Seismic waves are the traveling energy released from an earthquake, spreading out from the point of rupture in all directions. These waves directly cause the ground shaking and structural damage experienced during a seismic event. Determining which wave type is responsible for the most destruction requires looking at how they move and where they concentrate their energy. Seismologists generally agree that surface waves are the primary cause of damage.
Body Waves: The Initial Movement
The first waves to propagate after a fault rupture are the body waves, which travel through the Earth’s interior. These waves are composed of Primary (P) waves and Secondary (S) waves. P-waves are compressional waves that move the ground back and forth in the direction of wave travel, similar to a sound wave. They are the fastest seismic waves, often traveling between 5 to 8 kilometers per second in the crust, and cause the least damage due to their smaller amplitude.
S-waves, or shear waves, arrive next, moving slower than P-waves. These waves shake the ground perpendicular to the direction of travel, causing a side-to-side or up-and-down motion. S-waves cannot travel through liquids, a property used to map the Earth’s liquid outer core. While more destructive than P-waves, their damaging potential is secondary compared to surface waves. The time difference between the arrival of P and S waves forms the basis for earthquake early warning systems.
Surface Waves: The Primary Cause of Destruction
Surface waves are generated when P and S waves reach the Earth’s surface, traveling along the shallow layers of the crust. Although they are the slowest seismic waves, their destructive capability is greater because they concentrate energy near the surface where structures are located. Surface waves have a much higher amplitude, producing significantly larger ground movement than body waves.
The two main types are Love waves and Rayleigh waves, each having a distinct and damaging motion. Love waves move the ground horizontally and side to side, a shearing motion transverse to the direction of wave travel. This lateral shaking puts immense stress on building foundations and walls, which are often not designed to withstand powerful horizontal forces. Rayleigh waves create a rolling motion, moving the ground in an elliptical pattern that combines vertical and horizontal displacement. This complex motion causes the ground to repeatedly lift and settle, shaking structures in multiple directions and increasing the chance of structural failure.
Contextual Factors That Intensify Damage
The intensity of surface wave destruction is heavily influenced by the local environment, a phenomenon known as site effects. One significant factor is the underlying soil condition, as soft sediments and deep sedimentary basins can dramatically amplify the ground shaking. The difference in material stiffness between bedrock and loose soil layers can increase the amplitude of the waves multiple times over.
Another major concern is liquefaction, which occurs when loose, water-saturated granular soils temporarily lose their strength and behave like a liquid. Intense ground shaking increases the pore water pressure in these soils, causing a loss of bearing capacity. This leads to effects such as ground settlement, lateral spreading, and the floatation or sinking of buried structures. Damage can also be intensified by resonance, which happens when a building’s natural frequency matches the frequency of the seismic wave. When this occurs, the shaking is amplified, potentially leading to structural collapse.