Seismic Waves and Earth’s Interior

Posted on December 1, 2011 by Rashid Faridi

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.

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.

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.

P-waves are sound waves, it’s just that in seismology we are interested in frequencies that are lower than humans’ range of hearing (the speed of sound in air is about 0.3 km/sec). The vibration caused by P waves is a volume change, alternating from compression to expansion in the direction that the wave is traveling. P-waves travel through all types of media – solid, liquid, or gas.

S-Waves

As a transverse wave passes the ground perpendicular to the direction that the wave is propagating. S-waves are transverse waves.

Secondary , or S waves, travel slower than P waves and are also called “shear” waves because they don’t change the volume of the material through which they propagate, they shear it. S-waves are transverse waves because they vibrate the ground in a the direction “transverse”, or perpendicular, to the direction that the wave is traveling. Even though they are slower than P-waves, the S-waves move quickly. Typical S-wave propagation speeds are on the order of 1 to 8 km/sec. The lower value corresponds to the wave speed in loose, unconsolidated sediment, the higher value is near the base of Earth’s mantle.

An important distinguishing characteristic of an S-wave is its inability to propagate through a fluid or a gas because a fluids and gasses cannot transmit a shear stress and S-waves are waves that shear the material.

In general, earthquakes generate larger shear waves than compressional waves and much of the damage close to an earthquake is the result of strong shaking caused by shear waves.

Love Waves

Love waves are transverse and restricted to horizontal movement – they are recorded only on seismometers that measure the horizontal ground motion.

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. In general, earthquakes generate Love waves over a range of periods from 1000 to a fraction of a second, and each period travels at a different velocity but the typical range of velocities is between 2 and 6 km/second.

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

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. Typical speeds for Rayleigh waves are on the order of 1 to 5 km/s.

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.

Seismic Wave Propagation
Waves on a Seismogram

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.e. the rocks) are commonly observable on seismograms

  • Refraction
  • Reflection
  • Dispersion
  •  Diffraction
  •  AttenuationWe can examine the two simplest types of interaction refraction and reflection.
    RefractionAs 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. This speed decrease bends waves backwards and creates a “P-wave Shadow Zone” between about 100° and 140° distance (1° = 111.19 km).

Reflection

When a wave encounters a change in material properties (seismic velocities and or density) its energy is split into reflected and refracted waves.

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 100 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.

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.

Dispersion

A dispersed Rayleigh wave generated by an earthquake in Alabama near the Gulf coast, and recorded in Missouri.

It is 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.

P-Waves in Earth

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.

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.

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. The decrease in velocity from the lower mantle to the outer core casts a “shadow” on the P-waves that extends from about 100° to 140° distance. Other waves such as surface waves and body waves reflecting off the surface are recorded in the “shadow” region, but the P-wave “dies out” near 100°. Since the outer core is fluid, and S-waves cannot travel through a fluid, the “S-wave shadow zone” is even larger, extending from about 100° to 180°.

Source:http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/notes/waves_and_interior.html

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Podzolization of Soil

Posted on November 30, 2011 by Rashid Faridi

Podzolization encompasses the downward migration of Al and Fe, together with organic matter, from the surface areas and their accumulation in the profile’s deep areas.

This process is characterised by a strong acidity that causes the slow development of organic matter (which releases abundant organic compounds with an acidic nature) and an extreme alteration of the mineral phase (releasing abundant elements that are lixiviated by the drainage waters, while the medium is enriched with insoluble elements, such as Fe and Al, which are migrated downward by the organic compounds towards deeper horizons). In short, an eluvial horizon is formed on the surface with intense substance losses.

The organometallic complexes migrate to the subsurface horizons and they accumulate originating the Bh and Bs horizon of the podzols, in short leaving a very differentiated profile with a very complete and very noticeable horizon consequence: O/A/E/Bh/Bs.

The evidence that the podzolization process has developed in a soil is reflected in the profile’s spectacular micromorphology, with abundant coverings of organic matter on the sand grains in horizon Bh.

 Through this process, the overlying eluvial horizons are getting bleached. The complexes are moving to the brown, red or black horizon, which consist of cemented sesquioxides and/or organic compounds. The process of podzolization occurs usually under low pH value. The corresponding soil type is called Podzol. But there exist synonyms for Podzol. In China and United States of America it is called Spodosoils, in Brazil it’s called Espodossolos and in Australia Pedosols. The Podzols are typical soils for humid boreal and humid temperate zones. Podzols cover about 485 million ha worldwide and are usually found under coniferous forest or under heather . Podzols are able to occur on almost any rock and form on quartz-rich sands and sandstones, or on sedimentary debris from magmatic rocks, provided that there is a high precipitation. Podzols count as unattractive soil for farming. The reason is the sandy fraction of the soil, which means a low level of moisture and nutrients. A low pH makes it even worse. Furthermore, phosphate deficit and aluminium toxicity are other problems. Podzols can be left idle under their natural fauna, or else the best use of Podzols is grazing.

Links and Sources:

Website, University of Granada. Spain

Wikipedia

Image Link

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New Study:Earth’s Core Deprived of Oxygen

Posted on November 27, 2011 by Rashid Faridi

The composition of Earth‘s core remains a mystery. Scientists know that the liquid outer core consists mainly of iron, but it is believed that small amounts of some other elements are present as well. Oxygen is the most abundant element in the planet, so it is not unreasonable to expect oxygen might be one of the dominant “light elements” in the core. However, new research from a team including Carnegie’s Yingwei Fei shows that oxygen does not have a major presence in the outer core. This has major implications for our understanding of the period when Earth formed through the accretion of dust and clumps of matter.

read it here

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Countries That Beat United States in Science

Posted on November 26, 2011 by Rashid Faridi

Guest Post from Kaitlyn Cole

The U.S. stands out from the crowd in many ways, but average test scores in science and math usually don’t place our nation in the top tier. While there are many top-notch educational facilities in our country, from elementary all the way up to college, overall, the American educational system could use a major overhaul. Students simply haven’t been keeping up with those in other nations when it comes to learning the basics of math and science, a fact that could leave us behind in an economy driven by technology.

While there are many nations with amazing educational systems that you might expect to outpace American students in science, there are a number of other, more surprising, countries that fare better as well. Here, we’ve listed just a few of the countries you might not expect to best us in science scores, but that national studies have found actually do — with some beating Americans quite soundly.
Czech Republic: The Czech Republic has a tumultuous past, having been renamed and reorganized several times during the past century. As recently as 1993, the Czech Republic was part of the larger nation of Czechoslovakia, but through a peaceful dissolution it became its own nation. While the Czech Republic may have had an interesting past, even over the past two decades, today the nation is one of the most peaceful, democratic, and healthy in Europe, though many in the U.S. might know little about it. Education is also blossoming in the Czech Republic, and the education system is currently ranked 15th in the world by the Programme for International Student Development. Science scores easily outpace those here, with students in eighth (Czech eighth graders are second in the world in science test scores) and 12th grade scoring higher than U.S. students.
Hungary: At various points in history, Hungary was one of the cultural centers of the Western world, but after World War I, the nation lost more than 70% of its territory and more than 33% of its population. The years that followed were full of political and economic tumult, but today Hungary is a stable and thriving nation. While many U.S. students struggle with science education, those in Hungary score quite well on international assessments. For example, Hungarian eighth graders average 554 in science, while U.S. students were at 534 in the same year. U.S. students do catch up in high school, however.
Bulgaria: This small nation in the Balkans must be doing something right when it comes to science education. Despite major conflicts during World War II and political upheaval until the late 1980s, the nation boasts great science and math test scores for students. On the International Mathematics and Science Study, Bulgarian students scored well in science, especially at the eighth grade level, when the nation ranked fifth in the world (of the nations studied), a fair amount above the U.S.’s 17th place finish for the same grade.
Slovenia: Bordering Italy and Croatia, most in the U.S. likely know little of this small European nation. Slovenia is a relatively new country, declaring independence from Yugoslavia in 1991, but it has a long history and culture. While Slovenia may be relatively new, its education system is getting things right, and is currently ranked the 12th best in the world and the 4th best in the EU. Students in Slovenia scored better than U.S. students in science both in eighth and 12th grade assessments by the TIMSS. They also did better in the Programme for International Student Assessment, ranking 17th in science, with U.S. students coming in 23rd.
Estonia: Estonia is a tiny Baltic nation that in recent years had a tough battle to get independence from Soviet Russia, finally becoming autonomous in 1991. Today, it is one of the wealthiest former Soviet republics and boasts a great deal of political, economic, and educational freedom. Students in Estonia can expect to get an education in science that may be markedly better than that of U.S. students. According to PISA tests, Estonia ranks ninth in the world for science scores, outpacing nations like Australia, Germany, and Australia.
Macau: Macau is technically part of China, but is one of the nation’s two special administrative regions (the other being Hong Kong), and Macau operates its legal, economic, and immigration departments. Macau is a nation perhaps best known for gambling and tourism, so it might be surprising that it produces such standout scores in math and science. It’s even more surprising when you consider that the nation has no universal education system, and few residents have a secondary or college education. Still, students who do pursue education in Macau do well, ranking the small nation 18th in the world in science scores.
Poland: Poland was virtually destroyed during World War II, and its culture, economy, population, infrastructure, and stability all took a major hit. It took decades to recover, but today Poland has a very high standard of living and a respectable education system. While Poland’s education system is ranked 23rd in the world, its science scores earn it a higher spot at 19th, just a few spots higher than the U.S.
Russia: Russia may not be known for its education system, but that doesn’t mean students in the Russian Federation aren’t getting the knowledge they need, especially when it comes to science. This former Soviet nation has free education for all citizens, but getting into colleges and universities is extremely competitive, and as a result students put a great deal of effort into boosting their science and technology knowledge. That studying pays off, earning Russian students average scores on TIMSS that are just a few points higher than those in the U.S.
Australia: That famous laid-back Aussie attitude may be applied to many aspects of life in Australia, but education isn’t one of them. Australia currently ranks 10th in the world for science education via the PISA evaluation and scores exceptionally well on the TIMSS assessment as well, outperforming the U.S. in all grades except for fourth, where it doesn’t lag far behind.
China: It comes as no surprise that China is best ranked in the world when it comes to science education (and math, and reading), but what may be surprising is just how much better students score than those in the U.S. Fourth graders in Hong Kong scored an average of 554 on world math and science assessments, where students in the U.S. score 539. The PISA assessment found an even bigger gap, with science students in Shanghai averaging 575 and U.S. students just 502.

Of course, these scores and rankings should be taken in stride, as they don’t represent every student or every school. It’s also important to note that the U.S. beats out other nations with stellar educational systems like Norway, Iceland, and France when it comes to science education, so for everything the U.S. system is doing wrong, there’s also something they’re doing right.

First Published Here

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