That means that we can estimate the distance an earthquake is from a seismometer. The earthquake can be in any direction, but must be the estimated distance away. Geometrically that means that the earthquake must be located on a circle surrounding the seismometer, and the radius of the circle is about eight times the observed wave travel-time difference in kilometers. If we have two other seismometers which recorded the same earthquake, we could make a similar measurement and construct a circle of possible locations for each seismometer.
Since the earthquake location since it must lie on each circle centered on a seismometer, if we plot three or more circles on a map we could find that the three circles will intersect at a single location - the earthquake's epicenter.
Using the "S minus P arrival time" to locate an earthquake. You need at least three stations and some idea of the P and S velocities between the earthquake and the seismometers. In practice we use better estimates of the speed than our simple rule of thumb and solve the problem using algebra instead of geometry.
We also can include the earthquake depth and the time that earthquake rupture initiated called the "origin time" into the problem. Love waves are transverse waves that vibrate the ground in the horizontal direction perpendicular to the direction that the waves are traveling. They are formed by the interaction of S waves with Earth's surface and shallow structure and are dispersive waves.
The speed at which a dispersive wave travels depends on the wave's period. Love waves are transverse and restricted to horizontal movement - they are recorded only on seismometers that measure the horizontal ground motion. 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.
Related questions How does liquefaction occur and what dangers are associated with it? What is the seismic moment of an earthquake and what is it used for? What is a seismic wave? What is the focus of an earthquake? What is an epicenter? What causes earthquakes? What is the Ring of Fire and how does it relate to earthquake distribution across the globe? What is an earthquake? S-waves cannot travel through liquids or gases.
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. Improve this page Learn More. Skip to main content. Module 8: Earthquakes. Search for:. Reading: Body Waves Body waves travel through the interior of the earth. P-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.
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 propagating across a plane left and from a point source right.
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 propagating across a plane left and from a point source right : S-waves can travel only through solids, because only solids have rigidity.
S-waves travel through materials with rigidity and density greater rigidity faster S-waves greater density slower S-waves Contribute!
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