Seismic waves are vibrations that travel through Earth’s layers, typically originating from sudden energy releases such as earthquakes, volcanic eruptions, or even large landslides and man-made explosions. Scientists study these waves to understand Earth’s internal structure and the processes that cause seismic events.
Primary and Secondary Waves
Seismic energy initially propagates through Earth’s interior as body waves, which consist of two main types: Primary (P) waves and Secondary (S) waves. P waves are compressional waves that move by pushing and pulling the material they travel through, similar to how sound waves propagate. They are the fastest seismic waves, arriving first at monitoring stations, and can travel through solids, liquids, and gases. P waves can reach speeds of approximately 1 to 14 kilometers per second, depending on the material’s density and stiffness.
S waves, also known as shear waves, arrive after P waves due to their slower speed, typically around 60% of the P wave velocity in any given material. These waves cause particles to move perpendicular to the direction of wave propagation, shaking the ground side to side or up and down. A distinguishing characteristic of S waves is their inability to travel through liquids or gases, as these mediums do not support the necessary shear stresses. This property is significant for understanding Earth’s liquid outer core, as S waves cannot pass through it.
Surface Waves: The Slowest Seismic Waves
Following the arrival of body waves, surface waves travel along the Earth’s surface. They are the slowest seismic waves. These waves are confined to the outer layers of the Earth and generally have lower frequencies compared to body waves. There are two primary types of surface waves: Love waves and Rayleigh waves.
Love waves, named after British mathematician A.E.H. Love, cause horizontal shifting of the ground perpendicular to the direction of wave travel. Their motion is entirely horizontal, and their amplitude is greatest at the surface, diminishing with depth. Rayleigh waves, named after Lord Rayleigh, exhibit a more complex rolling motion, similar to waves on the ocean. They cause the ground to move both up and down and forward and backward in an elliptical pattern. They are slower because they interact with the complex topography and varied materials of the Earth’s crust, and their energy disperses over a larger area as they travel.
Why Surface Waves Are So Damaging
Despite being the slowest to arrive, surface waves are responsible for the most significant ground shaking and structural damage during an earthquake. Their particle motion is larger than that of body waves, meaning they displace the ground more substantially. This larger amplitude, combined with their lower frequency, allows them to impart greater energy to structures.
Surface waves also have a prolonged duration, lasting longer than P or S waves, which extends the period of intense shaking. The horizontal motion of Love waves can be particularly destructive to building foundations, as many structures are not designed to withstand severe sideways forces. Similarly, the rolling motion of Rayleigh waves causes both horizontal and vertical displacement, further stressing buildings. The energy of surface waves is concentrated at the Earth’s surface, contributing to their destructive potential compared to body waves that spread out through the Earth’s volume.