Even though the technology does not exist to travel into all of Earth’s layers, scientists can still learn a great deal about Earth’s structure through seismic waves. Seismic waves are vibrations in the earth that transmit energy and occur during seismic activity such as earthquakes, volcanic eruptions, and even man-made explosions. There are two types of seismic waves, primary waves and secondary waves. Primary waves, also known as P waves or pressure waves, are longitudinal compression waves similar to the motion of a slinky (SF Fig. 7.1 A). Secondary waves, or S waves, are slower than P waves. The motion of secondary waves is perpendicular to the direction of the wave travel, similar to the motion of vigorously shaking a rope (SF Fig. 7.1 B). Show SF Fig. 7.1 C shows primary or P waves (on top) and secondary or S waves (on bottom) in motion. Scientists use seismometers (Fig. 7.2) to measure seismic waves. Seismometers measure the vibrations of the ground, relative to a stationary instrument. Data from a seismometer, also called a seismogram, shows velocity on the y axis and time on the x axis (Fig. 7.3). Note in SF Fig. 7.3 that the P wave occurs first, because they travel at a higher velocity. SF Table 7.1 shows that P waves have a higher velocity than S waves when traveling through several mineral types. The speed at which seismic waves travel depends on the properties of the material that they are passing through. For example, the denser a material is, the faster a seismic wave travels (SF Table 7.1). P waves can travel through liquid and solids and gases, while S waves only travel through solids. Scientists use this information to help them determine the structure of Earth. For example, if an earthquake occurs on one side of Earth, seismometers around the globe can measure the resulting S and P waves.
Courtesy of Stanford Rock Physics Laboratory SF Fig. 7.4 shows wave propagation through Earth. Note that P waves pass through all layers of the earth, while S waves cannot pass through the solid core of the earth, resulting in an S wave shadow on the opposite side of the earthquake. To understand P waves, we have to first look into the basics of seismology and seismic waves. The waves of energy that travel through the earth and cause earthquakes and related phenomena are seismic waves. There are two types of seismic waves : Body waves are the waves that can travel through the layers of the earth. They are the fastest waves and as a result, the first waves that seismographs can record. Body waves can move through all states of matter including rocks and molten lava. Surface waves can only travel on the surface of the earth. Their frequency is lower than body waves. Body WavesBody waves can be further sub-categorized into
P Waves and S Waves P WavesP waves, or Primary waves, are the first waves to arrive at a seismograph. P waves are the fastest seismic waves and can move through solid, liquid, or gas. They leave behind a trail of compressions and rarefactions on the medium they move through. P waves are also called pressure waves for this reason. Certain animals, such as dogs, can feel the P waves much before an earthquake hits the crust (surface waves arrive). Humans can only feel the ramifications it has on the crust. S WavesS waves, or secondary waves, are the second waves to arrive during an earthquake. They are much slower than P waves and can travel only through solids. It is after studying the trajectory of S waves through the layers of earth, scientists were able to conclude that the earth’s outer core is liquid. Following is the table explaining concepts related to waves:
Difference between S Waves and P Waves
P Wave And S Wave FormulaFollowing is the table for P and S waves formula based on the velocity of P and S waves:
Following is the table for P and S waves formula based on the distance between the earthquake location and the location of the seismic station:
Notations Used In The P And S Waves Formula
An earthquake is the trembling or shaking of the Earth when multiple tectonic plates suddenly slip past each other. The waves or bursts of energy that propagate through the Earth and instigate earthquakes are called seismic waves. Body waves and surface waves are the two types of seismic waves. P waves and S waves are the two types of body waves. P waves are the first waves that are detected by a seismograph. They are the fastest seismic waves and can travel through gases, liquids, or solids. Stay tuned with BYJU’S to learn more about Physics-related concepts.
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