By Cristie Chau
When an earthquake occurs, the shockwaves of released energy that shake the Earth are called seismic waves. Seismic waves are caused by the sudden movement of the Earth’s tectonic plates, such as slip along a fault during an earthquake. There are also many other causes of seismic waves, including volcanic eruptions, explosions, landslides, avalanches, and rushing rivers.
There are three basic types of seismic waves: P-waves, S-waves and surface waves.
P-waves, also called primary waves or pressure waves, travel through the Earth at the fastest velocity. When travelling through air, P-waves take the form of sound waves and travel at the speed of sound (330 ms-1), but may travel at 5000 ms-1 in granite. In a P wave, the rock particles are alternately compressed and dilatated, so they are also called compressional waves. P waves can travel through solids, liquids, and gases, and can travel through the liquid outer core.
S-waves, also known as secondary waves, shear waves or shaking waves, are transverse waves, so the particle motion is perpendicular to the direction of wave propagation. They travel slower than P-waves, the speed being typically around 60% of that of P-waves, and cannot travel through air or liquid. However, they are more destructive than P-waves because of their larger amplitudes.
Surface waves are similar in nature to water waves and travel just below the Earth’s surface. In general, they occur when the earthquake source is close to the Earth's surface. Although surface waves travel slower than S-waves and P-waves, they can be much larger in amplitude and are the most destructive type of seismic wave.
There are two main types of surface waves: Love waves and Rayleigh waves.
Love waves, also known as 'Q-waves', after their discoverer, A.E.H. Love, cause the ground to move in a horizontal direction, perpendicular to the direction of wave propagation. They travel slower than S-waves and P-waves, but usually travel slightly faster than Rayleigh waves. They move in a shearing motion and can cause significant ground shaking and damage.
Rayleigh waves, also called ground rolls, travel as ripples similar to those on water surfaces. The motion of particles in Rayleigh waves is opposite to the motion of ocean waves, despite the fact that they appear to roll like ocean waves. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
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