Seismic waves are vibrations of energy released during events like earthquakes or explosions that travel through the Earth’s interior. These waves allow scientists to study the physical properties of the planet’s layers. There are two types of body waves, known as P-waves and S-waves, which possess distinct characteristics regarding their speed, motion, and the materials they can pass through. Understanding the behavior of these fundamental waves is the basis for modern seismology.
Primary Waves: Characteristics and Movement
Primary waves, or P-waves, are known for being the fastest seismic waves, and are the first to be recorded by seismographs after an earthquake. They are characterized by longitudinal or compressional motion, causing particles in the material to move back and forth in the same direction as the wave propagates. This motion involves alternating cycles of compression and expansion, similar to how a sound wave travels through air.
P-waves generally travel between 5 to 8 kilometers per second near the Earth’s surface. Their speed increases deeper in the mantle due to greater pressure and density. A defining feature of P-waves is their ability to pass through solids, liquids, and gases. This is because their compressional motion only requires the medium to resist changes in volume, a property found in all three states of matter.
As P-waves encounter boundaries between different materials, such as the interface between the solid mantle and the liquid outer core, they refract or bend. This change in direction occurs because the wave velocity slows dramatically when entering the less rigid liquid medium.
Secondary Waves: Characteristics and Limitations
Secondary waves, or S-waves, are the second type of body wave and arrive at a seismograph station after the faster P-waves. These waves are also called shear waves because they move particles with a transverse motion. This means the particles oscillate perpendicular to the direction the wave is traveling.
S-waves travel slower than P-waves, typically moving between 1 to 8 kilometers per second depending on the material’s composition. The most significant characteristic of S-waves is their inability to travel through liquids or gases. This is because the transverse motion requires the medium to have rigidity, or shear strength, to maintain its shape as the wave passes through.
Since liquids lack the necessary shear strength, S-waves are completely blocked and absorbed when they encounter a liquid layer. This fundamental limitation means that S-waves are only detected when traveling through the Earth’s solid layers. The distinct behavior of S-waves provides a powerful natural probe for distinguishing between solid and liquid regions deep within the planet.
Determining the Earthquake’s Location
The difference in speed between P and S waves provides seismologists with a precise method for calculating the distance to an earthquake’s origin. The P-wave always arrives first, followed by the S-wave; the time gap between their arrivals is known as the S-P interval or lag time. Since P and S waves travel at known speeds through the Earth’s layers, the duration of this interval increases the farther the seismic station is from the earthquake’s focus.
By measuring the S-P interval on a seismogram, scientists use pre-calculated travel-time curves to determine the distance from that station to the earthquake’s epicenter. This distance defines the radius of a circle around the recording station where the earthquake could have occurred. To pinpoint the specific location on the Earth’s surface, data from at least three separate seismic stations are required.
The process of using three distance measurements to find a single point of intersection is known as triangulation. When circles representing the distance from the epicenter are drawn around the three stations, the point where all three circles overlap is identified as the earthquake’s epicenter. This technique allows for the rapid and accurate location of seismic events globally.
Revealing the Earth’s Internal Structure
The contrasting behavior of P and S waves has been instrumental in mapping the deep structure of the Earth’s interior. As seismic waves travel from a distant earthquake, their paths and speeds change dramatically when they encounter boundaries between layers of different density and state. This phenomenon is especially pronounced at the boundary between the mantle and the core.
The complete absence of S-waves in a wide zone on the side of the planet opposite an earthquake provides definitive evidence of a liquid outer core. This S-wave shadow zone, extending beyond an angular distance of about 103 degrees from the earthquake’s origin, exists because the shear waves are entirely blocked by the vast liquid layer, confirming the molten state of the outer core.
P-waves are also affected as they pass through the liquid core. Their path is bent, or refracted, significantly as they enter the liquid outer core, creating a separate P-wave shadow zone between approximately 104 and 140 degrees from the epicenter. By analyzing the precise speed changes and bending angles of the P-waves, seismologists can map the boundaries of the inner core and the mantle, revealing the planet’s layered composition.