Earthquakes generate seismic waves that travel through the Earth, carrying energy away from their source and causing ground shaking. Two primary categories of seismic waves are P-waves and S-waves. Understanding their properties helps explain how earthquake energy impacts the surface and which wave type causes more destruction.
Understanding P-Waves
P-waves, also known as primary waves, are compressional or longitudinal waves. They are the fastest seismic waves and are the first to arrive at a seismic station after an earthquake. This speed allows them to travel through any type of material, including solids, liquids, and gases, similar to how sound waves move through air.
The motion of a P-wave involves particles in the ground oscillating back and forth in the same direction that the wave is traveling. This push-pull motion causes the ground to alternately compress and expand as the wave passes. When P-waves reach the surface, they cause a sudden jolt or a slight up-and-down motion, often described as a gentle shove, resulting in minimal damage.
Understanding S-Waves
S-waves, or secondary waves, are shear or transverse waves that travel slower than P-waves and arrive after them. Their speed is typically around 60% of P-wave speeds in the same material. A distinguishing characteristic of S-waves is their inability to travel through liquids or gases; they can only propagate through solid mediums. This property has been instrumental in determining that Earth’s outer core is liquid.
The motion of an S-wave causes particles to move perpendicular to the direction of wave propagation. This results in a side-to-side or up-and-down shaking of the ground. As these waves pass, they shear the medium, causing a more violent, swaying motion.
Why S-Waves Cause More Damage
S-waves are more destructive than P-waves due to their motion and energy characteristics. The shearing motion of S-waves, causing ground particles to move sideways or vertically, is particularly damaging to structures. Buildings are designed to withstand vertical loads, but they are less resistant to the horizontal and twisting forces exerted by S-waves. This lateral stress can cause rigid structures like buildings and bridges to deform or collapse.
S-waves often have larger amplitudes than P-waves, meaning they cause greater displacement of ground particles. This increased displacement translates to more intense ground shaking and greater energy transfer to structures. The sustained shaking and specific frequencies of S-waves can also lead to resonance in buildings, amplifying the destructive forces as the structure’s natural vibration frequency matches that of the incoming wave.
Although S-waves are slower, their arrival after the initial P-wave jolt can exacerbate damage. Structures may already be weakened or set into motion by the preceding P-waves, making them more vulnerable to the powerful shearing forces of the S-waves.