earthquake

The wave we are most familiar with is the observation of water waves. When a stone is thrown into the pond, the water surface is disturbed, and the ripples expand outwards centering on the stone. This wave train is caused by the movement of particles of water near the water wave. However, the water does not flow in the direction of water waves; if a cork floats on the surface, it will jump up and down, but it will not be removed from its original position. This disturbance is continuously transmitted by the simple back and forth movement of the water particles, from one particle to the movement of the preceding particle. In this way, the water waves carry the energy of the broken water surface to the pool and stimulate the waves on the shore. Earthquake movements are quite similar. The shaking we feel is the vibration of the elastic rock generated by the energy of the seismic wave.

The physical properties of the first type of wave are just like sound waves. Sound waves, and even ultrasonic waves, are transmitted by alternating extrusion and expansion in the air. Because liquids, gases, and solid rock can be compressed, the same type of wave can pass through water bodies such as oceans and lakes and solid earths. In the event of an earthquake, this type of wave propagates from the fracture at all directions in the same direction, alternately squeezing and stretching the rock they pass through, and the particles move forward and backward in the direction of propagation of these waves, in other words The motion of these particles is perpendicular to the wavefront. The amount of displacement forward and backward is called the amplitude. In seismology, this type of wave is called the P wave, the longitudinal wave, which is the wave that arrives first.

Elastic rocks differ from air in that they can be compressed but not cut, and elastic materials allow the second type of wave to propagate by shearing and twisting objects. The second wave of arrival of the earthquake is called the S wave. When the S wave passes, the performance of the rock is quite different from that during the P wave propagation. Because the S wave involves cutting rather than squeezing, the motion of the rock particles traverses the direction of migration. These rock movements can be in a vertical or horizontal plane, similar to the lateral motion of light waves. The simultaneous presence of P and S waves makes seismic wave trains a unique combination of properties that make them different from the physical manifestations of light waves or sound waves. Because shearing motion is unlikely to occur within a liquid or gas, S waves cannot propagate through them. The distinct properties of P and S waves can be used to detect the presence of deep fluid bands in the Earth.

The S wave has a polarization phenomenon, and only those light waves that vibrate laterally in a particular plane can pass through the polarizing lens. The light waves that pass through are called plane polarized light. The passage of sunlight through the atmosphere is unpolarized, i.e., there is no preferred lateral direction of vibration of the light waves. However, the refraction of the crystal or through specially made plastics such as polarized eyes can make unpolarized light into plane polarized light.

When S waves pass through the Earth, they refract or reflect when they encounter a discontinuous interface and polarize their vibration direction. When the rock particles of the polarized S wave occur only in the horizontal plane, they are called SH waves. When the rock particles move in a water quality plane containing the wave propagation direction, the S wave is called an SV wave.

Most rocks, if not forced to vibrate with too large amplitude, have linear elasticity, ie the deformation due to the force varies linearly with the force. This linear elastic performance is called obeying Hooke's law and is named after Robert Hook, a British mathematician of Newton's contemporaries. Similarly, the rock will increase the deformation proportionally to the increased force during an earthquake. In most cases, the deformation will maintain the elastic range of the line, and the rock will return to its original position at the end of the shaking. However, important exceptions sometimes occur during earthquake events. For example, when strong shaking occurs in soft soil, permanent deformation will remain. After fluctuating deformation, the soil will not always return to its original position. In this case, the earthquake The intensity is harder to predict.

Elastic motion provides an excellent revelation of how energy changes as a seismic wave passes through a rock. The energy associated with the compression or extension of the spring is the elastic potential, and the energy associated with the movement of the spring member is kinetic energy. The total energy at any time is the sum of both elastic energy and kinetic energy. For an ideal elastic medium, the total energy is a constant. At the position of the maximum amplitude, the energy is all elastic potential energy; when the spring oscillates to the intermediate equilibrium position, the energy is all kinetic energy. We have assumed that no friction or dissipative forces exist, so once the reciprocating elastic vibration begins, it will continue at the same magnitude. This is of course an ideal situation. During an earthquake, the friction between the moving rocks gradually heats up and dissipates some fluctuating energy. Unless new energy is added, like a vibrating spring, the Earth's vibration will gradually stop. The measurement of seismic wave energy dissipation provides important information about the inelastic properties of the Earth. However, in addition to frictional dissipation, there are other factors in the formation of seismic vibrations as the propagation distance increases and gradually weakens.

As the sound wave propagates with an expanding spherical surface in front of it, the sound carried is weakened as the distance increases. Similar to the water wave expanding outside the pond, we observe the height or amplitude of the water wave and gradually decrease outward. The amplitude reduction is due to the fact that the initial energy propagation is wider and wider and produces attenuation, which is called geometric diffusion. This type of diffusion also weakens seismic waves through the Earth's rocks. Unless there are special circumstances, the farther the seismic waves travel from the source, the more their energy is attenuated.