S-waves (secondary waves) are seismic body waves used to study the Earth’s interior. They are shear waves, transmitting energy by causing particles to oscillate sideways, perpendicular to the direction of travel. S-waves cannot propagate through ideal liquids or gases because these states of matter lack the physical property necessary for shear motion. This inability to pass through fluids makes S-wave behavior a powerful tool for understanding planetary interiors.
How S-Waves and P-Waves Move
P-waves (primary waves) are compressional waves that move by pushing and pulling the material in the same direction as the wave’s travel path. This “push-pull” motion is similar to how sound travels, causing alternating regions of compression and expansion in the medium. Since liquids and gases can be compressed, P-waves can travel through solids, liquids, and gases, although their speed changes significantly depending on the medium.
S-waves are transverse waves, shaking the material at right angles to the direction of wave propagation. This shearing motion requires the medium to resist a change in shape and rapidly spring back to its original configuration. The velocity of an S-wave is typically around 60% of the P-wave speed in the same material, which is why S-waves always arrive second at a seismic station.
The Requirement of Shear Strength in Materials
The fundamental difference in particle motion explains why S-waves are blocked by liquids while P-waves are not. S-waves depend entirely on a material’s rigidity, or its ability to withstand and recover from a shearing force. This property is mathematically represented by the shear modulus of the material. Solids, such as rock and metal, possess high rigidity and shear strength, allowing them to effectively transmit the side-to-side motion of an S-wave.
Liquids and gases have effectively zero shear strength or rigidity. When a shearing force is applied to a fluid, it simply flows or deforms permanently instead of resisting the change and snapping back. Because the S-wave’s motion relies on the medium’s elastic recoil, the wave cannot propagate without rigidity; the shear energy is dissipated, and the wave velocity becomes zero.
The inability to transmit S-waves is a defining physical characteristic of a fluid state. While P-waves rely on the bulk modulus (resistance to compression), S-waves rely on the shear modulus, making them highly sensitive to the phase of the material they encounter. This physical principle allows seismologists to distinguish between solid and liquid layers deep within the Earth.
Using S-Wave Behavior to Study Earth’s Interior
Seismologists use the S-wave’s inability to travel through liquid to map the structure of the Earth’s deep interior. When an earthquake occurs, S-waves travel through the solid mantle until they encounter the core-mantle boundary, where they are completely stopped by the molten outer core. This blockage creates a vast region on the opposite side of the planet, known as the S-wave shadow zone, where no direct S-waves are detected.
This shadow zone begins at an angular distance of about 103 degrees from the earthquake’s epicenter. The existence and size of this shadow zone provided the first compelling evidence that the Earth’s outer core is in a liquid state, composed primarily of molten iron and nickel.