A seismic wave is a surge of energy that travels through the Earth’s layers, typically originating from an earthquake, volcanic eruption, or explosion. These waves carry information about the planet’s interior and are detected by seismographs worldwide. Among the different types of seismic energy, the fastest traveling wave is the Primary wave, commonly known as the P-wave.
Primary (P) Waves and Their Speed Advantage
Primary waves earn their name because they are the first to arrive at any seismic station after an event. The P-wave motion is compressional, causing the material to oscillate back and forth in the same direction the wave is propagating, similar to how a sound wave travels. This push-and-pull motion is also described as longitudinal.
This specific mode of motion allows P-waves to travel through any medium—solids, liquids, and gases—because all materials can be compressed. P-waves are significantly faster than other seismic waves, with typical speeds ranging between 5 and 8 kilometers per second near the Earth’s surface. In any given material, P-waves are approximately 1.7 times faster than the next fastest wave type, the Secondary wave.
The speed advantage stems from the fact that P-wave velocity depends on the material’s resistance to both compression and shear deformation. P-waves utilize the material’s incompressibility (bulk modulus) for transmission, a property present in all states of matter, allowing them to achieve the highest velocity. For example, in granite, P-waves can travel at speeds around 5,000 meters per second.
Secondary (S) and Surface Waves
Following the P-waves are the slower Secondary waves, or S-waves, which travel through the Earth’s interior. S-waves move the material in a shearing motion, where the particles oscillate perpendicular to the direction the wave is traveling. This side-to-side or up-and-down motion requires the medium to have rigidity, or shear strength, to propagate the wave.
Because S-waves rely on a medium’s rigidity, they cannot travel through liquids or gases, which lack shear strength. This inability to travel through fluids is a key distinction, as the absence of S-waves in the Earth’s outer core provided evidence that this layer is liquid. Typical S-wave speeds are substantially lower than P-waves, often traveling at only 60% of the P-wave velocity in the same material.
The slowest seismic waves are the Surface waves, which are confined to traveling along the Earth’s surface and near-surface layers. These waves are categorized into two main types: Love waves and Rayleigh waves. Love waves cause a horizontal, side-to-side motion of the ground, while Rayleigh waves produce a rolling or ocean-like motion that is responsible for most of the shaking felt during an earthquake.
Surface waves travel slower than both P-waves and S-waves because they follow a more complex path along the boundary layers. Rayleigh waves are typically the slowest of all, with speeds roughly 90% of the S-wave velocity. Even though they are the slowest, surface waves are often the most destructive due to their high amplitude and prolonged shaking motion.
Factors Governing Wave Velocity
The speed of any seismic wave is fundamentally determined by the physical properties of the material it is passing through. Two properties are most influential: the material’s density and its elastic properties, specifically the bulk modulus and the shear modulus. The bulk modulus measures a material’s resistance to compression, which governs the P-wave speed.
The shear modulus measures a material’s resistance to shearing, which controls the S-wave speed. Higher values for these moduli, corresponding to greater incompressibility and rigidity, result in faster wave velocities. Although density is a factor, denser rocks are typically harder and more rigid, meaning their elastic moduli increase at a faster rate, leading to faster wave speeds in deep, solid rock.
The mathematical relationship ensures that the compressional motion of a P-wave, which utilizes both the bulk and shear properties of the material, will always yield a faster result than the S-wave, which is governed only by the shear modulus. This dependency on the medium’s elastic properties explains the velocity hierarchy: P-waves are fastest, followed by S-waves, and then the Surface waves.