Seismic waves are vibrations that travel through the Earth or along its surface, typically generated by a sudden release of energy, such as an earthquake. These waves provide the only direct physical means of probing the planet’s deep interior. Seismic energy propagates in both transverse and longitudinal forms simultaneously, carried primarily by two distinct types of body waves: P-waves and S-waves. The difference in their motion, either longitudinal or transverse, is fundamental to how they behave and how scientists use them to study the planet.
Understanding Transverse and Longitudinal Motion
Wave motion is fundamentally classified by the relationship between the direction the wave travels and the direction the particles of the medium move. In one form of motion, the medium’s particles oscillate back and forth parallel to the path of energy transfer. This motion involves the material being alternately squeezed together and stretched apart as the wave passes through. Sound waves traveling through the air are a common example of this type of wave action.
The second type of motion involves the particles moving at right angles, or perpendicularly, to the direction the energy is propagating. This perpendicular oscillation is defined by a shearing action, where the material is temporarily displaced sideways from its resting position. These two distinct types of particle movement determine the characteristics of the seismic waves that shake the ground during an earthquake.
Primary Waves: The Longitudinal Compressional Force
The Primary wave, or P-wave, is the fastest type of seismic wave and is the first to be recorded by seismographs, which is how it earned its name. P-waves exhibit longitudinal motion, meaning they move the material in the same line as the wave’s path of travel. This movement consists of alternating compressions and rarefactions, pushing and pulling the rock like a giant piston.
Because this motion relies only on the material’s resistance to compression, P-waves can travel through any state of matter: solids, liquids, and gases.
The speed of a P-wave is determined by the density and elastic properties of the medium it passes through, generally traveling faster in denser, more rigid materials. This ability to propagate through the entire planet makes them an invaluable tool for global seismology.
Secondary Waves: The Transverse Shearing Force
The Secondary wave, or S-wave, arrives after the P-wave because it travels at a slower speed, typically about 60% of the P-wave velocity in the same material. S-waves are characterized by a transverse, or shear, motion, causing particles to move perpendicular to the wave’s direction of travel. This motion causes the material to twist and deform sideways.
S-waves are unable to travel through liquids or gases. This is because these fluid states lack sufficient shear strength; they cannot resist the side-to-side deformation that S-waves require for propagation. S-waves can only move through solid rock materials, making them highly sensitive to changes in the Earth’s internal structure. The energy of S-waves often contributes significantly to the destructive shaking experienced at the surface near an earthquake epicenter.
Mapping Earth’s Interior Using Wave Differences
The distinct properties of P-waves and S-waves are the primary means by which scientists have determined the internal layering of the Earth. Seismologists use the difference in arrival times between the faster P-waves and the slower S-waves to calculate the distance from a recording station to the earthquake’s source. By comparing these travel times across multiple stations, the exact epicenter of the seismic event can be precisely located.
The most significant discovery enabled by these waves is the confirmation of the Earth’s liquid outer core. S-waves entirely disappear at a depth of about 2,900 kilometers, creating a measurable “shadow zone” on the opposite side of the planet from the earthquake.
This abrupt termination of S-wave transmission proves that the material at that depth, the outer core, must be in a molten or liquid state, as it cannot sustain the necessary shear motion. The analysis of both the speed and the presence or absence of these wave types allows for the construction of detailed maps of the planet’s layered structure, including the solid mantle, liquid outer core, and solid inner core.