When an earthquake occurs, the energy it releases travels through the planet as seismic waves. The speed of this energy is highly variable, constantly changing depending on the material it is passing through. Tracking these variations in speed is the foundation of modern seismology, providing a non-invasive way to study the planet’s deep structure. Wave velocity can range from as slow as 1.5 kilometers per second in loose, shallow sediment to over 13 kilometers per second deep within the mantle. This variability is governed by the physical properties of the Earth’s layers, which change with depth and composition.
The Two Primary Types of Seismic Waves
The energy released from an earthquake travels through the Earth’s interior as two main types of body waves: Primary waves (P-waves) and Secondary waves (S-waves). P-waves are the fastest traveling waves and are always the first to arrive at a seismograph station. Their motion involves compressing and expanding the material in the direction the wave is traveling, similar to how sound moves through the air. P-waves can transmit energy through solids, liquids, and gases because all these states of matter resist changes in volume. In the Earth’s crust, P-waves typically travel at speeds between 6 and 7 kilometers per second.
S-waves travel more slowly and are the second waves to be recorded. Their motion involves a shearing or shaking movement that is perpendicular to the direction of wave travel. Because S-waves rely on a material’s rigidity to propagate, they can only move through solid matter. They are unable to travel through liquids or gases because fluids lack the shear strength necessary to spring back into shape. S-waves are generally about half the speed of P-waves in the same material, typically moving around 3.5 kilometers per second in the crust. The time difference between the arrival of the P-wave and the S-wave indicates the distance to the earthquake’s origin.
Factors Influencing Wave Velocity
The speed of a seismic wave is controlled by the elastic properties and density of the rock it passes through. The most influential factors are pressure and temperature deep within the Earth. As depth increases, the weight of the overlying rock causes pressure to rise significantly, making the rock more compact and rigid. This greater rigidity allows seismic waves to travel faster, leading to a general increase in velocity with depth throughout the crust and mantle.
Temperature works in opposition to pressure by decreasing wave speed. Hotter rock is less rigid and less dense, which slows transmission. In most of the mantle, the effect of increasing pressure is dominant, causing wave speeds to climb steadily deeper into the planet. P-wave velocity, for example, increases from about 8 kilometers per second at the top of the mantle to over 13 kilometers per second near the core-mantle boundary.
Dramatic changes in speed occur at boundaries where the material’s composition or state changes abruptly. The most significant boundary is the transition from the solid mantle to the liquid outer core at a depth of about 2,900 kilometers. As P-waves cross into this liquid layer, their velocity drops sharply from around 13.7 kilometers per second to about 8 kilometers per second. S-waves completely disappear at this boundary, confirming that the outer core is a fluid layer that lacks the rigidity to support shear motion.
Mapping Earth’s Interior Using Wave Speeds
Seismologists use the varied speeds of P-waves and S-waves to map the planet’s hidden layers. By tracking the arrival times of these waves at seismic stations across the globe, scientists calculate the paths the waves have taken through the Earth. When a wave encounters a boundary between two different layers, such as the crust and the mantle, it can be reflected (bouncing back toward the surface) or refracted (bending as it passes through the interface).
Analyzing these refractions and reflections allows researchers to locate the depths of major internal discontinuities. For instance, the Mohorovičić discontinuity, or Moho, is identified by a sudden jump in P-wave velocity. This jump marks the boundary between the less dense crust and the denser mantle. Travel-time curves are used to measure these velocity changes.
The discovery that S-waves vanish when entering the outer core was the defining evidence that this layer is liquid. The observation that P-waves speed up again upon reaching the inner core, at a depth of about 5,150 kilometers, provides evidence that the inner core is solid. Variations in earthquake energy speed act as a planet-scale diagnostic imaging system, revealing the composition and physical state of layers thousands of kilometers beneath the surface.