Earthquakes release immense amounts of energy that travel through the planet as seismic waves. These mechanical vibrations propagate away from the earthquake’s focus, or hypocenter, the point of rupture within the Earth. The specific properties of these waves, including their speed and how they move the material they pass through, determine their potential to cause damage at the surface. Understanding the distinct characteristics of these body waves is the first step toward appreciating the forces that cause structural failure during a seismic event.
Understanding Primary Wave Movement
The first seismic wave type to arrive at any location is the Primary wave, or P-wave, due to its superior speed through the Earth’s interior. P-waves travel by compressing and expanding the material in the same direction the wave is moving, a motion described as longitudinal or compressional. P-waves can propagate through solids, liquids, and gases, making them the fastest-traveling seismic waves within the Earth’s crust and mantle.
When P-waves reach the surface, they typically cause a sudden, sharp jolt or a vertical up-and-down motion. This initial shaking is often felt as a quick, low-amplitude disturbance and serves as a natural warning signal before the more intense shaking begins. Though they can create loud, booming noises, the forces they exert are generally well-tolerated by most structures. Buildings are inherently strong in compression, meaning they are designed to withstand significant weight pressing down on them, which limits the destructive potential of the P-wave’s push-pull action.
Understanding Secondary Wave Movement
Following the initial jolt of the P-waves, the Secondary waves, or S-waves, arrive, bringing a different and more violent type of ground motion. S-waves are slower than P-waves, traveling at speeds about 1.7 times less than their primary counterparts. The defining characteristic of S-waves is their shear or transverse motion, where the ground vibrates perpendicular to the direction the wave is traveling. This motion can manifest as either a horizontal side-to-side shake or a vertical up-and-down oscillation.
S-waves are unique because their propagation requires the medium to resist a change in shape, a property absent in fluids. Consequently, S-waves cannot travel through liquids, such as the Earth’s outer core. They are responsible for significantly more intense ground shaking than P-waves. This lateral and vertical oscillation introduces significant stress into the foundations and frameworks of buildings.
Why Shear Motion Causes Greater Destruction
The difference in motion between the two body waves explains why S-waves are the more destructive force. Structural materials like concrete and masonry are engineered to handle immense compressional loads, making them highly resistant to the push-pull forces of P-waves. However, these materials are much weaker when subjected to shear stress, the twisting and lateral force exerted by S-waves. The side-to-side motion of S-waves attempts to rip a structure apart by forcing different parts of the foundation to move in opposite directions.
This lateral stress can cause columns to buckle, walls to crack diagonally, and structural connections to fail completely. The intense, high-amplitude shear motion forces the entire building to sway and twist, often exceeding the elastic limits of the construction materials. The massive shear stress delivered by S-waves is the primary mechanism that leads to critical structural damage and eventual collapse. While S-waves are the most damaging waves traveling through the Earth’s interior, the surface waves they generate upon reaching the ground often cause the greatest total destruction.