Earthquakes can cause significant material damage and lead to a large number of deaths in the disaster area. However, for scientists they are a valuable source of information about the Earth’s structure. Earthquakes release energy stored over long period of time in layers of rock. This energy is transmitted from the release zone (the epicenter) in the form of vibrations and elastic seismic waves. They spread in all directions and reach the Earth’s surface, causing soil shifts, formation of cracks, changes in riverbeds, and destruction.
Seismologists distinguish between body waves, which pass through the Earth, and surface waves, which propagate parallel to its surface. They follow one after another and are recorded on seismograph readings. Some of the characteristics of waves (speed of propagation, amplitude) change not only depending on the amount of energy released, but also the geological structures they intersect. Thus, by studying earthquakes and recording seismic wave parameters, scientists can determine the nature of Earth’s crust structures.
Main types of seismic waves
Seismologists identify two main types of waves generated in the Earth as a result of an earthquake. These include:
- body waves, which propagate throughout the Earth’s crust and can be detected by seismographs in different regions. Body waves are divided into two types: shear waves (S-waves) and compressional waves (P-waves);
- surface waves (Love and Rayleigh waves). They propagate along the surface layers of the Earth. This type of wave is responsible for all the damage associated with earthquakes.
Let’s consider the effect of each type of seismic wave generated during an earthquake.
Longitudinal waves
They are also called compression waves. They deform rock formations by changing their volume and consist of vibrations that alternate between compression and expansion. The movement of P-waves creates zones of decompression and high pressure in the Earth’s crust. In this case, rock particles move back and forth in the direction of the wave propagation, parallel to it.
P-waves are the fastest seismic waves, with an average speed of 6 km/s. They can propagate through solids, liquids, and gases. Therefore, P-waves are the first vibrations detected by seismographs after the onset of an earthquake.
Shear waves
Secondary S-waves deform rocks by changing their shape. These are shear waves that are transmitted only by solids, as liquids lack the elasticity to return to their original shape after deformation. S-waves consist of particle oscillations perpendicular to the wave direction. They are detected by seismographs after P-waves because their speed in the Earth’s crust is approximately 3.5 km/s. Hence their name – secondary seismic waves.
Seismic waves, similar to light and sound waves, can propagate, reflect, and refract. They reflect off the irregularities of Earth’s rock layers, while refraction is associated with changes in the speed and direction of wave propagation.
If the Earth had a homogeneous composition and its density gradually increased with depth, the speed of seismic waves would constantly increase. Reflection and refraction at zones of sharp density changes in rocks lead to a gradual decrease in the velocity of their propagation (attenuation). The propagation of S-waves abruptly ceases at the boundary between the core and mantle. This indicates that the outer core of our planet is liquid.
Surface waves
These waves travel along the Earth’s surface. They are slower than body waves, but their amplitude is usually higher than the peaks of P and S oscillations. Surface seismic waves concentrate the maximum energy, therefore they are responsible for all the destruction during an earthquake. There are two types:
- L-waves (Love waves) – generating horizontal movements parallel to the Earth’s surface, they propagate only in heterogeneous solids. These are transverse vibrations polarized in the horizontal plane. Waves of this type cause horizontal vibrations, causing numerous destructions, breaking walls and foundations of buildings;
- Rayleigh waves – generate elliptical movements of the ground, propagate near the surface, regardless of soil and rock homogeneity. They arise from the interference of the vertical components of P and S waves and belong to the group of electro-acoustic oscillations.
The average propagation speed of Love waves is about 4 km/s, but the amplitude of Rayleigh waves is greater than that of other types of seismic vibrations. For this reason, Rayleigh waves are considered the most destructive.