Earthquakes generate vibrations known as seismic waves that travel deep beneath the surface. These waves act as natural probes, carrying information about the Earth’s interior to seismographs across the globe. By studying how these waves move and what materials they pass through, scientists map the structure of our planet, from the crust to the core. P-waves and S-waves have distinct motions that reveal the Earth’s composition.
Understanding Longitudinal and Transverse Waves
Waves are categorized by how the material they travel through moves relative to the direction the energy propagates. A longitudinal wave is characterized by particle motion that is parallel to the direction of wave travel. Pushing and pulling a Slinky causes the coils to compress and stretch along the same path the energy moves. These areas of compression and stretching are known as compressions and rarefactions.
A transverse wave involves particle motion that is perpendicular to the direction the wave is propagating. Shaking a rope up and down causes the wave to travel horizontally while the rope segments move vertically. This motion creates peaks (crests) and valleys (troughs). The material’s ability to resist a change in shape, or its shear strength, allows it to transmit this shearing motion.
Defining S-Waves and Their Motion
S-waves are transverse waves. S-waves (Secondary or Shear waves) are named because they arrive at seismograph stations after the faster P-waves (Primary waves). The particle motion of an S-wave is a distinct shearing action where the material oscillates perpendicular to the path of energy transfer.
This side-to-side or up-and-down shaking is the S-wave’s characteristic motion. Unlike longitudinal P-waves, S-waves require the medium to have rigidity to propagate. P-waves travel through solids, liquids, and gases because they rely on volume changes. S-waves travel roughly 1.7 times slower than P-waves but often cause more damage at the surface during an earthquake due to their shearing motion.
Practical Implications of S-Wave Behavior
The transverse nature of S-waves affects how scientists study the Earth’s internal structure. Because their motion is shearing, S-waves can only travel through materials that possess shear strength, which includes all solids. Liquids and gases lack this rigidity and cannot resist the change in shape required to propagate the wave’s perpendicular motion. When an S-wave encounters a liquid, it is blocked or absorbed.
Seismologists utilize this limitation to map the planet’s layers. When an earthquake occurs, a large region opposite the epicenter receives no direct S-waves. This area, known as the S-wave shadow zone, proved that the Earth’s outer core must be liquid. The inability of S-waves to pass through this deep layer provides evidence for the physical state of the Earth’s interior.