Overview
This lecture explains seismic waves, their classifications, properties, and effects on the Earth's surface, focusing on Body Waves (P and S waves) and Surface Waves (Love and Rayleigh waves).
Classification of Seismic Waves
- Seismic waves are classified into Body Waves and Surface Waves.
- Body Waves travel through the Earth's interior in all directions and depths.
- Surface Waves travel only along the Earth's surface and do not go deep.
Body Waves
- Body Waves are divided into Primary (P) Waves and Secondary (S) Waves.
Primary Waves (P Waves)
- Also called Primary, Push-Pull, Longitudinal, or Compressional waves.
- P Waves propagate by compressing and dilating the ground in the direction of travel.
- They are the first waves to arrive at a seismic station during an earthquake.
- Typical speeds are 8 to 13 km/s.
- P Waves can travel through solids, liquids, and gases.
- Velocity formula for P Waves involves Young's modulus (E), Poisson's ratio (B), and density (ρ).
Secondary Waves (S Waves)
- Also called Shear, Secondary, or Transverse waves.
- S Waves arrive after P Waves; they are about 1.7 times slower.
- They travel in a transverse (perpendicular) direction to wave propagation.
- S Waves travel only through solids, as they require shear strength.
- The ground is displaced perpendicular to wave direction.
- S Waves’ velocity formula is similar to P Waves.
Surface Waves
- Surface Waves are confined to the Earth's surface layers (crust).
- They are slower than Body Waves (speed: 4 to 5 km/s).
- Two main types: Love Waves and Rayleigh Waves.
Love Waves
- Cause horizontal shearing of the ground, similar to a snake’s motion.
- Ground moves perpendicular to wave motion when viewed from above (plan view).
- Love Waves are very destructive.
Rayleigh Waves
- Cause both horizontal and vertical rolling movements of the ground.
- Produce a rolling motion that is highly destructive.
Key Terms & Definitions
- Seismic Waves — vibrations caused by earthquakes, classified as Body Waves or Surface Waves.
- Body Waves — seismic waves that travel through the Earth’s interior (P and S waves).
- P (Primary) Waves — fastest waves, compress and expand the ground, travel through solid, liquid, and gas.
- S (Secondary) Waves — slower than P waves, move ground perpendicular to direction, travel only through solids.
- Surface Waves — seismic waves confined to the Earth's crust (Love and Rayleigh waves).
- Love Waves — surface waves causing horizontal ground movement.
- Rayleigh Waves — surface waves causing rolling ground motion.
Action Items / Next Steps
- Review previous lectures on elastic rebound theory, plate tectonics, continental drift, and internal earth structure.
- Watch provided animations for wave motion understanding.
- Comment with questions if any clarification is needed.