Seismic waves are energy waves that travel through the Earth, typically generated by a sudden movement, such as an earthquake. This energy radiates outward from the source, causing the ground to shake. Seismic energy is a composite of different wave types, each with unique motion characteristics. These waves are fundamentally divided based on the direction of particle movement relative to the wave’s travel path.
Defining Transverse and Longitudinal Motion
The distinction between wave types rests on how the material particles move as the energy passes through. A transverse wave is one where the particles of the medium oscillate perpendicular to the direction the energy is propagating.
A longitudinal wave involves particle motion that is parallel to the direction of wave travel. The particles move back and forth in the same direction the wave is moving, creating sections of compression and expansion.
Primary Waves (P-waves) and Compression
Primary waves, or P-waves, are a type of body wave that travels through the Earth’s interior and is the fastest seismic wave recorded. P-waves are purely longitudinal waves, meaning their motion is compressional. They alternately push and pull the material they move through, creating areas of high pressure (compression) and low pressure (rarefaction).
This motion allows P-waves to travel through any medium, including solids, liquids, and gases. In the Earth’s crust, they travel at speeds ranging from approximately 5 to 8 kilometers per second. P-waves are always the first to arrive at a seismograph station following an earthquake.
Secondary Waves (S-waves) and Shear Motion
Secondary waves, or S-waves, are the second type of body wave and are fundamentally transverse waves. They are often called shear waves because they move the material perpendicular to the wave’s direction of travel, causing a shearing motion. The particles oscillate up and down or horizontally.
S-waves cannot travel through liquids or gases because these materials lack the necessary shear strength. Consequently, S-waves are blocked by the Earth’s liquid outer core, which helps map the planet’s internal structure. S-waves are slower than P-waves, traveling at roughly 60% of the P-wave velocity in the same material.
The Unique Characteristics of Surface Waves
When P and S waves reach the surface, they generate a third class of seismic energy called surface waves. These waves travel along the surface, are slower than body waves, but often cause the most significant shaking and damage. Surface waves are categorized into two main types: Love waves and Rayleigh waves.
Love waves are purely transverse, causing the ground to move horizontally and side-to-side perpendicular to the wave’s path. This motion is a form of horizontal shear confined to the surface layers. Love waves are typically the fastest of the two surface wave types.
Rayleigh waves exhibit a complex rolling motion, similar to waves on the surface of water. Their movement combines both transverse (vertical) and longitudinal (horizontal) components, resulting in an elliptical path for the ground particles. This complex motion contributes to the destructive nature of Rayleigh waves.
How Seismologists Use Wave Differences
The distinct travel speeds and properties of P and S waves provide seismologists with tools to locate and analyze earthquakes. Since P-waves are faster than S-waves, the time difference between their arrival at a seismic station, known as the S-P interval, increases with distance from the earthquake’s origin. By measuring this time difference at three or more stations, scientists can pinpoint the exact location of the earthquake’s epicenter through triangulation.
The behavior of S-waves is also used to determine the physical state of the Earth’s deep interior. The observation that S-waves cannot pass through the outer core confirms that this layer is liquid, as only solids can transmit shear waves. This difference in wave behavior is fundamental to understanding the composition and structure of the deep Earth.