When an earthquake occurs, it generates energy that travels through the Earth in the form of seismic waves. These waves are the primary tool seismologists use to study the planet’s structure and activity. P waves (Primary waves) and S waves (Secondary waves) are the two main categories of body waves, meaning they travel through the Earth’s interior rather than along its surface. These two wave types are categorized based on three core physical differences that define how they move, their speed, and the materials they can pass through.
Classification by Particle Movement
The most fundamental physical difference between P waves and S waves lies in how they cause the material they travel through to move. P waves are categorized as longitudinal waves, also known as compressional waves. In a P wave, the particles of the medium oscillate back and forth parallel to the direction the wave is propagating. This motion involves alternating cycles of compression, where particles are squeezed together, and rarefaction, where they are pulled apart. P waves efficiently transmit energy by temporarily changing the volume of the material they pass through, which is why they are the fastest type of seismic wave.
In contrast, S waves are categorized as transverse waves, also known as shear waves. The particle motion in an S wave is perpendicular to the direction of wave propagation. As the wave moves forward, the particles move up and down or side to side. This shearing motion causes a change in the shape of the material rather than a change in its volume.
Classification by Relative Velocity
The names Primary and Secondary are direct categorizations based on the relative speeds of the waves and their arrival times at a seismic monitoring station. P waves travel faster than S waves through any given Earth material. This higher velocity ensures that P waves are always the first to be recorded by a seismograph following an earthquake, earning them the designation “Primary.”
S waves travel at a slower pace in the same medium. Because they arrive after the P waves, they are designated as “Secondary” waves. The arrival time difference between the two waves is directly proportional to the distance they have traveled from the earthquake’s source. The actual speed of both P and S waves depends on the density and rigidity of the material they are traversing. For instance, both waves travel faster through the dense, rigid mantle than they do through the crust. Regardless of the material, the P wave always maintains its velocity advantage over the S wave.
Classification by Required Medium
A significant categorization of P and S waves involves the physical states of matter they are capable of traversing. P waves can propagate through solids, liquids, and gases. This is because the mechanism of compression and rarefaction is possible in all three states, allowing the material’s particles to be squeezed together and then spread apart.
S waves, on the other hand, are strictly limited to traveling through solids. This restriction exists because S waves rely on the medium’s ability to resist shearing forces, a property known as rigidity. Liquids and gases lack the necessary rigidity to support this shearing motion. If a liquid were subjected to a shear force, it would simply flow rather than spring back to its original shape, meaning the transverse wave cannot be transmitted.
Practical Applications of Wave Differences
Seismologists utilize the difference in velocity between the two waves to precisely locate the epicenter of an earthquake. The time interval between the arrival of the faster P wave and the slower S wave is known as the S-P interval. This interval increases with distance from the source.
By measuring the S-P interval at multiple seismic stations, scientists can calculate the distance from each station to the earthquake. Using a process called triangulation, where three or more distances intersect, the exact location of the earthquake’s origin can be determined.
The difference in the required medium is the primary tool for mapping the Earth’s deep interior structure. Since S waves cannot pass through liquids, their absence in certain regions provides undeniable evidence of liquid layers. The existence of the liquid outer core, for example, was inferred because S waves generated by an earthquake fail to reach seismic stations positioned in a specific angular zone on the opposite side of the planet. This S-wave shadow zone confirms that a large, non-rigid layer exists deep within the Earth.
Analyzing the velocity changes of both waves as they refract and reflect allows scientists to create detailed models of the crust, mantle, and core. This provides information about the density, temperature, and phase changes within the Earth’s deep layers. The unique mechanical and propagational properties of P and S waves are indispensable for both hazard assessment and planetary science.