Which wave arrives first

In S or shear waves, rock oscillates perpendicular to the direction of wave propagation. 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. P waves are recorded earlier than S waves , because they travel at a higher velocity. 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. All surface waves travel slower than body waves and Rayleigh waves are slower than Love waves. Seismic waves can be classified into two basic types: body waves which travel through the Earth and surface waves , which travel along the Earth's surface.

Those waves that are the most destructive are the surface waves which generally have the strongest vibration. 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. S waves travel by particles trying to slide past each other similar to when one shakes a rope up and down or from side to side. P waves can travel through solid and fluid materials, S waves can only travel through solids.

P waves travel faster than S waves. There are two types of body waves: P-waves travel fastest and through solids, liquids, and gases ; S-waves only travel through solids. Surface waves are the slowest, but they do the most damage in an earthquake. Explain that earthquake waves move particles of material in different ways: whereas, compressional waves create a back and forth motion parallel to the direction of the waves, shear waves create a back and forth motion perpendicular to the direction of the waves.

Seismic P waves are also called compressional or longitudinal waves, they compress and expand oscillate the ground back and forth in the direction of travel , like sound waves that move back and forth as the waves travel from source to receiver. P wave is the fastest wave. In the Earth, P waves travel at speeds from about 6 km 3. As the waves enter the core, the velocity drops to about 8 km 5 miles per second.

A P wave primary wave or pressure wave is one of the two main types of elastic body waves , called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any affected location or at a seismograph. A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion.

Seismic waves may travel either along or near the earth's surface Rayleigh and Love waves or through the earth's interior P and S waves. 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 mathematics behind wave propagation is elegant and relatively simple, considering the fact that similar mathematical tools are useful for studying light, sound, and seismic waves.

We can solve these equations or an appropriate approximation to them to compute the paths that seismic waves follow in Earth.

The diagram below is an example of the paths P-waves generated by an earthquake near Earth's surface would follow. The paths of P-wave energy for a shallow earthquake located at the top of the diagram. The main chemical shells of Earth are shown by different colors and regions with relatively abrupt velocity changes are shown by dashed lines.

The curves show the paths of waves, and the lines crossing the rays show mark the wavefront at one minute intervals. Note the curvature of the rays in the mantle, the complexities in the upper mantle, and the dramatic impact of the core on the wavefronts. We have already discussed the main elements in Earth's interior, the core, the mantle, and the crust. By studying the propagation characteristics travel times, reflection amplitudes, dispersion characteristics, etc.

Great progress was made quickly because for the most part Earth's interior is relatively simple, divided into a sphere the inner core surrounded by roughly uniform shells of iron and rock. Models that assume the Earth is perfectly symmetric can be used to predict travel times of P-waves that are accurate to a few seconds for a trip all the way across the planet.

The diagram below is a plot of the P- and S-wave velocities and the density as a function of depth into Earth. The top of the Earth is located at 0 km depth, the center of the planet is at km.

Velocity and density variations within Earth based on seismic observations. The main regions of Earth and important boundaries are labeled.

Several important characteristics of Earth's structure are illustrated in the chart. First note that in several large regions such as in the lower mantle, the outer core, and inner core, the velocity smoothly increases with depth.

The increase is a result of the effects of pressure on the seismic wave speed. Although temperature also increases with depth, the pressure increase resulting from the weight of the rocks above has a greater impact and the speed increases smoothly in these regions of uniform composition.

The shallow part of the mantle is different; it contains several important well-established and relatively abrupt velocity changes. In fact, we often divide the mantle into two regions, upper and lower, based on the level of velocity heterogeneity.

In this depth range the minerals that make up the mantle silicate rocks are transformed by the increasing pressure. The atoms in these rocks rearrange themselves into compact structures that are stable at the high pressures and the result of the rearrangement is an increase in density and elastic moduli, producing an overall increase in wave speed.

The two largest contrasts in material properties in the Earth system are located near the surface and the core-mantle boundary. Both are compositional boundaries and the core-mantle boundary is the larger contrast. Other sharp contrasts are observable, the inner-core outer-core boundary is relatively sharp, and velocities increase from the liquid to the solid. More recent efforts have focused on estimating the lateral variations in wave speed within the shells that make up the reference model.

These approaches are often based on seismic tomography, which is a way of mapping out the variations in structure using observations from large numbers of seismograms. The basic idea is to use observed delayed or early arrival times delayed with respect to the reference model to locate regions of relatively fast and relatively slow seismic wave speed. The idea is illustrated in the cartoon to the left. Waves are represented by arrows and are traveling from left to right.

Those that travel through the slow region are slowed down, and hence will be recorded later on the a seismogram. The same ideas are used in medical CAT scan imaging of human bodies, but the observed quantity in a CAT scan is not a travel time, but the amount of x-ray absorption.

Ultrasound imaging is identical to P-wave tomography, it's just that in seismology we don't have the choice of where are wave sources are located - we just exploit earthquakes. In the two decades tomography has been applied to Earth studies on many scales, from looking at small regions of Earth's crust that may contain petroleum, to imaging the entire planet. On a global scale, we might expect that the shallow parts of the mantle would correlate with the major structural features we can observe at the surface - the plate boundaries.

In regions where material is rising from the mantle, it should be warmer, and the velocity should be lower, in regions that are old and cold, such as beneath many of the old parts of continents, we would expect to see faster regions assuming that temperature is the only difference.

The actual variations are influenced by both temperature and composition variations, but they agree well with the ideas of plate tectonics, particularly at the divergent boundaries or oceanic spreading ridges. Map of the variations in seismic shear-wave speed with respect to the value in PREM at km depth.

The warm colors red, orange, and yellow show regions with slower than normal speeds, the darker regions are faster than normal. Note the correlation with plate boundaries and surface heat flow. Model S12 WM13, from W. Su, R. Woodward and A.