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8.3: Seismic Waves

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The point on a fault within earth’s crust where the fracturing begins and most slippage occurs is called the focus of the earthquake. Another name for it is the hypocenter. The point on the earth’s surface directly above the focus is the epicenter. The epicenter is not where the earthquake originated. Earthquakes originate within the earth. The epicenter is the point on the surface of the earth directly above where the earthquake originated.

When an earthquake occurs, some of the energy it releases is turned into heat within the earth. Some of the energy is expended in breaking and permanently deforming the rocks and minerals along the fault. The rest of the energy, which is most of the energy, is radiated from the focus of the earthquake in the form of seismic waves.

Seismic waves fall into two general categories: body waves, which travel through the interior of the earth, and surface waves, which travel only at the earth’s surface.

There are two types of body waves: P-waves and S-waves. The P in P-waves stands for primary, because these are the fastest seismic waves and are the first to be detected once an earthquake has occurred. P-waves travel through the earth’s interior many times faster than the speed of a jet airplane, taking only a few minutes to travel across the earth.

P-waves are predominantly compressional waves. As a P-wave passes, material compresses in the same direction the wave is moving, and then extends back to its original thickness once the wave has passed. The speed at which P-waves travel through material is determined by:

• rigidity—how strongly the material resists being bent sideways and is able to straighten itself out once the shearing force has passed – the more rigid the material, the faster the P-waves
• compressibility—how much the material can be compressed into a smaller volume and then recover its previous volume once the compressing force has passed; the more compressible the material, the faster the P-waves
• density—how much mass the material contains in a unit of volume; the greater the density of the material, the slower the P-waves

The animations below show P-waves propogating across a plane (left) and from a point source (right). They are from Wikipedia.org/wiki/P-wave uploaded November, 2006 by Christophe Dang Ngoc Chan.

P-waves travel through liquids and gases as well as through solids. Although liquids and gases have zero rigidity, they have compressibility, which enables them to transmit P-waves. Sound waves are P-waves moving through the air.

Because the earth’s mantle becomes more rigid and compressible as the depth below the asthenosphere increases, P-waves travel faster as they go deeper in the mantle. The density of the mantle also increases with depth below the asthenosphere. The higher density reduces the speed of seismic waves. However, the effects of increased rigidity and compressibility in the deep mantle are much greater than the effect of the increased density.

The S in S-waves stands for secondary, because they are the second-fastest seismic waves and the second type to be detected once an earthquake has occurred. Although S-waves are slower than P-waves, they still travel fast, over half the speed of P-waves, moving at thousands of kilometers per hour through the earth’s crust and mantle.

S-waves are shear waves (though that is not what the S stands for). They move by material flexing or deforming sideways (shearing) from the direction of wave travel, and then returning to the original shape once the wave passes. The speed at which S-waves travel through material is determined only by:

• rigidity — how strongly the material resists being bent sideways and is able to straighten itself out once the shearing force has passed – the more rigid the material, the faster the S-waves
• density — how much mass the material contains in a unit of volume – the greater the density of the material, the slower the S-waves

The animations below show S-waves propogating across a plane (left) and from a point source (right). They are from Wikipedia.org/wiki/S-wave uploaded November, 2006 by Christophe Dang Ngoc Chan.

S-waves can travel only through solids, because only solids have rigidity. S-waves cannot travel through liquids or gases.

Because the earth’s mantle becomes more rigid as its depth below the asthenosphere increases, S-waves travel faster as they go deeper in the mantle. The density of the mantle also increases at greater depth, which has the effect of reducing the speed of seismic waves, but the increase in rigidity is much greater than the increase in density, so S-waves speed up as they get deeper in the mantle, in spite of the increased density.

SURFACE WAVES

There are two types of surface waves, Rayleigh waves and Love waves. Rayleigh waves are named after Lord Rayleigh (John Strutt), an English aristocrat who, in his work as a scientist and mathematician, developed a detailed mathematical accounting of the type of surface wave named after him. Rayleigh waves are set off by the combined effect of P- and S-waves on the earth’s surface. Rayleigh waves are sometimes called rolling waves. In Rayleigh waves the surface of the earth rises up and sinks down in crests and troughs, similar to waves on the surface of water. People who are outdoors during a major earthquake commonly see Rayleigh waves moving across the surface of the earth, and can feel the ground rising and falling as the waves pass beneath them.

Love waves, sometimes called L-waves, are named after Augustus Love, an English mathematician and physicist who first modeled them mathematically. Love waves involve the surface shearing sideways and then returning to its original form as each wave passes.

All surface waves travel slower than body waves and Rayleigh waves are slower than Love waves.

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Original content from Kimberly Schulte (Columbia Basin College) and supplemented by Lumen Learning . The content on this page is copyrighted under a Creative Commons Attribution 4.0 International license.

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Compare-Contrast-Connect: Seismic Waves and Determining Earth’s Structure

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).

SF Fig. 7.1. ( A ) Primary or "P" waves show longitudinal compression similar to a slinky.

Image by Narrissa Spies

SF Fig. 7.1. ( B ) Secondary or "S" waves have motion perpendicular to the direction of the waive, similar to a rope.

Image courtesy of CK-12 Foundations, Wikipedia Commons

SF Fig. 7.1  ( C ) Primary or P waves (on top) and secondary or S waves (on bottom) in motion

Image courtesy of Actualist, Wikipedia Books

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 Fig. 7.2. Seismometers are used to measure seismic waves.

Image courtesy of Yamaguchi先生, Wikimedia Commons

SF Fig. 7.3. A seismogram shows the data from a seismograph. Wave velocity is measured on the y axis, and time in seconds is measured on the x axis. P waves are recorded earlier than S waves, because they travel at a higher velocity.

Image courtesy of Crickett, Wikimedia Commons

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.

SF Fig. 7.4. This diagram shows hypothetical S and P wave propagation through the earth from an earthquake. P waves (arrows in yellow) can penetrate through the mantle and core, but S waves (arrows in red) can only travel through the mantle.

Image by Byron Inouye

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.

• What are seismic waves? Use your own words to describe them.  
• Why do you think that waves traveling through basalt have a higher seismic velocity than a wave traveling through sand?  
• How have scientists used seismic waves to determine structure of Earth?  
• Think of additional objects, in addition to a slinky or rope tied to a tree, that have a similar motion to a P wave and an S wave.

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Science News Explores

Explainer: seismic waves come in different ‘flavors’.

When Earth begins to rock and roll and shimmy, it’s important to know what the various types of shakes mean

There are several different types of seismic waves, some of which are much more damaging than others.

Sebastien_B/iStockphoto

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By Sid Perkins

January 12, 2018 at 6:30 am

Earthquakes and underground explosions can release a lot of energy. That energy ripples away from its source in a variety of ways. Some of those vibrations will move forward  and back through the material they travel through . Other waves travel just like ocean waves, where they make the material they pass through move up and down compared to the direction the wave is traveling. And while some of these waves travel deep within the planet, still others move only along the surface. Studying where these various flavors of waves are and how they move not only can help scientists pinpoint where an earthquake or explosion occurred, but also can shed light on the structure of our inner planet.

Seismic waves are vibrations in the ground. These can be generated by a number of phenomena, including earthquakes, underground explosions, landslides or collapsing tunnels inside a mine. There are four major types of seismic waves, and each typically travels at different rates of speed. That’s one big reason why scientists are able to tell them apart. If the waves arrived at vibration-detecting instruments — seismometers (Sighs-MAH-meh-turz) — all at the same time, it would be difficult to tell them apart.

Another major difference between these types of waves is how a material will move as the wave passes through it. With these differences in mind, let’s review the major types of seismic waves.

P versus S waves

Seismologists are scientists who study earthquakes. They also study how a quake’s energy spreads through Earth’s crust, as well as the deeper layers of our planet. The fastest seismic waves are known as P waves. That “p” stands for primary. And early seismologists called them that because these waves were the first to arrive at seismometers from some distant quake.

At Earth’s surface, P waves travel somewhere between 5 and 8 kilometers per second (3.1 and 5 miles per second). Deeper within the planet, where pressures are higher and material is typically more dense, these waves can travel up to 13 kilometers per second (8.1 miles per second).

P waves travel through rock the same way that sound waves do through air. That is, they move as pressure waves. When a pressure wave passes a certain point, the material it is passing through moves forward, then back, along the same path that the wave is traveling.

P waves can travel through solids, liquids and gases. That’s one big difference between them and the other types of seismic waves, which typically travel only through solids (such as rock).

The next-fastest type of seismic waves are “secondary.” They earned that name because they were typically the second set to reach seismometers from a distant quake. Not surprisingly, they’re known as S waves.

In general, S waves are only 60 percent as fast as p waves. So, along Earth’s surface they move at speeds of between 3 and 4.8 kilometers per second (1.9 and 3 miles per second).

As an S wave passes through a material, the site of its passing moves from side to side or up and down (as compared to the direction the wave is traveling). This is why S waves are also known as transverse waves . “Transverse” comes from the Latin words for “turned across.”)

S waves cannot travel through liquids or gases. That’s because the types of stresses set up by those waves can only be transmitted through solid materials.

Distinguishing earthquakes from nuclear shakes

Because P waves and S waves travel through Earth — not just along its surface — they are also known as “body waves.” This trait makes them useful in a number of ways. For one, scientists can use P waves and S waves to identify where an earthquake began. To do that, they need to have data gathered by seismic instruments at three or more different locations. That lets them triangulate to find the source of Earth’s shimmying.

Triangulation is only possible when there are accurate measurements of the times at which P waves and S waves show up at each seismometer. Some techniques use only the P waves. Others also consider the time difference between the arrival of the first P waves and S waves. (The farther the distance between the seismometer and the source of the quake, the more exaggerated that time difference will be.)

Whatever method is used, it gives scientists only an estimate of how far from a seismometer the earthquake’s source happens to be. So with a seismometer as a center, scientists draw a circle of the proper size on a map. But using only one seismometer, there is no way to tell in which direction the source was. It could be anywhere along the outer edge of that circle. By plotting the circles for at least three instruments on the same map, however, there will be a single point where those circles overlap. That marks the point on Earth’s surface above the quake site.

Most quakes occur deep within Earth’s crust. The point where a quake originates is called its hypocenter . The point on Earth’s surface directly above the hypocenter is the quake’s epicenter .

But scientists don’t just use these waves to map earthquakes. Those same seismic waves also can be generated by underground explosions. These might arise from a small blast inside an underground coal mine, for example. Or, they might signal the test detonation of a nuclear weapon (such as several that recently took place in North Korea). And P waves, in particular, can strongly point to whether the seismic waves come from a natural quake or an unnatural blast.

Here’s why: When a natural earthquake occurs, one side of a fault zone slides in one direction; the other side slides in the opposite way. (A fault zone is a fracture in Earth’s crust, or a boundary between two tectonic plates, where slippage can occur and seismic energy can be released.) Now, imagine that an earthquake occurs in an area that’s covered with a network of seismometers. For some of the instruments, the first P waves to arrive will be a “push” from the quake. But for others, the first P waves to arrive will be a “pull.”

For seismic vibrations generated by an unnatural explosion, the first P wave to arrive at every seismometer will provide a “push.” Not only that, the P waves generated by an unnatural explosion are typically sharp and sudden. So they die away pretty quickly. Vibrations produced by a natural earthquake instead tend to rumble for quite a while. That’s because the slippage along fault zones in a natural quake doesn’t happen all at once, like an explosion does.

Still more flavors of seismic waves

At first, all of a quake’s energy travels from its source deep within the planet as P waves and S waves. But when that energy reaches the surface, it now can spread as either of two different types of waves.

Think of a quake’s energy as a bubble rising from the bottom of a pond. The surface waves are much like the ripples in the pond’s surface. Here, the waves spread from the quake’s epicenter. These waves also are typically larger and cause much more damage than P waves and S waves.

The faster of these surface waves was named after British mathematician A.E.H. Love. More than 100 years ago, he worked out the math that explains how such waves move. The second type of surface waves were named for a British physicist who, in the 1880s, predicted their existence. This scientist was named John William Strutt. His father had been a British noble dubbed Lord Rayleigh. At his father’s death in the 1870s, Strutt inherited the title, becoming the next Lord Rayleigh. The waves he predicted are now known as Rayleigh waves.

Of these two surface waves, the Love type travels a bit faster.

Like S waves, Love waves shake the ground from side to side compared to the direction they’re headed. (In other words, for a Love wave traveling north, the ground shakes back and forth from east to west.) Rayleigh waves, on the other hand, cause ground movements in two directions at once. One of those motions is up and down, very much like waves on the ocean’s surface. The other is a push-pull movement along the same path that the wave is traveling. Together, those motions generate a rolling action that can cause extreme damage to buildings and other structures.

Other uses for seismic waves

Geoscientists often use seismic waves to map details of the inner structure of our planet. For instance, the time it takes P waves and S waves to travel down into Earth and then return to the surface helps scientists calculate how deep the boundaries of Earth’s major layers are. (Those calculations are made possible, in large part, because researchers have measured the speed of seismic waves through rocks under immense pressure in the lab.)

P waves and S waves tell scientists a lot more than the depth ranges of Earth’s major layers. In some cases, they also provide strong clues about the type and density of materials in those layers. For example, at distances of between 11,570 and 15,570 kilometers (7,190 to 9,670 miles) from a major earthquake, seismometers don’t record any S waves coming directly from that quake. That’s a big clue that Earth’s outer core is made of liquid, scientists say. (In areas more than 15,570 kilometers away from a quake’s epicenter, seismometers do detect S waves. Those waves develop when the energy of P waves that have traveled through Earth’s outer core once again enter the mostly solid mantle. That’s the very thick layer that lies between Earth’s outer core and its crust.)

At shallow depths in Earth’s crust, all types of seismic waves can be used to map out relatively small geological structures. These include things such as faults and sediment-filled basins. (Sediment-filled basins are broad bowls of solid rock where loose material accumulates. Such areas can be especially affected by earthquakes. That’s because seismic waves can get trapped and bounce around inside that basin, making the sediment shake like jelly in a bowl.) Again, the time it takes for a seismic wave to travel to a structure and then echo back helps scientists estimate how far away that structure is.

Even people setting off small explosions of dynamite can trigger seismic waves. That means these can be mapped from afar. It’s also possible to use data gathered by seismometers over a long period of time. Although such signals may be faint, they can be assembled into stronger signals (much in the same way that photographers can take photos in dim light by leaving their camera’s shutter open for minutes or even hours at a time).

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Body waves inside the earth

Two kinds of waves are generated by earthquakes and travel through solid rock:

In P or compressional waves, the vibration of the rock is in the direction of propagation. P waves travel fastest and are the first to arrive from the earthquake. In S or shear waves, rock oscillates perpendicular to the direction of wave propagation. In rock, S waves generally travel about 60% the speed of P waves, and the S wave always arrives after the P wave. For example, sound waves are P waves at a high enough frequency to hear with your ear. An example of an S wave is wiggling or shaking a rope which is tied down at one or both ends.

Both P and S waves travel outward from an earthquake focus inside the earth. The waves are often seen as separate arrivals recorded on seismographs at large distances from the earthquake. The direct P wave arrives first because its path is through the higher speed, dense rocks deeper in the earth. The PP (one bounce) and PPP (two bounces) waves travel more slowly than the direct P because they pass through shallower, lower velocity rocks. The different S waves arrive after the P waves.

The slowest (and latest to arrive on seismograms) are surface waves, such as the L wave. L waves are named for the Cambridge mathematician A.E.H. Love who first described them. The surface waves are generally the largest recorded from an earthquake. Body waves in the earth's interior lose their amplitude rapidly as they get farther from the earthquake because they spread out inside the volume of the earth. Surface waves, however, spread out more slowly and only on the earth's surface. The energy from surface waves is confined to a smaller volume at the surface and the wave amplitude to carry that energy is therefore larger than body waves.

A reference for further reading is:

Inside the Earth, Evidence from Earthquakes; by Bruce A. Bolt, W.H. Freeman & Co., San Francisco, first published 1982.

NOTIFICATIONS

Seismic waves.

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When an earthquake occurs, the shockwaves of released energy that shake the Earth and temporarily turn soft deposits, such as clay, into jelly ( liquefaction ) are called seismic waves, from the Greek ‘seismos’ meaning ‘earthquake’. Seismic waves are usually generated by movements of the Earth’s tectonic plates but may also be caused by explosions, volcanoes and landslides.

Seismologists use seismographs to record the amount of time it takes seismic waves to travel through different layers of the Earth. As the waves travel through different densities and stiffness, the waves can be refracted and reflected. Because of the different behaviour of waves in different materials, seismologists can deduce the type of material the waves are travelling through.

The results can provide a snapshot of the Earth’s internal structure and help us to locate and understand fault planes and the stresses and strains acting on them.

This wave behaviour can also be used on a smaller scale by recording waves generated by explosions or ground vibrators in the search for oil and gas.

Types of seismic waves

There are three basic types of seismic waves – P-waves, S-waves and surface waves. P-waves and S-waves are sometimes collectively called body waves.

P-waves, also known as primary waves or pressure waves, travel at the greatest velocity through the Earth. When they travel through air, they take the form of sound waves – they travel at the speed of sound (330 ms -1 ) through air but may travel at 5000 ms -1 in granite. Because of their speed, they are the first waves to be recorded by a seismograph during an earthquake.

They differ from S-waves in that they propagate through a material by alternately compressing and expanding the medium, where particle motion is parallel to the direction of wave propagation – this is rather like a slinky that is partially stretched and laid flat and its coils are compressed at one end and then released.

S-waves, also known as secondary waves, shear waves or shaking waves, are transverse waves that travel slower than P-waves. In this case, particle motion is perpendicular to the direction of wave propagation. Again, imagine a slinky partially stretched, except this time, lift a section and then release it, a transverse wave will travel along the length of the slinky.

S-waves cannot travel through air or water but are more destructive than P-waves because of their larger amplitudes

Surface waves

Surface waves are similar in nature to water waves and travel just under the Earth’s surface. They are typically generated when the source of the earthquake is close to the Earth’s surface. Although surface waves travel more slowly than S-waves, they can be much larger in amplitude and can be the most destructive type of seismic wave. There are two basic kinds of surface waves:

• Rayleigh waves, also called ground roll, travel as ripples similar to those on the surface of water. People have claimed to have observed Rayleigh waves during an earthquake in open spaces, such as parking lots where the cars move up and down with the waves.
• Love waves cause horizontal shearing of the ground. They usually travel slightly faster than Rayleigh waves

What can seismic waves tell us?

Studies of the different types of seismic waves can tell us much about the nature of the Earth’s structure.

For example, seismologists can use the direction and the difference in the arrival times between P-waves and S-waves to determine the distance to the source of an earthquake. If the seismographs are too far away from the event to record S-waves, several recordings of P-waves can be crunched in a computer program to give an approximate location of the source.

Activity idea

In the activity Earthquake location , students are introduced to some of the methods scientists use to record earthquakes. They extract data from seismograms to locate the epicentre of an earthquake, which they plot on a map of New Zealand. Students then consider the location and predict possible damage.

Useful links

Watch these videos on YouTube, from GNS scientists:

• Yoshihiro Kaneko models the propagation of seismic waves across New Zealand
• John Ristau explains of how seismic waves are used to locate an earthquake .

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16.1 Traveling Waves

Learning objectives.

By the end of this section, you will be able to:

• Describe the basic characteristics of wave motion
• Define the terms wavelength, amplitude, period, frequency, and wave speed
• Explain the difference between longitudinal and transverse waves, and give examples of each type
• List the different types of waves

We saw in Oscillations that oscillatory motion is an important type of behavior that can be used to model a wide range of physical phenomena. Oscillatory motion is also important because oscillations can generate waves, which are of fundamental importance in physics. Many of the terms and equations we studied in the chapter on oscillations apply equally well to wave motion ( Figure 16.2 ).

Types of Waves

A wave is a disturbance that propagates, or moves from the place it was created. There are three basic types of waves: mechanical waves, electromagnetic waves, and matter waves.

Basic mechanical wave s are governed by Newton’s laws and require a medium. A medium is the substance mechanical waves propagate through, and the medium produces an elastic restoring force when it is deformed. Mechanical waves transfer energy and momentum, without transferring mass. Some examples of mechanical waves are water waves, sound waves, and seismic waves. The medium for water waves is water; for sound waves, the medium is usually air. (Sound waves can travel in other media as well; we will look at that in more detail in Sound .) For surface water waves, the disturbance occurs on the surface of the water, perhaps created by a rock thrown into a pond or by a swimmer splashing the surface repeatedly. For sound waves, the disturbance is a change in air pressure, perhaps created by the oscillating cone inside a speaker or a vibrating tuning fork. In both cases, the disturbance is the oscillation of the molecules of the fluid. In mechanical waves, energy and momentum transfer with the motion of the wave, whereas the mass oscillates around an equilibrium point. (We discuss this in Energy and Power of a Wave .) Earthquakes generate seismic waves from several types of disturbances, including the disturbance of Earth’s surface and pressure disturbances under the surface. Seismic waves travel through the solids and liquids that form Earth. In this chapter, we focus on mechanical waves.

Electromagnetic waves are associated with oscillations in electric and magnetic fields and do not require a medium. Examples include gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves. Electromagnetic waves can travel through a vacuum at the speed of light, v = c = 2.99792458 × 10 8 m/s . v = c = 2.99792458 × 10 8 m/s . For example, light from distant stars travels through the vacuum of space and reaches Earth. Electromagnetic waves have some characteristics that are similar to mechanical waves; they are covered in more detail in Electromagnetic Waves .

Matter waves are a central part of the branch of physics known as quantum mechanics. These waves are associated with protons, electrons, neutrons, and other fundamental particles found in nature. The theory that all types of matter have wave-like properties was first proposed by Louis de Broglie in 1924. Matter waves are discussed in Photons and Matter Waves .

Mechanical Waves

Mechanical waves exhibit characteristics common to all waves, such as amplitude, wavelength, period, frequency, and energy. All wave characteristics can be described by a small set of underlying principles.

The simplest mechanical waves repeat themselves for several cycles and are associated with simple harmonic motion. These simple harmonic waves can be modeled using some combination of sine and cosine functions. For example, consider the simplified surface water wave that moves across the surface of water as illustrated in Figure 16.3 . Unlike complex ocean waves, in surface water waves, the medium, in this case water, moves vertically, oscillating up and down, whereas the disturbance of the wave moves horizontally through the medium. In Figure 16.3 , the waves causes a seagull to move up and down in simple harmonic motion as the wave crests and troughs (peaks and valleys) pass under the bird. The crest is the highest point of the wave, and the trough is the lowest part of the wave. The time for one complete oscillation of the up-and-down motion is the wave’s period T . The wave’s frequency is the number of waves that pass through a point per unit time and is equal to f = 1 / T . f = 1 / T . The period can be expressed using any convenient unit of time but is usually measured in seconds; frequency is usually measured in hertz (Hz), where 1 Hz = 1 s −1 . 1 Hz = 1 s −1 .

The length of the wave is called the wavelength and is represented by the Greek letter lambda ( λ ) ( λ ) , which is measured in any convenient unit of length, such as a centimeter or meter. The wavelength can be measured between any two similar points along the medium that have the same height and the same slope. In Figure 16.3 , the wavelength is shown measured between two crests. As stated above, the period of the wave is equal to the time for one oscillation, but it is also equal to the time for one wavelength to pass through a point along the wave’s path.

The amplitude of the wave ( A ) is a measure of the maximum displacement of the medium from its equilibrium position. In the figure, the equilibrium position is indicated by the dotted line, which is the height of the water if there were no waves moving through it. In this case, the wave is symmetrical, the crest of the wave is a distance + A + A above the equilibrium position, and the trough is a distance − A − A below the equilibrium position. The units for the amplitude can be centimeters or meters, or any convenient unit of distance.

The water wave in the figure moves through the medium with a propagation velocity v → . v → . The magnitude of the wave velocity is the distance the wave travels in a given time, which is one wavelength in the time of one period, and the wave speed is the magnitude of wave velocity. In equation form, this is

This fundamental relationship holds for all types of waves. For water waves, v is the speed of a surface wave; for sound, v is the speed of sound; and for visible light, v is the speed of light.

Transverse and Longitudinal Waves

We have seen that a simple mechanical wave consists of a periodic disturbance that propagates from one place to another through a medium. In Figure 16.4 (a), the wave propagates in the horizontal direction, whereas the medium is disturbed in the vertical direction. Such a wave is called a transverse wave . In a transverse wave, the wave may propagate in any direction, but the disturbance of the medium is perpendicular to the direction of propagation. In contrast, in a longitudinal wave or compressional wave, the disturbance is parallel to the direction of propagation. Figure 16.4 (b) shows an example of a longitudinal wave. The size of the disturbance is its amplitude A and is completely independent of the speed of propagation v .

A simple graphical representation of a section of the spring shown in Figure 16.4 (b) is shown in Figure 16.5 . Figure 16.5 (a) shows the equilibrium position of the spring before any waves move down it. A point on the spring is marked with a blue dot. Figure 16.5 (b) through (g) show snapshots of the spring taken one-quarter of a period apart, sometime after the end of` the spring is oscillated back and forth in the x -direction at a constant frequency. The disturbance of the wave is seen as the compressions and the expansions of the spring. Note that the blue dot oscillates around its equilibrium position a distance A , as the longitudinal wave moves in the positive x -direction with a constant speed. The distance A is the amplitude of the wave. The y -position of the dot does not change as the wave moves through the spring. The wavelength of the wave is measured in part (d). The wavelength depends on the speed of the wave and the frequency of the driving force.

Waves may be transverse, longitudinal, or a combination of the two. Examples of transverse waves are the waves on stringed instruments or surface waves on water, such as ripples moving on a pond. Sound waves in air and water are longitudinal. With sound waves, the disturbances are periodic variations in pressure that are transmitted in fluids. Fluids do not have appreciable shear strength, and for this reason, the sound waves in them are longitudinal waves. Sound in solids can have both longitudinal and transverse components, such as those in a seismic wave. Earthquakes generate seismic waves under Earth’s surface with both longitudinal and transverse components (called compressional or P-waves and shear or S-waves, respectively). The components of seismic waves have important individual characteristics—they propagate at different speeds, for example. Earthquakes also have surface waves that are similar to surface waves on water. Ocean waves also have both transverse and longitudinal components.

Example 16.1

Wave on a string.

• The speed of the wave can be derived by dividing the distance traveled by the time.
• The period of the wave is the inverse of the frequency of the driving force.
• The wavelength can be found from the speed and the period v = λ / T . v = λ / T .
• The first wave traveled 30.00 m in 6.00 s: v = 30.00 m 6.00 s = 5.00 m s . v = 30.00 m 6.00 s = 5.00 m s .
• The period is equal to the inverse of the frequency: T = 1 f = 1 2.00 s −1 = 0.50 s . T = 1 f = 1 2.00 s −1 = 0.50 s .
• The wavelength is equal to the velocity times the period: λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m . λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m .

Significance

Check your understanding 16.1.

When a guitar string is plucked, the guitar string oscillates as a result of waves moving through the string. The vibrations of the string cause the air molecules to oscillate, forming sound waves. The frequency of the sound waves is equal to the frequency of the vibrating string. Is the wavelength of the sound wave always equal to the wavelength of the waves on the string?

Example 16.2

Characteristics of a wave.

• The amplitude and wavelength can be determined from the graph.
• Since the velocity is constant, the velocity of the wave can be found by dividing the distance traveled by the wave by the time it took the wave to travel the distance.
• The period can be found from v = λ T v = λ T and the frequency from f = 1 T . f = 1 T .
• The distance the wave traveled from time t = 0.00 s t = 0.00 s to time t = 3.00 s t = 3.00 s can be seen in the graph. Consider the red arrow, which shows the distance the crest has moved in 3 s. The distance is 8.00 cm − 2.00 cm = 6.00 cm . 8.00 cm − 2.00 cm = 6.00 cm . The velocity is v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s . v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s .
• The period is T = λ v = 8.00 cm 2.00 cm/s = 4.00 s T = λ v = 8.00 cm 2.00 cm/s = 4.00 s and the frequency is f = 1 T = 1 4.00 s = 0.25 Hz . f = 1 T = 1 4.00 s = 0.25 Hz .

Check Your Understanding 16.2

The propagation velocity of a transverse or longitudinal mechanical wave may be constant as the wave disturbance moves through the medium. Consider a transverse mechanical wave: Is the velocity of the medium also constant?

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Home → Science → Geology

The main types of seismic waves: P, S, and surface waves

Seismic waves can either be body waves or surface waves -- but the full story is far more complex.

Generally speaking, there are two types of waves: body waves (which comprise of P or Primary waves and S or Secondary waves) and surface waves (Love and Rayleigh). But the long story is more complex — and much more interesting.

Seismic waves are produced by earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions. They are a form of acoustic wave, just like sound waves. The vast majority of them are associated with natural earthquakes.

What’s an earthquake, anyway?

In the broadest sense, an earthquake is just what the name suggests — any shaking of the Earth’s interior. Earthquakes can happen for a variety of reasons, but by far the most common cause is tectonic.

The Earth’s crust (the outermost layer) is split into rigid plates, all of which are moving relative to each other. The movement produces more and more stress on the ground until something eventually breaks along what’s called a geological fault. This is why, if you overlay a global tectonic plate map and a global earthquake map, you’ll see an almost perfect overlap between tectonic edges and temblors.

Volcanoes can also produce earthquakes, though they are generally less impactful than tectonic earthquakes.

Man-made explosions (for instance, atomic testing) can also produce earthquake-type features which produce seismic waves and can be detected — this is what allows remote monitoring of nuclear explosions.

For some earthquakes, the cause remains poorly understood, particularly in the case of intraplate tectonics (inside tectonic plates , not on the edges). Another cause of some very deep earthquakes is the so-called mineral phase change: atoms in minerals such as olivine can change their positions to become more tightly packed. Their chemistry remains the same, but their volume and density change dramatically  if an equilibrium is reached. This requires very particular conditions to happen, but if it does happen, it creates a type of “anti-crack” and can generate massive, deep earthquakes.

Types of seismic waves

Seismologists like to split seismic waves into several categories, but the main types of seismic waves come in two categories — body waves (which move throughout entire bodies, such as the Earth), and surface waves )(which travel only on different surfaces, not through the whole body). The main types of seismic waves are the following:

• Primary waves  (P-waves).  These are the “first” body waves — the ones that travel the fastest and through any type of medium (solid, liquid, gas). They propagate longitudinally on the propagation direction (think of an accordion) and are harmless in terms of earthquake damage.
• Secondary ( S-wave ). These are shear waves, which arrive after the P-waves. They’re also body waves but they only propagate through a solid medium. They also rarely do any significant damage.
• Surface waves — Rayleigh ( R-wave ). Surface waves (Rayleigh and Love) do by far the most damage. As opposed to body waves (S and P waves), they propagate on the surface and carry the vast majority of the energy felt on the surface — in other words, these are what you feel when you experience an earthquake. This happens because although they move slower than body waves, their particle movement is much more pronounced (see below). In the case of Rayleigh waves, the motion is of a rolling nature, similar to an ocean surface wave.
• Surface wave — Love ( L-wave ). Contrary to their name, there’s nothing really lovable about the Love waves — they were named thusly after Augustus Edward Hough Love, a Professor for Natural Philosophy at Oxford University who first described the movement of the waves named after him. Love waves have a transversal (perpendicular) movement and are the most destructive outside the immediate area of the epicenter. Love waves can be devastating

Why seismic waves are important

Studying and understanding seismic waves is more than a theoretical pursuit — it’s very important for a number of reasons, which flow quite logically.

• Detecting epicenters

There are numerous seismographs around the world, all of which measure the earthquake (seismic) waves to some extent. Because the different waves have different speeds, by detecting the arrival times at in different regions in the world, the position of the earthquake can be detected — the so-called hypocenter. Contrary to popular belief, the epicenter is not the place where the earthquake ruptures (that’s called the ‘hypocenter’), but rather is the projection of the earthquake on the surface, which can of course also be inferred from this data.

A historical map of epicenters gives a good starting point to assess the likelihood of future earthquakes and can serve as a basic preparation, allowing city planners and residents to prepare for the likelihood of seismic events. Naturally, this leads to the next reason for studying seismic earthquakes.

• Assessing hazards

Assessing hazards basically aims to predict the potential ground shaking intensity from future earthquakes. This can’t be done from studying seismic waves alone, it requires a lot of local geology input and external considerations (for instance, earthquakes can also cause indirect damage though processes such as landslides) — but seismology is the first step.

Precise earthquake prediction (pinpointing the exact time and place of a future earthquake) is not possible and will not be possible for the foreseeable future due to the sheer complexity of the problem — but this doesn’t mean that we can’t make some guesses. Scientists predict earthquakes in odds and intervals, not in exact values. A noteworthy situation is the estimation of volcanic hazard: volcano eruptions are typically predicted by a swarm of small earthquakes, which is why most of the world’s active volcanoes are surrounded by seismic detectors.

• Constructing better buildings

If you want to look for the best seismic engineers in the world, you’ll probably find them in places like Chile or Japan. Why? Because they  need  to be good, given that those are some of the seismically active places in the world.

Engineering seismology lays the bases for calculating seismic hazard, and it makes a big difference — for instance, Japan’s sophisticated engineering and strongly-enforced building codes have probably saved thousands of lives. The western coast of the US, for instance, is also quite earthquake-prone due to the San Andreas fault, and despite calls  for better preparation, the area remains vulnerable.

• 30-second warning

If you live in an earthquake-prone area, you probably have access to one type of “early” earthquake alert. Typically, these alerts can let you know when the earthquake is coming 30-60 seconds ahead of time — it’s not a lot, but in some cases, it could make all the difference.

In case you’re wondering how this is done, it has everything to do with the velocity of different seismic waves: if you recall, P-waves travel much faster than surface waves, but don’t do any real damage — the “30 seconds” are the interval between the arrival of the P waves and that of the surface (Love and Rayleigh) waves.

• Detecting explosions

If you’ve ever wondered why nations can’t just hide nuclear tests, it has a lot to do with seismic waves.

Man-made explosions generate types of waves which can be detected worldwide, and it’s essentially impossible to hide any massive explosion from the entire world (although seismic waves alone can’t reveal the nuclear or non-nuclear nature of the explosion).

Studying the Earth with seismic waves

Another completely different reason why it makes a lot of sense to study seismic waves is to study the Earth’s interior.

Since we’re kids, we’re taught that the Earth has a crust, a mantle, and a core… but how do we know that? The answer is, of course, through seismic waves.

Geologists use seismic waves to determine the depths and structures of different Earth layers. For instance, P waves travel through all types of medium, whereas S waves only travel through solid waves — this was used to deduct the fact that the mantle acts as a fluid (it’s not really a liquid, but it’s not exactly a solid either — think of it as extremely thick honey).

Seismic waves also get reflected and refracted when they travel from one medium to another. These transitions are governed by differences in density, which is why we know so much about the density of many structures deep inside the Earth. An interesting consequence of this property is that earthquakes have a “blind spot”: an area of the world where waves coming from them can’t be detected.

Much of what we know about the planet’s tectonics, the Earth’s deep structure, and even some features closer to the surface hinges on our understanding of seismic waves.[panel style=”panel-default” title=”Prospecting” footer=””]A few decades ago, people realized that they can mimic natural seismic waves through explosions or specialized machinery — at a much smaller scale. Similarly to how earthquake waves can reveal a lot about the subsurface at a large scale, these man-made waves are used to infer properties of at a smaller scale.

This is widely used as a prospection tool, particularly for oil and gas reservoirs, but to a smaller extent, also for mineral resources, water, and even environmental studies.[/panel]

Other types of seismic waves

If you’ve made it this far — first of all, congrats — you might be looking for a more detailed classification of seismic waves. Seismologists apparently love to draw up wave categories, not necessarily depicting different types of waves but rather describing where those waves have passed through. So while primary, secondary, Rayleigh, and Love waves are abbreviated by P, S, R, and L respectively, they can gain additional notations. For instance, a  g   notation indicates that the wave only travels through the crust, without any ocean floor in its path. Conversely, a w  indicates that the wave traveled or bounced on the ocean floor.

Going deeper, a  J wave is an S wave in the outer core, while a K wave is a P-wave in the outer core. A c indicates that the wave reflects off the outer core, while an  i  indicates that it bounces off the inner core .

In theory, there are an infinite of paths for waves to take — although in practice, their energy decays as the travel through the Earth. However, they can still reach an impressive number of bounces, and the notations add up. So you can end up with wave names such as PKiKP  or SKS .

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Anatomy of an Electromagnetic Wave

Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another. Examples of stored or potential energy include batteries and water behind a dam. Objects in motion are examples of kinetic energy. Charged particles—such as electrons and protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light.

What are Electromagnetic and Mechanical waves?

Mechanical waves and electromagnetic waves are two important ways that energy is transported in the world around us. Waves in water and sound waves in air are two examples of mechanical waves. Mechanical waves are caused by a disturbance or vibration in matter, whether solid, gas, liquid, or plasma. Matter that waves are traveling through is called a medium. Water waves are formed by vibrations in a liquid and sound waves are formed by vibrations in a gas (air). These mechanical waves travel through a medium by causing the molecules to bump into each other, like falling dominoes transferring energy from one to the next. Sound waves cannot travel in the vacuum of space because there is no medium to transmit these mechanical waves.

ELECTROMAGNETIC WAVES

Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet. A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves. Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations."

Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of a radio wave -- one cycle per second -- is named the hertz, in honor of Heinrich Hertz.

His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light! This proved that radio waves were a form of light! Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell's waves — electromagnetic waves.

WAVES OR PARTICLES? YES!

Light is made of discrete packets of energy called photons. Photons carry momentum, have no mass, and travel at the speed of light. All light has both particle-like and wave-like properties. How an instrument is designed to sense the light influences which of these properties are observed. An instrument that diffracts light into a spectrum for analysis is an example of observing the wave-like property of light. The particle-like nature of light is observed by detectors used in digital cameras—individual photons liberate electrons that are used for the detection and storage of the image data.

POLARIZATION

One of the physical properties of light is that it can be polarized. Polarization is a measurement of the electromagnetic field's alignment. In the figure above, the electric field (in red) is vertically polarized. Think of a throwing a Frisbee at a picket fence. In one orientation it will pass through, in another it will be rejected. This is similar to how sunglasses are able to eliminate glare by absorbing the polarized portion of the light.

DESCRIBING ELECTROMAGNETIC ENERGY

The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy. This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two. Radio and microwaves are usually described in terms of frequency (Hertz), infrared and visible light in terms of wavelength (meters), and x-rays and gamma rays in terms of energy (electron volts). This is a scientific convention that allows the convenient use of units that have numbers that are neither too large nor too small.

The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.

Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is the wavelength. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet!

An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts (eV). An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens. Consider a jump rope with its ends being pulled up and down. More energy is needed to make the rope have more waves.

Next: Wave Behaviors

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/02_anatomy

Science Mission Directorate. "Anatomy of an Electromagnetic Wave" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/02_anatomy

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When an earthquake occurs, it releases energy waves, known as Seismic waves. It is like the ripples created in water if you throw a stone in it. Seismic waves are like ripples that can travel through the inside of the earth and on the surface.

Types of Earthquake Waves

Based on the medium they travel in, earthquake waves can be classified under two categories:

• Surface waves

Body waves are those waves that travel through the earth. They originate at the epicentre of the earthquake and travel through the earth at amazing speeds. There are two types of body waves, namely,

Surface waves are those waves that travel on the surface of the earth. The destruction caused by earthquakes is primarily done by these waves.

S waves and P waves

S waves  also called secondary waves and shear waves, are the second waves to hit the seismographs. They are transverse waves, which means that the motion is perpendicular to the direction of wave propagation . S waves can only travel through solids, and scientists have successfully mapped the earth’s interior by studying the routes of these waves.

P waves  or Primary waves are the first waves to hit the seismographs when an earthquake strikes. They are longitudinal waves which means that the direction of motion and propagation are the same.

Difference between p waves and s waves

Frequently asked questions – faqs, what is an earthquake, what are seismic waves, what are the two types of seismic waves, what are the two types of body waves, what are p waves, watch the video and discover what an earthquake is and what causes it..

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why surface wave is important for geographical studies?

Seismic waves are important in geology and not geography. Geography explains how human culture impacts the natural environment and how different regions influence the people living there. Geology discusses the Earth’s structure, composition, and changes over time.

Geologists investigate the deep layers of the Earth’s crust by examining how seismic waves traverse through the Earth.

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