There are two types of body waves:. Primary waves or P waves are the fastest moving waves, traveling at 1 to 5 miles per second 1. They can pass through solids, liquids and gases easily. As they travel through rock, the waves move tiny rock particles back and forth -- pushing them apart and then back together -- in line with the direction the wave is traveling.
These waves typically arrive at the surface as an abrupt thud. Secondary waves also called shear waves, or S waves are another type of body wave. They move a little more slowly than P waves, and can only pass through solids. As S waves move, they displace rock particles outward, pushing them perpendicular to the path of the waves. In an earthquake, body waves produce sharp jolts, while rolling motions of surface waves do most of the damage in an earthquake.
Seismograms record seismic waves. Over the past century, scientists have developed several ways of measuring earthquake intensity. The currently accepted method is the moment magnitude scale, which measures the total amount of energy released by the earthquake.
At this time, seismologists have not found a reliable method for predicting earthquakes. A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram.
Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was.
Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer. If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake because those waves cannot travel through the liquid core of the earth.
The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section. In order to locate an earthquake epicenter, scientists must first determine the epicenter distance from three different seismographs. The longer the time between the arrival of the P-wave and S-wave, the farther away is the epicenter.
So the difference in the P and S wave arrival times determines the distance between the epicenter and a seismometer. Another important characteristic of Love waves is that the amplitude of ground vibration caused by a Love wave decreases with depth - they're surface waves. Like the velocity the rate of amplitude decrease with depth also depends on the period.
Rayleigh waves are the slowest of all the seismic wave types and in some ways the most complicated. Like Love waves they are dispersive so the particular speed at which they travel depends on the wave period and the near-surface geologic structure, and they also decrease in amplitude with depth.
Rayleigh waves are similar to water waves in the ocean before they "break" at the surf line. As a Rayleigh wave passes, a particle moves in an elliptical trajectory that is counterclockwise if the wave is traveling to your right.
The amplitude of Rayleigh-wave shaking decreases with depth. As you might expect, the difference in wave speed has a profound influence on the nature of seismograms. Since the travel time of a wave is equal to the distance the wave has traveled, divided by the average speed the wave moved during the transit, we expect that the fastest waves arrive at a seismometer first.
Thus, if we look at a seismogram, we expect to see the first wave to arrive to be a P-wave the fastest , then the S-wave, and finally, the Love and Rayleigh the slowest waves.
Although we have neglected differences in the travel path which correspond to differences in travel distance and the abundance waves that reverberate within Earth, the overall character is as we have described. The fact that the waves travel at speeds which depend on the material properties elastic moduli and density allows us to use seismic wave observations to investigate the interior structure of the planet. We can look at the travel times, or the travel times and the amplitudes of waves to infer the existence of features within the planet, and this is a active area of seismological research.
To understand how we "see" into Earth using vibrations, we must study how waves interact with the rocks that make up Earth. Several types of interaction between waves and the subsurface geology i. As a wave travels through Earth, the path it takes depends on the velocity.
Perhaps you recall from high school a principle called Snell's law, which is the mathematical expression that allows us to determine the path a wave takes as it is transmitted from one rock layer into another. The change in direction depends on the ratio of the wave velocities of the two different rocks.
When waves reach a boundary between different rock types, part of the energy is transmitted across the boundary. The transmitted wave travels in a different direction which depends on the ratio of velocities of the two rock types.
Part of the energy is also reflected backwards into the region with Rock Type 1, but I haven't shown that on this diagram. Refraction has an important affect on waves that travel through Earth. In general, the seismic velocity in Earth increases with depth there are some important exceptions to this trend and refraction of waves causes the path followed by body waves to curve upward. The overall increase in seismic wave speed with depth into Earth produces an upward curvature to rays that pass through the mantle.
A notable exception is caused by the decrease in velocity from the mantle to the core. The second wave interaction with variations in rock type is reflection. I am sure that you are familiar with reflected sound waves; we call them echoes. And your reflection in a mirror or pool of water is composed of reflected light waves.
In seismology, reflections are used to prospect for petroleum and investigate Earth's internal structure. In some instances reflections from the boundary between the mantle and crust may induce strong shaking that causes damage about km from an earthquake we call that boundary the "Moho" in honor of Mohorovicic, the scientist who discovered it.
A seismic reflection occurs when a wave impinges on a change in rock type which usually is accompanied by a change in seismic wave speed. Part of the energy carried by the incident wave is transmitted through the material that's the refracted wave described above and part is reflected back into the medium that contained the incident wave. When a wave encounters a change in material properties seismic velocities and or density its energy is split into reflected and refracted waves.
The amplitude of the reflection depends strongly on the angle that the incidence wave makes with the boundary and the contrast in material properties across the boundary.
For some angles all the energy can be returned into the medium containing the incident wave. The actual interaction between a seismic wave and a contrast in rock properties is more complicated because an incident P wave generates transmitted and reflected P- and S-waves and so five waves are involved.
Likewise, when an S-wave interacts with a boundary in rock properties, it too generates reflected and refracted P- and S-waves. I mentioned above that surface waves are dispersive - which means that different periods travel at different velocities. The effects of dispersion become more noticeable with increasing distance because the longer travel distance spreads the energy out it disperses the energy.
Usually, the long periods arrive first since they are sensitive to the speeds deeper in Earth, and the deeper regions are generally faster. A dispersed Rayleigh wave generated by an earthquake in Alabama near the Gulf coast, and recorded in Missouri. 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.
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