Seismic waves are the mechanical vibrations that carry energy released by an earthquake or an explosion through the Earth. The rate at which this energy travels is highly variable, changing constantly as the waves encounter different materials deep beneath the surface. The specific velocity of a seismic wave depends entirely on the physical properties of the medium it is passing through. This variability allows scientists to use the waves as a tool to map the planet’s internal structure.
The Two Primary Wave Types
The energy from an earthquake radiates outward in the form of body waves, which travel through the Earth’s interior. The two main types are Primary (P) waves and Secondary (S) waves, distinguished by their motion and speed difference. P-waves are compressional waves that move by pushing and pulling the material in the same direction the wave is traveling, similar to how sound waves move through air. These waves are the fastest, traveling at speeds ranging from approximately 1.5 kilometers per second in loose surface materials to over 8 kilometers per second in the deep crust and upper mantle.
S-waves are shear waves that move particles perpendicular to the direction of wave propagation, causing a side-to-side or up-and-down shaking motion. They are significantly slower than P-waves, traveling at roughly 60% of the P-wave speed in any given solid material. Near the surface, S-waves generally travel between 1.0 and 4.5 kilometers per second. This difference in velocity is a fundamental principle used in seismology.
Factors Determining Wave Velocity
A seismic wave’s speed is dictated by the physical characteristics of the rock it propagates through, primarily density, pressure, temperature, and state of matter. Higher density and increased pressure, which both rise with depth in the Earth, lead to faster wave speeds. The immense pressure deep within the Earth compacts the rock material, increasing its rigidity and allowing waves to transmit energy more efficiently.
Temperature, however, tends to have the opposite effect; increasing heat can slightly reduce a rock’s rigidity and slow wave velocity. The state of matter is a crucial factor, particularly for S-waves. Because S-waves require a medium that can resist a change in shape, they are unable to travel through any liquid medium. This inability of shear waves to pass through liquids is a key indicator used to determine the phase of Earth’s internal layers.
Speed Through Earth’s Layers
The application of these physical principles results in dramatic shifts in speed as waves cross the boundaries between the Earth’s major layers. In the shallow crust, P-waves travel at 6 to 7 kilometers per second. Their speed increases abruptly upon crossing the Mohorovičić discontinuity (Moho) into the solid upper mantle, where P-wave velocities jump to around 8 kilometers per second. Through the bulk of the mantle, increasing pressure causes P-wave speeds to steadily climb, reaching up to 10.4 kilometers per second just above the core-mantle boundary.
The most profound change occurs at the boundary with the Outer Core, which is composed of liquid iron and nickel. Here, S-waves stop entirely because the liquid cannot support the necessary shearing motion. P-waves, which can travel through liquid, slow down sharply, dropping from over 10 km/s in the lower mantle to about 8 kilometers per second upon entering the Outer Core. As P-waves continue inward, they speed up again at the boundary between the liquid Outer Core and the solid Inner Core, reaching velocities of up to 11 kilometers per second in the planet’s center.
Using Time and Speed to Locate Earthquakes
The reliable difference in speed between the two body waves provides seismologists with a precise method for calculating the distance to an earthquake’s source. P-waves always arrive first at a seismic station, followed by the slower S-waves. The time difference between the arrival of the faster P-wave and the slower S-wave is known as the S-P interval.
The duration of this S-P interval is directly proportional to the distance the waves have traveled from the earthquake’s origin, meaning a longer interval indicates a greater distance. By using established travel-time curves, scientists convert this measured time delay into a distance from the recording station to the epicenter. To pinpoint the exact location of the earthquake, data from at least three different seismic stations are required. Circles are drawn around each station with a radius equal to the calculated distance, and the single point where all three circles intersect is the earthquake’s epicenter.