An earthquake is the sudden, rapid shaking of the Earth caused by the release of energy in the Earth’s crust. This energy travels outward from the source, known as the hypocenter, in the form of seismic waves. All seismic waves fall into one of two fundamental categories based on the path they take through or along the Earth. These two primary types are known as Body Waves and Surface Waves.
Waves Traveling Through the Earth’s Interior
Body waves are the seismic waves that travel through the inner layers of the planet, including the crust, mantle, and core. They are generally of higher frequency than their surface counterparts and are recorded first by monitoring stations around the globe. Body waves are further divided into two distinct types: Primary waves, or P-waves, and Secondary waves, or S-waves.
P-waves are the fastest seismic waves. They are compressional waves that move through the material by pushing and pulling it in the same direction the wave is traveling, similar to how sound travels through air. P-waves can travel through any medium—solids, liquids, and gases—because the material only needs to be compressible for the wave to pass.
S-waves are slower than P-waves through the same material. These are shear waves that move the material perpendicular to the direction the wave is propagating, causing a side-to-side or up-and-down shaking motion. A defining characteristic of S-waves is their inability to travel through liquids or gases, as these media cannot sustain the necessary shear stress.
The speed difference between P-waves and S-waves is crucial for locating the origin of an earthquake. Because the P-wave always arrives first, the time delay between the two arrivals increases with the distance from the earthquake’s source. This measurable difference allows seismologists to calculate how far away an earthquake occurred from any single monitoring station.
Waves Moving Along the Surface
Surface waves travel along the Earth’s outer layer, similar to ripples on a pond, and are generated when body waves interact with the surface. Although they travel at slower speeds than body waves, they tend to have larger amplitudes and longer wavelengths. This concentration of energy and motion near the surface makes them responsible for the vast majority of the shaking and structural damage experienced during an earthquake. There are two main types of surface waves: Love waves and Rayleigh waves.
Love waves cause a distinct horizontal shearing motion, moving the ground from side to side in a direction perpendicular to the wave’s path. This motion is confined entirely to the horizontal plane. This side-to-side movement is particularly damaging to building foundations and structures, which are often less able to withstand strong lateral forces. Love waves are the fastest of the surface waves.
Rayleigh waves create a more complex, rolling motion that resembles waves on the ocean. The ground particles move in a retrograde elliptical path, combining both vertical up-and-down movement and horizontal movement in the direction of wave travel. This dual-axis shaking causes structures to experience both vertical and horizontal stresses simultaneously, contributing significantly to widespread damage. The energy of surface waves is confined to the shallow layers of the Earth, and their slower speed prolongs the period of shaking, increasing the likelihood of structural failure.
How Seismic Waves are Tracked and Analyzed
Scientists use specialized instruments called seismographs to detect and record the arrival of all these different seismic waves. A seismograph works by converting the ground’s vibrations into electrical signals, which are then displayed as a seismogram. Because the waves travel at different velocities, they arrive sequentially, creating a distinct pattern on the recording: P-waves first, followed by S-waves, and then the slower, larger-amplitude surface waves.
Locating the Epicenter
Seismologists use the calculated distance between the station and the earthquake’s epicenter to draw a circle on a map, with the radius representing that distance. The exact location of the epicenter cannot be determined from a single station, as the circle only indicates the distance, not the direction. To pinpoint the precise location of the earthquake, data from at least three different seismograph stations are required. By plotting the calculated distance circles from three separate stations, the circles will intersect at a single point. This point of intersection is the exact location of the earthquake’s epicenter on the Earth’s surface, a process known as triangulation.