Seismic waves are both longitudinal and transverse, depending on the type. An earthquake generates several distinct wave types, and they move the ground in fundamentally different ways. The two main body waves, P-waves and S-waves, represent textbook examples of longitudinal and transverse motion, respectively. Surface waves add further complexity with elliptical and side-to-side patterns.
P-Waves: The Longitudinal Ones
Primary waves, or P-waves, are longitudinal. They compress and expand the ground back and forth in the same direction the wave is traveling, exactly like sound waves moving through air. As a P-wave passes through rock, particles squeeze together (compression) and then spread apart (dilation) in a repeating cycle. Once the wave moves on, the material returns to its original shape.
P-waves are the fastest seismic waves. In Earth’s upper crust, they travel at roughly 5.5 km/s, speeding up to about 6.9 km/s in the lower crust and exceeding 8 km/s in the upper mantle. Because they arrive first at a seismograph station, they got the name “primary.” They can travel through solids, liquids, and gases, which means they pass through every layer of Earth’s interior, including the liquid outer core.
S-Waves: The Transverse Ones
Secondary waves, or S-waves, are transverse. Their particle motion is perpendicular to the direction the wave is traveling. Instead of compressing and expanding the ground, S-waves shear it, jerking rock up and down or side to side. Think of shaking a rope: the wave moves forward along the rope, but the rope itself moves at right angles to that direction.
S-waves are slower than P-waves, typically traveling at about 60% to 70% of the P-wave speed in the same material. This speed difference is what makes them “secondary,” since they always arrive after the P-waves. S-waves also come with a critical limitation: they cannot travel through liquids. Fluids have no rigidity, so they can’t sustain the shearing motion that defines a transverse wave. This is why S-waves stop entirely at Earth’s liquid outer core.
Surface Waves: A More Complex Picture
Beyond the two body waves, earthquakes also produce surface waves that travel along Earth’s outer layer rather than through its interior. These don’t fit neatly into a simple longitudinal-or-transverse label.
Love waves move particles horizontally, perpendicular to the direction the wave travels. That makes them purely transverse, similar to S-waves but confined to the surface. Rayleigh waves are more unusual: particles trace an elliptical path, rolling backward against the wave’s direction of travel. This combines both vertical and horizontal motion, so Rayleigh waves are neither purely longitudinal nor purely transverse.
Surface waves are slower than both P-waves and S-waves, but they carry more energy near the surface. They produce the largest ground displacements during an earthquake and cause the most structural damage to buildings.
How the Difference Shows Up on a Seismograph
Modern seismograph stations use three sensors oriented in different directions: one vertical and two horizontal. The design takes advantage of how longitudinal and transverse waves move the ground differently.
P-waves travel upward through Earth’s interior and arrive at a station with a strong vertical push, compressing the ground beneath the sensor. The vertical component of the seismograph picks them up clearly, while horizontal sensors barely register them. S-waves, on the other hand, arrive with a shearing side-to-side motion. They show up prominently on the horizontal components and register only weakly on the vertical one.
This separation makes it straightforward to identify each wave type on a seismogram, even during a complex earthquake.
Why the Speed Difference Matters
The fact that longitudinal P-waves travel faster than transverse S-waves is one of the most practically useful things about seismology. When an earthquake strikes, P-waves race ahead and arrive at monitoring stations first. S-waves follow seconds to minutes later, depending on distance. The gap between their arrival times tells seismologists how far away the earthquake occurred.
For example, if the P-wave and S-wave arrivals are 24 seconds apart at a given station, the earthquake’s epicenter was roughly 215 kilometers away. Repeating this measurement at three or more stations lets scientists triangulate the exact location. The entire method depends on the consistent speed difference between longitudinal and transverse waves.
How S-Waves Revealed Earth’s Liquid Core
The inability of transverse S-waves to pass through liquids gave scientists one of their biggest discoveries about Earth’s interior. When a large earthquake occurs, seismograph stations on the opposite side of the planet record P-waves but no direct S-waves at all. S-waves vanish completely beyond about 104 degrees from the earthquake’s epicenter because they cannot penetrate the liquid outer core.
P-waves also create a shadow zone between 104 and 140 degrees, where they are bent (refracted) by the liquid core and fail to reach the surface directly. But P-waves do eventually reappear beyond 140 degrees. S-waves never do. This contrast between longitudinal waves passing through liquid and transverse waves being blocked by it provided the clearest evidence that Earth has a molten outer core, a conclusion that still holds today.