Seismic waves are waves of energy that travel through Earth’s layers, carrying vibrations generated by geological phenomena like earthquakes, volcanic eruptions, and large human-made explosions. The study of these waves is fundamental to understanding the planet’s internal structure, as their behavior changes depending on the materials they pass through.
Primary Waves
Primary waves, or P-waves, are the fastest seismic wave type. They are the first to be detected by seismograph stations, traveling approximately 1.7 times faster than other wave types. P-waves exhibit a compressional motion, pushing and pulling material in the same direction as the wave, similar to how sound waves move through air. This back-and-forth movement causes particles to alternately compress and expand.
P-waves can travel through all states of matter: solids, liquids, and gases. They can propagate through Earth’s solid crust and mantle, and its liquid outer core. Their speed varies depending on the density and elasticity of the medium. In granite, P-waves can reach speeds of about 5,000 meters per second.
Secondary Waves
Secondary waves, or S-waves, arrive at seismic stations after P-waves. They are characterized by a shearing, or transverse, motion, where particles move perpendicular to the wave’s direction of travel. This can be visualized as a side-to-side or up-and-down shaking motion. S-waves travel roughly 60% the speed of P-waves in the same material.
S-waves cannot travel through liquids or gases. This is because fluids do not support the shear stresses necessary for S-wave propagation. Consequently, S-waves are absorbed or reflected when they encounter liquid layers, such as Earth’s outer core. This property is useful for deducing the physical state of Earth’s interior layers.
Surface Waves
Surface waves are generated when P and S waves reach the Earth’s surface. They travel along the Earth’s surface, much like ripples on water, and arrive after both P and S waves. While slower than body waves, surface waves carry larger amplitudes and are responsible for major ground shaking and destruction during an earthquake. Their energy diminishes with increasing depth.
There are two types of surface waves. Love waves, named after British mathematician A.E.H. Love, exhibit a horizontal, side-to-side shearing motion perpendicular to the wave’s direction of travel. This motion can cause structures to twist and shear, posing a threat to building foundations. Rayleigh waves, named after British physicist Lord Rayleigh, produce a rolling, elliptical motion similar to ocean waves. This movement involves both vertical and horizontal ground displacement, contributing to the shaking felt during an earthquake.
Significance of Seismic Waves
The study of seismic waves is fundamental to seismology, providing insights into Earth’s structure and dynamic processes. Seismologists use instruments called seismographs to detect and record these waves, converting ground vibrations into measurable signals. By analyzing the arrival times of P-waves and S-waves at multiple seismograph stations, scientists can pinpoint the epicenter and depth of an earthquake. The time difference between the arrival of P and S waves helps determine the distance to the earthquake source.
The varying speeds and behaviors of seismic waves as they travel through different materials enable mapping of Earth’s internal layers. For example, the fact that S-waves cannot pass through the outer core provides evidence that this layer is liquid. Analyzing how waves reflect and refract at boundaries reveals the composition, density, and physical state of the crust, mantle, and core. This understanding of Earth’s interior is applied to assessing seismic hazards, informing earthquake-resistant construction, and contributing to strategies for mitigating the impact of seismic events on communities.