Читать книгу Geology For Dummies - Alecia M. Spooner - Страница 67

One way scientists separate the layers of Earth’s geosphere is by physical properties, or whether the layers are liquid or solid.

Because geologists cannot see inside the earth, they make observations about Earth’s internal properties by proxy: by interpreting information from earthquake waves that can be used to make inferences about the physical properties of Earth’s interior.

When earthquakes occur, they send out waves. Two types of seismic waves, called S waves and P waves, are used by scientists to learn about the interior of the earth. These seismic waves are recorded by instruments called seismometers, which are buried underground all over the planet. When an earthquake occurs, the seismometer sends a signal from underground to a machine in a lab (a seismograph) that records the earthquake wave movements on a printout called a seismogram. Scientists watch the seismographs as they print the seismograms to see when the P waves and S waves arrive. Here’s why:

 P waves travel quickly through solid materials and slow down, slightly changing direction, as they move through liquid materials. By recording where each P wave starts and how long it takes to reach the other side of the planet, scientists have recognized that it must move through regions of solid and liquid materials within the earth.

 S waves travel through solid materials but cannot travel through liquid at all. When scientists record the path that S waves take through the earth, they find that some S waves never reach the other side — they simply disappear, suggesting that they have hit a section of liquid material.

Figure 4-2 illustrates how the earthquake waves would travel if the interior of the earth was made of one continuous, solid type of material. And Figure 4-3 illustrates how P waves and S waves actually travel through Earth, illustrating the different physical properties of its interior.

FIGURE 4-2: The path of wave travel if Earth’s interior were a continuous solid.

The areas on the other side of the globe where P waves or S waves do not appear because they either disappear or are refracted (change direction) are called shadow zones. For more details about P waves, S waves, and shadow zones and why geologists study them, be sure to read Chapter 10.

FIGURE 4-3: The recorded path of P waves and S waves.