Continued research into seismic wave behavior, coupled with improved building codes and early warning systems, is crucial for mitigating the risks associated with these natural phenomena, particularly in seismically active zones. Understanding the interplay between wave types and geological conditions is paramount for effective disaster preparedness.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.