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    how does the behaviour of s-waves and p-waves indicate the properties of the earth?


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    Seismic Evidence for Internal Earth Structure

    Evidence for Internal Earth Structure and Composition

    Seismic Waves When an earthquake occurs the seismic waves (P and S waves) spread out in all directions through the Earth's interior. Seismic stations located at increasing distances from the earthquake epicenter will record seismic waves that have traveled through increasing depths in the Earth.

    Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass. Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves move more slowly through a liquid than a solid. Molten areas within the Earth slow down P waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves.

    When seismic waves pass between geologic layers with contrasting seismic velocities (when any wave passes through media with distinctly differing velocities) reflections, refraction (bending), and the production of new wave phases (e.g., an S wave produced from a P wave) often result. Sudden jumps in seismic velocities across a boundary are known as .

    The Crust Mohorovicic Seismic DiscontinuitySeismic stations within about 200 km of a continental earthquake (or other seismic disturbance such as a dynamite blast) report travel times that increase in a regular fashion with distance from the source. But beyond 200 km the seismic waves arrive sooner than expected, forming a break in the travel time vs. distance curve. Mohorovicic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km from the earthquake source had passed through a lower layer with significantly higher seismic velocity.

    This seismic discontinuity is now know as the Moho (much easier than "Mohorovicic seismic discontinuity") It is the boundary between the felsic/mafic crust with seismic velocity around 6 km/sec and the denser ultramafic mantle with seismic velocity around 8 km/sec. The depth to the Moho beneath the continents averages around 35 km but ranges from around 20 km to 70 km. The Moho beneath the oceans is usually about 7 km below the seafloor (i.e., ocean crust is about 7 km thick).

    Properties of the Crust Continental Crust

    Depth to Moho: 20 to 70 km, average 30 to 40 km

    Composition: felsic, intermediate, and mafic igneous, sedimentary, and metamorphic rocks

    Age: 0 to 4 b.y.

    Summary: thicker, less dense, heterogeneous, old

    Oceanic Crust

    Depth to Moho: ~7 km

    Composition: mafic igneous rock (basalt & gabbro) with thin layer of sediments on top

    Age: 0 to 200 m.y.

    Summary: thin, more dense, homogeneous, young

    The Mantle Low Velocity ZoneSeismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure, and therefore density, with depth. However, seismic waves recorded at distances corresponding to depths of around 100 km to 250 km arrive later than expected indicating a zone of low seismic wave velocity. Furthermore, while both the P and S waves travel more slowly, the S waves are attenuated or weakened. This is interpreted to be a zone that is partially molten, probably one percent or less (i.e., greater than 99 percent solid).  Alternatively, it may simply represent a zone where the mantle is very close to its melting point for that depth and pressure that it is very "soft."  Then this represents a zone of weakness in the upper mantle. This zone is called the asthenosphere or "weak sphere."

    The asthenosphere separates the strong, solid rock of the uppermost mantle and crust above from the remainder of the strong, solid mantle below.  The combination of uppermost mantle and crust above the asthenosphere is called the lithosphere.  The lithosphere is free to move (glide) over the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.

    670 km Seismic DiscontinuityBelow the low velocity zone are a couple of seismic discontinuities at which seismic velocities increase.  Theoretical analyses and laboratory experiments show that at these depths (pressures) ultramafic silicates will change phase (atomic packing structure or crystalline structure) from the crystalline structure of olivine to tighter packing structures.  A discontinuity at around 670 km depth is particularly distinct. The 670 km discontinuity results from the change of spinel structure to the crystalline structure which remains stable to the base of the mantle. Perovskite (same chemical formula as olivine) is then the most abundant silicate mineral in the Earth. The 670 km discontinuity is thought to represents a major boundary separating a less dense upper mantle from a more dense lower mantle.

    The Core Gutenberg Seismic Discontinuity / Core-Mantle BoundarySeismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle (exceptions: see Low Velocity Zone and 670 km Discontinuity above). However, at arc distances of between about 103° and 143° no P waves are recorded. Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as the result of a molten core beginning at a depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted (bent).

    Source : www.columbia.edu

    P and S waves' paths through Earth

    Learn how knowledge of P and S waves can help scientists understand the structure of the Earth and how to locate the epicentre of an earthquake

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    Seismic waves

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    Physics (Single Science)Electricity, energy and waves

    P and S waves' paths through Earth

    The speed of P waves and S waves increases as they travel deeper into the Earth’s mantle.

    They travel through the Earth in curved paths, but they change direction suddenly when they pass through the boundary between substances in different states.

    The diagrams show what happens when P waves and S waves pass through the Earth.

    P waves

    Longitudinal Fast moving

    Travel through liquids and solids

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    P waves can pass through the Earth's core

    S waves


    Slower moving than P waves

    Travel through solids only

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    S waves do not pass through the Earth's core

    S waves cannot pass through the liquid outer core, but P waves can. The waves are refracted as they travel through the Earth due to a change in density of the medium. This causes the waves to travel in curved paths. When the waves cross the boundary between two different layers, there is a sudden change in direction due to refraction.


    Compare the properties of P waves, S waves and surface seismic waves.

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    BBC: Science and Environment

    BBC Earth

    BBC Tomorrow's World

    Isaac Physics Quizlet RevisioSUBSCRIPTION


    Science Museum

    Source : www.bbc.co.uk

    Seimic Waves and Earth’s Interior


    EAS-A193 Class Notes

    Seismic Waves and Earth's Interior


    When you look at a seismogram the wiggles you see are an indication that the ground is being, or was, vibrated by seismic waves. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions. You can picture this concept by recalling the circular waves that spread over the surface of a pond when a stone is thrown into the water. An earthquake is a more complicated process than a stone splashing into water, and the seismic waves that are set up during an earthquake are more varied than those on the pond.

    The are many different seismic waves, but all of basically of four types:

    Compressional or P (for primary)

    Transverse or S (for secondary)

    Love Rayleigh

    An earthquake radiates P and S waves in all directions and the interaction of the P and S waves with Earth's surface and shallow structure produces surface waves.

    Near an earthquake the shaking is large and dominated by shear-waves and short-period surface waves. These are the waves that do the most damage to our buildings, highways, etc. Even in large earthquakes the intense shaking generally lasts only a few tens of seconds, but it can last for minutes in the greatest earthquakes. At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake spreads throughout a larger volume of Earth. Also with increasing distance from the earthquake, the waves are separated apart in time and dispersed because P, S, and surface waves travel at different speeds.

    Seismic waves can be distinguished by a number of properties including the speed the waves travel, the direction that the waves move particles as they pass by, where and where they don't propagate. We'll go through each wave type individually to expound upon the differences.

    The first two wave types, P and S , are called body waves because they travel or propagate through the body of Earth. The latter two are called surface waves they the travel along Earth's surface and their amplitude decreases with depth into Earth.

    Wave Travel Times

    Travel times are best conceptualized of with an analogy of an auto trip. If you have to travel 120 miles and you drive 60 mph, you'll get to your destination in two hours, if you are forced to drive at a speed of 30 mph, it will take you twice as long to arrive at your destination. The mathematical formula we use in this problem is

    To apply those ideas to earthquake studies, think of the earthquake location as the starting point for the trip and the seismometer as the place where the trip concludes. Faster waves will travel the distance quicker and show up on the seismogram first.

    Travel time is a relative time, it is the number of minutes, seconds, etc. that the wave took to complete its journey. The arrival time is the time when we record the arrival of a wave - it is an absolute time, usually referenced to Universal Coordinated Time (a 24-hour time system used in many sciences). Here's an example to illustrate the difference: if two earthquakes occurred at the same place but exactly 24 hours apart, the wave travel times would be the same but the arrival times would differ by one day.

    Seismic Wave Speed

    Seismic waves travel fast, on the order of kilometers per second (km/s). The precise speed that a seismic wave travels depends on several factors, most important is the composition of the rock. We are fortunate that the speed depends on the rock type because it allows us to use observations recorded on seismograms to infer the composition or range of compositions of the planet. But the process isn't always simple, because sometimes different rock types have the same seismic-wave velocity, and other factors also affect the speed, particularly temperature and pressure. Temperature tends to lower the speed of seismic waves and pressure tends to increase the speed. Pressure increases with depth in Earth because the weight of the rocks above gets larger with increasing depth. Usually, the effect of pressure is the larger and in regions of uniform composition, the velocity generally increases with depth, despite the fact that the increase of temperature with depth works to lower the wave velocity.

    When I describe the different seismic wave types below I'll quote ranges of speed to indicate the range of values we observe in common terrestrial rocks. But you should keep in mind that the specific speed throughout Earth will depend on composition, temperature, and pressure.

    Compressional or P-Waves

    P-waves are the first waves to arrive on a complete record of ground shaking because they travel the fastest (their name derives from this fact - P is an abbreviation for primary, first wave to arrive). They typically travel at speeds between ~1 and ~14 km/sec. The slower values corresponds to a P-wave traveling in water, the higher number represents the P-wave speed near the base of Earth's mantle.

    The velocity of a wave depends on the elastic properties and density of a material. If we let k represent the bulk modulus of a material, m the shear-modulus, and r the density, then the P-wave velocity, which we represent by a, is defined by:

    Source : eqseis.geosc.psu.edu

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