An earthquake releases immense stored energy that travels away from the source in the form of seismic waves. These waves represent the propagation of energy through the Earth’s layers, causing the ground shaking we experience. Understanding the characteristics of these different wave types is necessary to grasp why some are relatively mild while others can cause widespread devastation.
Body Waves: The Initial Tremors
The first seismic waves to arrive are body waves, which travel through the Earth’s interior from the focus to the surface and are categorized into two main types. The fastest are Primary waves, or P-waves, which move through rock by alternately compressing and expanding the material in the same direction the wave is traveling. P-waves are analogous to sound waves and can propagate through solids, liquids, and gases, traveling at speeds that can range from approximately 4 to 8 kilometers per second in the Earth’s crust.
Following the P-waves are the Secondary waves, or S-waves, which are slower, traveling at roughly 2.5 to 4 kilometers per second. S-waves move the material they pass through in a shearing motion, causing particles to oscillate perpendicular to the direction of wave travel. This side-to-side or up-and-down shaking is often felt as a more intense jolt than the initial P-wave. S-waves cannot travel through liquids, which prevents them from passing through the Earth’s liquid outer core and provides insight into the planet’s internal structure.
The difference in arrival time between the faster P-waves and the slower S-waves allows scientists to estimate the distance to the earthquake’s epicenter. Both wave types attenuate their energy rapidly as they travel deeper into the Earth. Since their energy is spread throughout the entire volume of the Earth’s interior, their impact near the surface is less concentrated compared to the waves that follow them. The arrival of these body waves serves as the initial warning of a seismic event before the more sustained, powerful shaking begins.
Surface Waves: Mechanism of Ground Movement
Surface waves are generated when body waves reach the Earth’s surface, trapping energy along the boundary between the crust and the atmosphere. These waves travel much slower than P-waves and S-waves, but they are responsible for the vast majority of ground displacement felt during an earthquake. Because their energy is confined to a two-dimensional path along the surface, it diminishes less rapidly with distance compared to body waves, which spread out in three dimensions. This concentration of energy allows surface waves to maintain their destructive potential over much greater distances.
There are two primary types of surface waves, each with a distinct and complex motion. Love waves involve a purely horizontal shearing motion, causing the ground to move side-to-side perpendicular to the direction the wave is propagating. This motion is particularly damaging to building foundations and infrastructure not designed to withstand intense lateral forces. Love waves are typically faster than the other type of surface wave.
The second type, Rayleigh waves, have a much more complex, rolling motion that displaces the ground both vertically and horizontally. The particle motion is elliptical and retrograde, similar to how waves move on the surface of water, forcing the ground to roll up and down while simultaneously moving forward and backward. This combined vertical and horizontal displacement often produces the most dramatic and widely felt shaking during an earthquake. The amplitude of both Love and Rayleigh waves decreases significantly with depth, which further emphasizes that their effects are strongest right at the Earth’s surface.
Why Surface Waves Cause Catastrophic Destruction
Surface waves are the most destructive of all seismic waves due to a combination of three physical properties: amplitude, duration, and frequency. The most noticeable factor is their significantly greater wave amplitude, which translates directly to a much larger ground displacement than that caused by P- or S-waves. This larger amplitude means the ground is visibly and dramatically moved, which directly strains and overwhelms the structural capacity of buildings and bridges. Near the epicenter, the amplitude of surface waves can easily be several times larger than that of the body waves.
Another factor contributing to their immense power is their prolonged duration, as the train of surface waves often lasts much longer than the rapid jolts of the body waves. This sustained, high-amplitude shaking subjects structures to continuous stress, increasing the likelihood of structural fatigue and eventual collapse. The lower frequency of surface waves is also a significant element in their destructive capability. Lower frequency waves mean slower shaking, which can match the natural sway frequency of large structures like tall buildings and long bridges.
When the wave frequency matches a structure’s natural frequency, a phenomenon called resonance occurs, greatly amplifying the building’s oscillation and causing maximum stress. The slower, lower-frequency oscillations of surface waves are particularly effective at inducing damage in larger, more flexible structures. Furthermore, the specific side-to-side motion of Love waves and the rolling motion of Rayleigh waves combine to attack structures from multiple directions. This multi-directional, low-frequency, high-amplitude, and prolonged ground motion is why surface waves are the primary cause of catastrophic destruction.