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The earth is made up of different layers with different physical and chemical properties. However, we cannot observe these layers directly because they are too deep for us to access. So how do we know the layered structure of the earth? One way to do this is to use seismic waves.

Seismic waves can be divided into two main types: body waves and surface waves. Body waves propagate in the interior of the earth, while surface waves propagate on the surface of the earth. Body waves can be divided into longitudinal waves (P waves) and transverse waves (S waves). Longitudinal waves vibrate in the same direction as the propagation direction, while transverse waves vibrate in a direction perpendicular to the propagation direction. Longitudinal waves can pass through solids, liquids, and gases, but shear waves can only pass through solids.

When an earthquake occurs, it produces compressional and shear waves that radiate outward from the source. A seismograph is an instrument that measures ground motion and can detect these seismic waves. By analyzing the time and amplitude at which these waves arrive at different locations around the world, we can understand how they interact with different layers of the Earth.

An important observation is that in certain areas of the Earth's surface seismographs cannot pick up direct seismic waves from a given earthquake. These areas are called shadow bands, and they indicate that certain layers of the Earth refract or reflect these waves away from their original path. For example, at a depth of about 2900 km below the Earth's surface, there is a boundary between two layers: the mantle and the outer core.

The mantle is a thick layer of solid rock that makes up most of the Earth's volume. The outer core is a thin layer of liquid metal surrounded by another layer of material called the inner core. When longitudinal waves reach this boundary, they decelerate rapidly because liquids have lower elastic moduli than solids, which causes them to be refracted from their original direction to a steeper angle. Some compressional waves are also reflected back into the mantle at this boundary. When the shear waves reach this boundary, they disappear completely, because the fluid cannot withstand shear stress, which means that the shear waves cannot pass through this boundary.

Another important observation is that in certain regions of the Earth's surface, seismographs pick up only weak or distorted longitudinal waves. These regions indicate that certain layers of the Earth have properties, such as density or composition, that differ from their surroundings. For example, at a depth of about 5150 km below the Earth's surface, there is a boundary between two layers: the outer core and the inner core.

The core is a solid sphere of metal, dense and hot. When the longitudinal waves reach this boundary, they accelerate again because solids have higher elastic moduli than liquids. This causes them to bend at a shallower angle in the original direction. Some longitudinal waves are also reflected back into the outer core at this boundary.

By measuring the extent to which these boundaries affect the velocity, direction, and amplitude of seismic waves, we can infer their depth, thickness, and density. We can also infer their composition by comparing them with laboratory experiments or theoretical models. For example, we know that the outer core is primarily iron because its density and magnetic properties match those expected of liquid iron under high pressure and temperature conditions.

However, models of the earth's structure that rely on seismic waves do not reflect reality perfectly, one reason being that there are different ways of processing and interpreting seismic data. There are many methods for seismic wave measurement and analysis, such as traveltime tomography, waveform inversion, receiver function analysis, etc. Each method has its advantages and limitations, depending on the type of wave, frequency range, noise level, etc. Furthermore, each method relies on certain assumptions about the structure or physics of the Earth, which may not hold true in all cases.

For example, some methods assume that the Earth is isotropic, while others consider anisotropy. These assumptions will affect how well the data fits the model and may lead to errors in our model. However, as new data becomes available, some assumptions are confirmed or excluded, and our model is constantly updated and refined. Every once in a while, we hear news of new findings about the Earth's interior.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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