When an earthquake occurs, the sudden release of stored energy sends vibrations, known as seismic waves, traveling through the Earth’s interior and along its surface. These waves propagate the energy away from the rupture point, causing the ground shaking we feel. Seismic waves are categorized based on their distinct motion and speed. The varying velocities dictate the order in which they arrive at a recording station, which is fundamental for understanding the planet’s structure and locating earthquakes.
Primary Waves and Maximum Velocity
The fastest seismic waves are Primary waves (P-waves), which are the first to be recorded by seismographs after an earthquake. As body waves, they travel through the Earth’s interior. P-waves move through rock via compression and dilation, creating a push-pull motion that vibrates particles in the same direction as the wave travels. This longitudinal motion is the most efficient way to transmit energy, allowing P-waves to achieve maximum velocity.
The speed of P-waves ranges from 1.5 to 8 kilometers per second (km/s) in the crust and up to 14 km/s deep within the mantle. Because their motion changes the volume of the material, P-waves can propagate through any medium, including solids, liquids, and gases. Their ability to travel through the liquid outer core is a defining characteristic.
Secondary Waves and Shear Motion
Secondary waves (S-waves) arrive second at a seismic station. Like P-waves, S-waves are body waves, but their motion is distinctly different and less efficient for energy transfer. S-waves are shear waves, moving rock particles perpendicular to the direction the wave propagates, causing side-to-side or up-and-down shaking. This transverse motion requires the medium to resist a change in its shape.
This dependence on a material’s shear strength is why S-waves are slower, typically moving at about 60 to 70 percent of the speed of P-waves. Crucially, S-waves cannot travel through liquids or gases because these fluids lack the necessary shear strength. The inability of S-waves to pass through the liquid outer core was key evidence used by seismologists to determine the core’s physical state.
Surface Waves and Destructive Speed
The final category of seismic energy to arrive are surface waves, which are the slowest because they are confined to the outer layers of the Earth. These waves travel along the surface boundary, concentrating their energy near human structures. Surface waves are divided into two types: Love waves and Rayleigh waves.
Love waves move the ground in a purely horizontal, side-to-side motion. Rayleigh waves create a rolling, elliptical motion that combines both vertical and horizontal movement, similar to an ocean wave. Despite their slower speed, surface waves are responsible for the vast majority of structural damage during an earthquake. Their confinement to the surface allows them to maintain a larger amplitude and causes a prolonged shaking duration compared to body waves.
Using Arrival Times to Locate Earthquakes
The difference in speed between the body waves is a practical tool seismologists use to pinpoint the origin of an earthquake. The time interval between the arrival of the P-wave and the subsequent arrival of the S-wave is known as the S-P interval. This interval is directly proportional to the distance between the seismic recording station and the earthquake’s epicenter.
A shorter S-P interval indicates a closer earthquake, while a longer interval means the station is farther away. By measuring the S-P interval on seismograms from at least three different stations, seismologists calculate the distance to the epicenter from each station. The precise location of the earthquake’s epicenter is marked by the intersection point when a circle is drawn around each station with a radius equal to the calculated distance.