Seismic Waves: How Vibrations Reveal Earth's Hidden Interior

When an earthquake strikes, a sudden release of energy sends ripples through the planet. These ripples, known as seismic waves, are mechanical waves of acoustic energy that travel through the Earth or other planetary bodies. While most commonly associated with earthquakes, these waves can also be triggered by volcanic eruptions, magma movement, large landslides, or even significant man-made explosions.

Seismologists study these vibrations using specialized instruments such as seismometers, hydrophones (for underwater detection), and accelerometers. By analyzing how these waves move, scientists can map the internal structure of our planet, much like an ultrasound allows doctors to see inside the human body.

Key Facts

Seismic waves are categorized into two main groups: body waves and surface waves.
P waves (Primary) are the fastest and can travel through solids, liquids, and gases.
S waves (Secondary) move slower and can only travel through solid materials.
The inability of S waves to pass through the Earth's outer core provides evidence that the core is liquid.
Surface waves generally travel more slowly than body waves but cause more significant ground damage.

The Two Main Categories of Seismic Waves

Seismic waves are broadly divided based on the path they take through the Earth. This distinction is fundamental to understanding how energy is distributed during a seismic event.

Body Waves

Body waves travel through the interior of the Earth. Their paths are dictated by the density and elasticity (stiffness) of the materials they encounter. Because temperature, composition, and material phase change as you go deeper, the velocity of these waves changes accordingly. This process is similar to the refraction of light as it passes through different mediums.

Surface Waves

Surface waves travel along the Earth's surface rather than through its deep interior. While they propagate more slowly than body waves, they decay more slowly with distance. Because their particle motion is larger, surface waves are typically responsible for the most intense damage during an earthquake.

Deep Dive: Body Waves (P and S Waves)

Body waves are further divided into two types based on their particle motion, a distinction first recognized by mathematician Siméon Denis Poisson in 1830.

Primary Waves (P Waves)

P waves are compressional, longitudinal waves. They act as pressure waves, meaning they push and pull the material they travel through. Because they are the fastest, they are the first to arrive at a seismograph station. P waves are highly versatile, capable of traveling through solids, liquids, and gases. For example, in air, they move at the speed of sound (roughly 330 m/s), while in granite, they can reach speeds of approximately 5000 m/s.

Secondary Waves (S Waves)

S waves are shear waves that are transverse in nature. They arrive after P waves and displace the ground perpendicular to the direction the wave is moving. A critical characteristic of S waves is that they can only travel through solids; they cannot propagate through fluids like liquids or gases because these substances do not support shear stress. This unique property is a cornerstone of geophysics: the fact that S waves disappear when hitting the Earth's outer core proves that the outer core is liquid.

Velocity of seismic waves in Earth versus depth.[1] The negligible S-wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S-wave velocity is non-zero

Comparison of Primary and Secondary Waves
Feature	P Wave (Primary)	S Wave (Secondary)
Motion Type	Compressional (Longitudinal)	Shear (Transverse)
Relative Speed	Fastest (approx. 1.7x faster than S)	Slower (approx. 60% of P wave speed)
Mediums	Solids, Liquids, and Gases	Solids only

Patterns of seismic wave travel through Earth's mantle and core. S waves can not travel through the liquid outer core, so they leave a shadow on Earth's far side. P waves do travel through the core, but P wave refraction bends seismic waves away from P wave shadow zones.

Understanding Surface Waves

Surface waves are complex and can be categorized into several specific types based on their motion and the medium through which they travel.

Rayleigh Waves

Named after Lord Rayleigh, these waves move in a manner similar to ripples on the surface of water, though the particle motion at shallow depths is typically retrograde. They are generally slower than body waves, traveling at about 90% of the S-wave velocity.

Love Waves

Named after Augustus Edward Hough Love, these are horizontally polarized shear waves. They can only exist in a layered medium and typically travel slightly faster than Rayleigh waves.

Stoneley Waves

These are boundary waves that propagate along the interface between a solid and a fluid, or sometimes between two solids. They are particularly important in borehole studies.

The scheme of motion for spheroidal 0S2 oscillation. Dashed lines give nodal (zero) lines. Arrows give the sense of motion.

Earth's Free Oscillations (Normal Modes)

When massive earthquakes occur, they can cause the entire Earth to vibrate in standing waves known as normal modes. These are the result of interference between surface waves traveling in opposite directions. These oscillations are classified into two types:

Spheroidal oscillations (S): Result from the interference of Rayleigh waves. Examples include the "breathing" mode (0S0) and the "rugby" mode (0S2).
Toroidal oscillations (T): Result from the interference of Love waves, involving a twisting motion.

The sense of motion for toroidal 0T1 oscillation for two moments of time.

Locating the Source: The Hypocenter and Epicenter

By measuring the difference in arrival times between P and S waves, scientists can calculate the distance to an earthquake. For a precise location, data from at least three different seismic stations are required. The point within the Earth where the earthquake originates is the hypocenter, while the point directly above it on the surface is the epicenter.

The hypocenter/epicenter of an earthquake is calculated by using the seismic data of that earthquake from at least three different locations. The hypocenter/epicenter is found at the intersection of three circles centered on three observation stations, here shown in Japan, Australia and the United States. The radius of each circle is calculated from the difference in the arrival times of P and S waves at the corresponding station.

Frequently Asked Questions

Why can't S waves travel through the Earth's outer core?

S waves are shear waves, which require a medium with rigidity to propagate. Because the Earth's outer core is liquid, it cannot support the shear stresses necessary for S waves to move through it.

What is the main difference between a P wave and an S wave?

The primary difference is speed and medium capability: P waves are faster and can travel through solids and fluids, whereas S waves are slower and can only travel through solids.

How do scientists use seismic waves to study the Earth's interior?

Scientists observe how seismic waves refract (bend) or reflect when they hit different layers of the Earth. By analyzing these changes in velocity and direction, they can map the density and state (solid or liquid) of various internal layers.

Which type of wave causes the most damage during an earthquake?

Surface waves, such as Rayleigh and Love waves, typically cause the most damage because they have larger particle motions and decay more slowly over distance compared to body waves.

What is the difference between an epicenter and a hypocenter?

The hypocenter is the actual location within the Earth where the seismic energy is released, while the epicenter is the point on the Earth's surface located directly above the hypocenter.