Seismic Waves and Earth’s Interior

Introduction

• Seismograms show vibrations indicating ground movement by seismic waves.
• Seismic waves carry energy from the source outward in all directions, similar to ripples in water from a stone.
• Earthquakes emit various seismic waves: P (Primary), S (Secondary), Love, and Rayleigh waves.

Types of Seismic Waves

Body Waves

1. P-Waves (Primary Waves)

   • Fastest seismic waves, traveling at 1-14 km/sec.
   • Called compressional waves, causing volume change in the direction of travel.
   • Can travel through solids, liquids, and gases.

2. S-Waves (Secondary Waves)

   • Slower than P-waves, speed ranges from 1-8 km/sec.
   • Called shear waves; vibrate ground perpendicular to travel direction.
   • Cannot propagate through fluids or gases.

Surface Waves

1. Love Waves

   • Transverse waves vibrating ground horizontally, perpendicular to travel direction.
   • Speeds between 2-6 km/sec; amplitude decreases with depth.

2. Rayleigh Waves

   • Slowest waves, with speed 1-5 km/sec.
   • Cause elliptical movement in particles, decreasing in amplitude with depth.

Wave Travel Times and Speeds

• Travel time = distance/seismic wave speed.
• Arrival time is absolute, while travel time is relative.
• Wave speed depends on rock composition, temperature, and pressure.
  • Pressure increases speed; temperature decreases speed.

Using Seismic Waves to Locate Earthquakes

• P and S waves travel at different speeds; this difference helps locate earthquakes.
• Distance from the epicenter can be estimated by examining the arrival time gap between P and S waves.
• Three or more seismometer readings help triangulate the earthquake's epicenter.

Interaction with Earth's Interior

• Seismic waves interact with Earth’s subsurface, exhibiting refraction, reflection, dispersion, diffraction, and attenuation.

Refraction and Reflection

1. Refraction

   • Wave paths bend due to velocity changes at material boundaries.
   • Velocity generally increases with depth in Earth.

2. Reflection

   • Part of the wave energy reflects at material boundaries (echoes).
   • Used to explore Earth’s structure and locate petroleum.

Seismic Wave Propagation

• Differences in wave speed influence the order of arrival on seismograms.
• Fastest waves (P-waves) arrive first, followed by S-waves, then surface waves.

Earth’s Interior Structure

• Earth’s interior includes core, mantle, and crust, studied via seismic wave characteristics.
• The Preliminary Earth Reference Model (PREM) helps understand Earth’s structure.
• Pressure and composition diversely affect wave velocity with depth.

Models of Earth's Heterogeneity

• Tomography reveals lateral variations in seismic wave speeds within Earth’s layers.
• Surface tectonics correlate with shallow features, not with deeper layers.

Summary

• Seismic waves reveal much about Earth’s interior structure.
• Variations in wave speed help map Earth’s layers and locate earthquakes.