Earthquakes are sudden releases of energy within the Earth’s crust, generating vibrations that travel away from the point of rupture as seismic waves. These waves are categorized into body waves, which move through the Earth’s interior, and surface waves, which travel along the planet’s surface. The two main types of body waves are Primary waves (P-waves) and Secondary waves (S-waves).
Primary Waves: Speed and Motion
Primary waves are the fastest seismic waves generated by an earthquake, making them the first to arrive and useful for early warning systems. P-waves are compressional waves, moving ground particles back and forth parallel to the direction of travel. This push-pull motion causes a momentary change in volume in the material they pass through. P-waves generally cause minimal structural damage because their amplitude is low and their frequency is high. They can travel through solids, liquids, and gases.
Secondary Waves: Shear and Amplitude
Secondary waves (S-waves) are the second type of body wave to arrive, traveling slower than P-waves, typically at about half the speed. S-waves are shear waves that move particles perpendicular to the direction of propagation, causing side-to-side or up-and-down oscillation. This motion transfers more energy and results in a larger ground displacement, or amplitude, compared to the P-wave. S-waves can only propagate through solid materials because liquids and gases cannot sustain the necessary shear stress.
Comparing Ground Movement
S-waves are significantly more destructive than P-waves. P-waves cause a change in volume, a force that structures are generally designed to resist due to their ability to bear vertical load. The shear motion of S-waves generates intense lateral forces highly damaging to buildings. This shaking attempts to twist and tear structural frames apart, a force modern construction is less resistant to.
S-wave damage is amplified by the wave’s lower frequency and higher amplitude. If the S-wave frequency aligns with a building’s natural frequency, the shaking is magnified through resonance, potentially leading to catastrophic failure. This powerful shearing action is why engineers focus on designing structures to withstand intense lateral forces. S-waves can also trigger liquefaction in loose, water-saturated soil, causing the soil to temporarily behave like a liquid and resulting in the collapse of buildings.
The Ultimate Factor in Destruction
While S-waves are more damaging than P-waves, the majority of widespread destruction in large earthquakes is caused by surface waves. These waves, which include Love and Rayleigh waves, travel along the Earth’s outer layers and are the slowest to arrive. Their energy is concentrated near the surface where infrastructure is located, making their effects the most intense.
Surface waves possess the largest amplitude and last for a longer duration, prolonging the shaking. Love waves cause horizontal shearing confined to the surface, damaging foundations and roads. Rayleigh waves produce a rolling, elliptical motion that stresses structures with both vertical and horizontal forces simultaneously. These combined motions make surface waves the most significant threat during a major seismic event, exceeding the destructive capacity of body waves.