9 interior of the earth

7/29/2019 9 Interior of the Earth

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Chapter 9

Earth's Internal StructureCrust - Mantle - Core

374

TeachingPlate

Tectonics

Teaching Plate Tectonics Earth's Internal Structure

Divergent Boundary

Convergent Boundary Transform Boundary Tectonic Features Map

Three Parts of Earth's Interior: Knowledge of earth's interior is essential for understanding plate tectonics. Agood analogy for teaching about earth's interior is a piece of fruit with a large pit such as a peach or a plum. Moststudents are familiar with these fruits and have seen them cut in half. In addition the sizes of the features are verysimilar.

If we cut a piece of fruit in half we will see that it is composed of three parts: 1) a very thin skin, 2) a seed of

significant size located in the centre and 3) most of the mass of the fruit being contained within the flesh. Cuttingthe earth we would see: 1) a very thin crust on the outside, 2) a core of significant size in the centre, and 3) mostof the mass of the Earth contained in the mantle.

Earth's Crust: There are two different types of crust: thin oceanic crust that underlies the ocean basins andthicker continental crust that underlies the continents. These two different types of crust are made up of different
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types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust iscomposed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on themuch higher density mantle below.

Earth's Mantle: Earth's mantle is thought to be composed mainly of olivine-rich rock. It has differenttemperatures at different depths. The temperature is lowest immediately beneath the crust and increases withdepth. The highest temperatures occur where the mantle material is in contact with the heat-producing core. Thissteady increase of temperature with depth is known as the geothermal gradient. The geothermal gradient isresponsible for different rock behaviors and the different rock behaviors are used to divide the mantle into twodifferent zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft(but not molten). Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes.However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking. The lowerlimit of brittle behavior is the boundary between the upper and lower mantle.

Earth's Core: Earth's Core is thought to be composed mainly of an iron and nickel alloy. This composition is

assumed based upon calculations of its density and upon the fact that many meteorites (which are thought to be

portions of the interior of a planetary body) are iron-nickel alloys. The core is earth's source of internal heat

because it contains radioactive materials which release heat as they break down into more stable substances.

The core is divided into two different zones. The outer core is a liquid because the temperatures there are

adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher

than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough

to crowd the atoms tightly together and prevents the liquid state.

earthquake
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EarthquakeView Poster

Headlines

Aid Flows in to Haiti

Glimmers of Hope Amid Devastation in Haiti

How Haiti's Earthquake Unfolded

An earthquake is atremorof the earth's surface usually triggered by the release of underground

stress alongfaultlines. This release causes movement in masses of rock and resulting shock waves.

In spite of extensive research and sophisticated equipment, it is impossible to predict an

earthquake, although experts can estimate the likelihood of an earthquake occurring in a particular

region.

In 1935, American seismologist Charles Richter developed a scale that measures the magnitude

ofseismic waves. Called theRichter scale, it rates earth tremors on a scale from 1 to 9, with 9

being the most powerful and each number representing an increase of ten times the energy over

the previous number. According to this scale, any quake that is higher than 4.5 can cause damage

to stone buildings; quakes rated a magnitude of 7 and above are considered very severe. A less-

known scale, theMercalli scale, was devised by Italian seismologist Giuseppe Mercalli to measurethe severity of an earthquake in terms of its impact on a particular area and its inhabitants and

buildings.

Some earthquakes are too small to be felt but can cause movement of the earth, opening up holes

and displacing rocks. Shock waves from a very powerful earthquake can trigger smaller quakes

hundreds of miles away from theepicenter. Approximately 1,000 earthquakes measuring 5.0 and

above occur yearly. Earthquakes of the greatest intensity happen about once a year and major

earthquakes (7.0-7.9) occur about 18 times a year. Strong earthquakes (6.0-6.9) occur about 10

times a month and moderate earthquakes (5.0-5.9) happen more than twice daily. Most

earthquakes are not even noticed by the general public, since they happen either under the ocean

or in unpopulated areas. Sometimes an earthquake under the ocean can be so severe, it will cause

a tsunami, responsible for far greater damage.

The greatest danger of an earthquake comes from falling buildings and structures and flying glass,

stones and other objects.

Read more:http://www.answers.com/topic/earthquake#ixzz1hNcWQ5Qp

What causes earthquakes?
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The short answer is that earthquakes are caused by faulting, a sudden lateral orvertical movement of rock along a rupture (break) surface.

Here's the longer answer: The surface of the Earth is in continuous slow motion.This is plate tectonics--the motion of immense rigid plates at the surface of the

Earth in response to flow of rock within the Earth. The plates cover the entiresurface of the globe. Since they are all moving they rub against each other in some

places (like the San Andreas Fault in California), sink beneath each other in others

(like the Peru-Chile Trench along the western border of South America), or spreadapart from each other (like the Mid-Atlantic Ridge). At such places the motion isn'tsmooth--the plates are stuck together at the edges but the rest of each plate is

continuing to move, so the rocks along the edges are distorted (what we call"strain"). As the motion continues, the strain builds up to the point where the rockcannot withstand any more bending. With a lurch, the rock breaks and the two

sides move. An earthquake is the shaking that radiates out from the breaking rock.

People have known about earthquakes for thousands of years, of course, but theydidn't know what caused them. In particular, people believed that the breaks in the

Earth's surface--faults--which appear after earthquakes, were caused *by* theearthquakes rather than the cause *of* them. It was Bunjiro Koto, a geologist in

Japan studying a 60-mile long fault whose two sides shifted about 15 feet in thegreat Japanese earthquake of 1871, who first suggested that earthquakes werecaused by faults. Henry Reid, studying the great San Francisco earthquake of 1906,

took the idea further. He said that an earthquake is the huge amount of energy

released when accumulated strain causes a fault to rupture. He explained that rocktwisted further and further out of shape by continuing forces over the centuries

eventually yields in a wrenching snap as the two sides of the fault slip to a new

position to relieve the strain. This is the idea of "elastic rebound" which is nowcentral to all studies of fault rupture.

Dr. Gerard Fryer

Earthquakes and the Earth's Interior

This page last updated on 24-Oct-2003

Earthquakes

Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This

release of energy causes intense ground shaking in the area near the source of the earthquake

and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can

be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes

are definitely a geologic hazard for those living in earthquake prone areas, but the seismic

waves generated by earthquakes are invaluable for studying the interior of the Earth.


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Origin of Earthquakes

Most natural earthquakes are caused by sudden

slippage along a fault zone. The elastic

rebound theory suggests that if slippage along

a fault is hindered such that elastic strain

energy builds up in the deforming rocks on

either side of the fault, when the slippage does

occur, the energy released causes an

earthquake. This theory was discovered by

making measurements at a number of points

across a fault. Prior to an earthquake it was

noted that the rocks adjacent to the fault were

bending. These bends disappeared after anearthquake suggesting that the energy stored in

bending the rocks was suddenly released

during the earthquake.

Seismology, The Study of Earthquakes

When an earthquake occurs, the elastic energy is released and sends out vibrations that travel

throughout the Earth. These vibrations are called seismic waves. The study of how seismicwaves behave in the Earth is calledseismology.

Seismographs - Seismic

waves travel through the

Earth as vibrations. A

seismometer is an

instrument used torecord these vibrations

and the resulting graph

that shows the

vibrations is called a

seismograph. The

seismometer must be

able to move with the

vibrations, yet part of it

must remain nearly

stationary.

This is accomplished by isolating the recording device (like a pen) from the rest of the


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Earth using the principal of inertia. For example, if the pen is attached to a large mass

suspended by a spring, the spring and the large mass move less than the paper which is

attached to the Earth, and on which the record of the vibrations is made.

Seismic Waves. The source of an

earthquake is called thefocus,

which is an exact location within

the Earth where seismic waves are

generated by sudden release of

stored elastic energy. The

epicenter is the point on the surface

of the Earth directly above the

focus. Sometimes the media get

these two terms confused. Seismic

waves emanating from the focus

can travel in several ways, and thusthere are several different kinds of

seismic waves.

o Body Waves - emanate

from the focus and

travel in all directions

through the body of the

Earth. There are two

types of body waves:

P - waves - are Primary waves. They travel with a velocity that depends

on the elastic properties of the rock through which they travel.

Vp = [(K + 4/3 )/ ]

Where, Vp is the velocity of the P-wave, K is the incompressibility of the

material, is the rigidity of the material, and is the density of the

material.

P-waves are the same thing as sound waves. They move through the


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material by compressing it, but after it has been compressed it expands,

so that the wave moves by compressing and expanding the material as it

travels. Thus the velocity of the P-wave depends on how easily the

material can be compressed (the incompressibility), how rigid the

material is (the rigidity), and the density of the material. P-waves have

the highest velocity of all seismic waves and thus will reach allseismographs first.

S-Waves - Secondary waves, also called shear waves. They travel with a

velocity that depends only on the rigidity and density of the material

through which they travel:

Vs = [( )/ ]

S-waves travel through material by shearing it or changing its shape in

the direction perpendicular to the direction of travel. The resistance to

shearing of a material is the property called the rigidity. It is notable that

liquids have no rigidity, so that the velocity of an S-wave is zero in a

liquid. (This point will become important later). Note that S-waves travel

slower than P-waves, so they will reach a seismograph after the P-wave.

o Surface Waves - Surface waves differ from body waves in that they do not

travel through the Earth, but instead travel along paths nearly parallel to the

surface of the Earth. Surface waves behave like S-waves in that they cause up

and down and side to side movement as they pass, but they travel slower than S-

waves and do not travel through the body of the Earth.

The record of an

earthquake, a

seismograph, as

recorded by a

seismometer, will

be a plot of

vibrations versus

time. On the

seismograph, time is

marked at regularintervals, so that we

can determine the

time of arrival of

the first P-wave and

the time of arrival

of the first S-wave.

(Note again, that because P-waves have a higher velocity than S-waves, the P-waves arrive at

the seismographic station before the S-waves).


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Location of Earthquakes - Inorder to determine the location

of an earthquake, we need to

have recorded a seismographof the earthquake from at least

three seismographic stations at

different distances from the

epicenter of the quake. In

addition, we need one further

piece of information - that is

the time it takes for P-waves

and S-waves to travel through

the Earth and arrive at a

seismographic station. Such

information has been collected

over the last 80 or so years,

and is available as travel time

curves.

From the seismographs at each

station one determines the S-P

interval (the difference in the

time of arrival of the first S-

wave and the time of arrival of

the first P-wave. Note that on

the travel time curves, the S-P

interval increases with

increasing distance from theepicenter. Thus the S-P interval

tells us the distance to the

epicenter from the

seismographic station where

the earthquake was recorded.

Thus, at each station we can

draw a circle on a map that has

a radius equal to the distance

from the epicenter.

Three such circles will intersect in a point that locates the epicenter of the earthquake.


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Magnitude of Earthquakes - Whenever a large destructive earthquake occurs in the

world the press immediately wants to know where the earthquake occurred and how big

the earthquake was (in California the question is usually - Was this the Big One?). The

size of an earthquake is usually given in terms of a scale called the Richter Magnitude.

Richter Magnitude is a scale of earthquake size developed by a seismologist namedCharles F. Richter. The Richter Magnitude involves measuring the amplitude(height) of the largest recorded wave at a specific distance from the earthquake. While it

is correct to say that for each increase in 1 in the Richter Magnitude, there is a tenfold

increase in amplitude of the wave, it is incorrect to say that each increase of 1 inRichter Magnitude represents a tenfold increase in the size of the Earthquake (as is

commonly incorrectly stated by the Press).

A better measure of the size of an earthquake is the amount of energy released by the

earthquake. The amount of energy released is related to the Richter Scale by the following

equation:

Log E = 11.8 + 1.5 M

Where Log refers to the logarithm to the base 10, E is the energy released in ergs, and M is the

Richter Magnitude.

Anyone with a hand calculator can solve this equation by plugging in various values of M and

solving for E, the energy released. I've done the calculation for you in the following table:

Richter Magnitude Energy

(ergs)

Factor

1 2.0 x 1031 x

2 6.3 x 10

3 2.0 x 1031 x

4 6.3 x 10

5 2.0 x 1031 x

6 6.3 x 10

7 2.0 x 1031 x

8 6.3 x 10

From these calculations you can see that each increase in 1 in Richter Magnitude

represents a 31 fold increase in the amount of energy released. Thus, a magnitude 7

earthquake releases 31 times more energy than a magnitude 6 earthquake. A magnitude

8 earthquake releases 31 x 31 or 961 times more energy than a magnitude 6

earthquake.


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The Hiroshima atomic bomb released an amount of energy equivalent to a magnitude

5.5 earthquake. The largest earthquake recorded, the Alaska earthquake in 1964, had a

Richter Magnitude of about 8.6. Note that larger earthquakes are possible, but have not

been recorded by humans.

Earthquake Risk

The risk that an earthquake will occur close to where you live depends on whether or not

tectonic activity that causes deformation is occurring within the crust of that area. For the

U.S., the risk is greatest in the most tectonically active area , that is near the plate margin in

the Western U.S. Here, the San Andreas Fault which forms the margin between the Pacific

Plate and the North American Plate, is responsible for about 1 magnitude 8 or greater

earthquake per century. Also in the western U.S. is the Basin and Range Province, where

extensional stresses in the crust have created many normal faults that are still active.

Historically, large earthquakes have also occurred in the area of New Madrid, Missouri;

Charleston, South Carolina; and an area extending from New Jersey to Massachusetts. (See

igure 10.10 in your text). Why earthquakes occur in these other areas is not well understood. If

earthquakes have occurred before, they are expected to occur again.

Earthquake Damage

Many seismologists have said that "earthquakes don't kill people, buildings do". This is because

most deaths from earthquakes are caused by buildings or other human construction falling

down during an earthquake. Earthquakes located in isolated areas far from human population

rarely cause any deaths. Thus, in earthquake prone areas like California, there are strict

building codes requiring the design and construction of buildings and other structures that will

withstand a large earthquake. While this program is not always completely successful, one fact

stands out to prove its effectiveness. In 1986 an earthquake near San Francisco, California witha Richter Magnitude of 7.1 killed about 40 people. Most were killed when a double decked

freeway collapsed. About 10 months later, an earthquake with magnitude 6.9 occurred in the

Armenia, where no earthquake proof building codes existed. The death toll in the latter

earthquake was about 25,000!

Damage from earthquakes can be classified as follows:

Ground Shaking - Shaking of the ground caused by the passage of seismic waves near

the epicenter of the earthquake is responsible for the collapse of most structures. The

intensity of ground shaking depends on distance from the epicenter and on the type of

bedrock underlying the area.

o In general, loose unconsolidated sediment is subject to more intense shaking

than solid bedrock.

o Damage to structures from shaking depends on the type of construction.

Concrete and masonry structures, because they are brittle are more susceptible

to damage than wood and steel structures, which are more flexible.

Ground Rupture - Ground rupture only occurs along the fault zone that moves during

the earthquake. Thus structures that are built across fault zones may collapse, whereas

structures built adjacent to, but not crossing the fault may survive.


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Fire - Fire is a secondary effect of earthquakes. Because power lines may be knocked

down and because natural gas lines may rupture due to an earthquake, fires are often

started closely following an earthquake. The problem is compounded if water lines are

also broken during the earthquake since there will not be a supply of water to extinguish

the fires once they have started. In the 1906 earthquake in San Francisco more than

90% of the damage to buildings was caused by fire.

Rapid Mass-Wasting Processes - In mountainous regions subjected to earthquakes

ground shaking may trigger rapid mass-wasting events like rock and debris falls, rock

and debris slides, slumps, and debris avalanches.

Liquefaction -

Liquefaction is a

processes that occurs in

water-saturated

unconsolidatedsediment due to

shaking. In areas

underlain by such

material, the ground

shaking causes the

grains to loose grain

to grain contact, and

thus the material tends

to flow.

You can demonstrate this process to yourself next time your go the beach. Stand on thesand just after an incoming wave has passed. The sand will easily support your weight

and you will not sink very deeply into the sand if you stand still. But, if you start to

shake your body while standing on this wet sand, you will notice that the sand begins to

flow as a result of liquefaction, and your feet will sink deeper into the sand.

Tsunamis - Tsunamis are giant ocean waves that can rapidly travel across oceans, as we

discussed in the Oceans and Their Margins. Earthquakes that occur along coastal areas

can generate tsunamis, which can cause damage thousands of kilometers away on the

other side of the ocean.

World Distribution of Earthquakes

The distribution of earthquakes is referred to asseismicity. Most earthquakes occur along

relatively narrow belts that coincide with plate boundaries (see figure 10.15 in your text).


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This makes sense, since

plate boundaries are

zones along which

lithospheric plates move

relative to one another.

Earthquakes along thesezones can be divided into

shallow focus

earthquakes that have

focal depths less than

about 70 km and deep

focus earthquakes that

have focal depths

between 75 and 700 km.

Earthquakes at Diverging Plate Boundaries. Diverging plate boundaries are zones where

two plates move away from each other, such as at oceanic ridges. In such areas thelithosphere is in a state of tensional stress and thus normal faults and rift valleys occur.

Earthquakes that occur along such boundaries show normal fault motion and tend to be

shallow focus earthquakes, with focal depths less than about 20 km. Such shallow focal

depths indicate that the brittle lithosphere must be relatively thin along these diverging

plate boundaries.

Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate

boundaries where lithospheric plates slide past one another in a horizontal fashion. The

San Andreas Fault of California is one of the longer transform fault boundaries known.

Earthquakes along these boundaries show strike-slip motion on the faults and tend to be

shallow focus earthquakes with depths usually less than about 50 km.

Earthquakes at Converging Plate Boundaries - Convergent plate boundaries are

boundaries where two plates run into each other. Thus, they tend to be zones where

compressional stresses are active and thus reverse faults or thrust faults are common.

There are two types of converging plate boundaries. (1) subduction boundaries, where

oceanic lithosphere is pushed beneath either oceanic or continental lithosphere; and (2)

collision boundaries where two plates with continental lithosphere collide.

o Subduction boundaries -At subduction boundaries cold oceanic lithosphere is

pushed back down into the mantle where two plates converge at an oceanic

trench. Because the subducted lithosphere is cold it remains brittle as it descendsand thus can fracture under the compressional stress. When it fractures, it

generates earthquakes that define a zone of earthquakes with increasing focal

depths beneath the overriding plate. This zone of earthquakes is called

theBenioff Zone. Focal depths of earthquakes in the Benioff Zone can reach

down to 700 km.


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o Collision boundaries - At collisional boundaries two plates of continentallithosphere collide resulting in fold-thrust mountain belts. Earthquakes occur

due to the thrust faulting and range in depth from shallow to about 200 km.

The Earth's Internal Structure

Much of what we know about the interior of the Earth comes from knowledge of seismic wave

velocities and their variation with depth in the Earth. Recall that body wave velocities are as

follows:

Vp = [(K + 4/3 )/ ]

Vs = [( )/ ]

Where K = incompressibility

= rigidity

= density

If the properties of the earth, i.e. K, , and where the same throughout, then Vp and Vs wouldbe constant throughout the Earth and seismic waves would travel along straight line pathsthrough the Earth. We know however that density must change with depth in the Earth, because

the density of the Earth is 5,200 kg/cubic meter and density of crustal rocks is about 2,500

kg/cubic meter. If the density were the only property to change, then we could make estimates

of the density, and predict the arrival times or velocities of seismic waves at any point away

from an earthquake. Observations do not follow the predictions, so, something else must be

happening. In fact we know that K, , and change due to changing temperatures, pressures

and compositions of material. The job of seismology is, therefore, to use the observed seismic

wave velocities to determine how K, , and change with depth in the Earth, and then infer

how P, T, and composition change with depth in the Earth. In other words to tell us something

about the internal structure of the Earth.


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Reflection and Refraction of Seismic Waves.

If composition (or physical properties) change abruptly at some interface, then seismic wave

will both reflect off the interface and refract (or bend) as they pass through the interface. Twocases of wave refraction can be recognized.

1. If the seismic wavevelocity in the rock

above an interface is less

than the seismic wave

velocity in the rockbelow the interface, the

waves will be refracted

or bent upward relative

to their original path.

If the seismic wave velocity decreases when passing into the rock below the interface,

the waves will be refracted down relative to their original path.


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If the seismic wave velocities

gradually increase with depth in

the Earth, the waves will

continually be refracted along

curved paths that curve back

toward the Earth's surface.

One of the earliest

discoveries ofseismology was a

discontinuity at a depth

of 2900 km where the

velocity of P-waves

suddenly decreases. This

boundary is the boundary

between the mantle and

the core and was

discovered because of a

zone on the opposite side

of the Earth from an

Earthquake focus

receives no direct P-

waves because the P-

waves are refracted

inward as a result of the

sudden decrease in

velocity at the boundary.

This zone is called a P-

wave shadow zone.


16/50

This discovery wasfollowed by the

discovery of an S-wave

shadow zone. The S-

wave shadow zone

occurs because no S-

waves reach the area on

the opposite side of the

Earth from the focus.

Since no direct S-wavesarrive in this zone, it

implies that no S-waves

pass through the core.

This further implies the

velocity of S-wave in the

core is 0. In liquids =

0, so S-wave velocity is

also equal to 0. From this

it is deduced that the

core, or at least part of

the core is in the liquid]state, since no S-waves

are transmitted through

liquids. Thus, the S-wave

shadow zone is best

explained by a liquid

outer core.

Seismic Wave Velocities in the Earth

Over the years seismologists have collected data on how seismic wave velocities vary with

depth in the Earth. Distinct boundaries, called discontinuities are observed when there is sudden

change in physical properties or chemical composition of the Earth. From these discontinuities,

we can deduce something about the nature of the various layers in the Earth. As we discussed

way back at the beginning of the course, we can look at the Earth in terms of layers of differing

chemical composition, and layers of differing physical properties.


17/50


18/50

amounts of Nickel.

Layers of Different Physical Properties

o At a depth of about 100 km there is a sudden decrease in both P and S-wave

velocities. This boundary marks the base of the lithosphere and the top of the

asthenosphere. The lithosphere is composed of both crust and part of the upper

mantle. It is a brittle layer that makes up the plates in plate tectonics, and appears

to float and move around on top of the more ductile asthenosphere.

o At the top of the asthenosphere is a zone where both P- and S-wave velocities

are low. This zone is called theLow-Velocity Zone (LVZ). It is thought that the

low velocities of seismic waves in this zone are caused by temperaturesapproaching the partial melting temperature of the mantle, causing the mantle in

this zone to behave in a very ductile manner.

o At adepth of 400 km there is an abrupt increase in the velocities of seismic

waves, thus this boundary is known as the 400 - Km Discontinuity. Experiments

on mantle rocks indicate that this represents a temperature and pressure where

there is a polymorphic phase transition, involving a change in the crystal

structure of Olivine, one of the most abundant minerals in the mantle.

o Another abrupt increase in seismic wave velocities occurs at a depth of 670 km.

It is uncertain whether this discontinuity, known as the 670 Km Discontinuity, isthe result of a polymorphic phase transition involving other mantle minerals or a

compositional change in the mantle, or both.

Return to EENS 111 Page

Seismic wave properties

Seismic waves are waves of energy that travel through the earth, and are a result ofanearthquake,explosion, or a volcano that imparts low-frequency acoustic energy. Many othernatural and anthropogenic sources create low amplitude waves commonly referred to asambientvibrations. Seismic waves are studied byseismologistsandgeophysicists. Seismic wave fields aremeasured by aseismograph,geophone,hydrophone(in water), oraccelerometer.

The propagationvelocityof the waves depends ondensityandelasticityof the medium. Velocity

tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth'scrustup to

13 km/s in the deepmantle.
http://www.tulane.edu/~sanelson/geol111/index.htmlhttp://www.tulane.edu/~sanelson/geol111/index.htmlhttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Explosionhttp://en.wikipedia.org/wiki/Explosionhttp://en.wikipedia.org/wiki/Explosionhttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/Geophysicshttp://en.wikipedia.org/wiki/Geophysicshttp://en.wikipedia.org/wiki/Geophysicshttp://en.wikipedia.org/wiki/Seismographhttp://en.wikipedia.org/wiki/Seismographhttp://en.wikipedia.org/wiki/Geophonehttp://en.wikipedia.org/wiki/Geophonehttp://en.wikipedia.org/wiki/Geophonehttp://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Signal_velocityhttp://en.wikipedia.org/wiki/Signal_velocityhttp://en.wikipedia.org/wiki/Signal_velocityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Mantle_(geology)http://en.wikipedia.org/wiki/Mantle_(geology)http://en.wikipedia.org/wiki/Mantle_(geology)http://en.wikipedia.org/wiki/Mantle_(geology)http://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Signal_velocityhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Hydrophonehttp://en.wikipedia.org/wiki/Geophonehttp://en.wikipedia.org/wiki/Seismographhttp://en.wikipedia.org/wiki/Geophysicshttp://en.wikipedia.org/wiki/Seismologyhttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Ambient_Vibrationshttp://en.wikipedia.org/wiki/Explosionhttp://en.wikipedia.org/wiki/Earthquakehttp://www.tulane.edu/~sanelson/geol111/index.html


19/50

Earthquakes create various types of waves with different velocities; when reaching seismic

observatories, their differenttravel timeenables the scientists to locate theepicenter. In geophysics

the refraction or reflection of seismic waves is used for research of the Earth's interior, and artificial

vibrations to investigate subsurface structures.

[edit]Types of seismic waves

There are two types of seismic waves, body waveand surface waves. Other modes of wave

propagation exist than those described in this article, but they are of comparatively minor importance

for earth-borne waves, although they are important in the case ofasteroseismology, especiallyJason

Chervanev.

[edit]Body waves

Body waves travel through the interior of the Earth. They follow raypaths refracted by the

varyingdensityandmodulus(stiffness) of the Earth's interior. The density and modulus, in turn, vary

according to temperature, composition, and phase. This effect is similar to therefractionoflight

waves.

[edit]Primary waves

Main article:P-wave

Primary waves (P-waves) are compressional waves that arelongitudinalin nature. P waves are

pressure waves that are the initial set of waves produced by an earthquake. These waves can travel

through any type of material, and can travel at nearly twice the speed of S waves. In air, they take the

form of sound waves, hence they travel at thespeed of sound. Typical speeds are 330 m/s in air,

1450 m/s in water and about 5000 m/s ingranite.

[edit]Secondary waves

Main article:S-wave

Secondary waves (S-waves) are shear waves that aretransversein nature. These waves typically

follow P waves during an earthquake and displace the ground perpendicular to the direction of

propagation. Depending on the propagational direction, the wave can take on different surface

characteristics; for example, in the case of horizontally polarized S waves, the ground moves

alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids

and gases) do not support shear stresses. S waves are slower than P waves, and speeds are

typically around 10% of that of P waves in any given material.

[edit]Surface waves

Main article:Surface wave

Surface waves are analogous to water waves and travel along the Earth's surface. They travel slower

than body waves. Because of their low frequency, long duration, and large amplitude, they can be the

most destructive type of seismic wave. There are two types of surface waves:Rayleigh

wavesandLove waves.

[edit]Rayleigh waves

Main article:Rayleigh wave
http://en.wikipedia.org/wiki/Propagation_speedhttp://en.wikipedia.org/wiki/Propagation_speedhttp://en.wikipedia.org/wiki/Propagation_speedhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=1http://en.wikipedia.org/wiki/Asteroseismologyhttp://en.wikipedia.org/wiki/Asteroseismologyhttp://en.wikipedia.org/wiki/Asteroseismologyhttp://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=2http://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=3http://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/Speed_of_soundhttp://en.wikipedia.org/wiki/Speed_of_soundhttp://en.wikipedia.org/wiki/Speed_of_soundhttp://en.wikipedia.org/wiki/Granitehttp://en.wikipedia.org/wiki/Granitehttp://en.wikipedia.org/wiki/Granitehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=4http://en.wikipedia.org/wiki/S-wavehttp://en.wikipedia.org/wiki/S-wavehttp://en.wikipedia.org/wiki/S-wavehttp://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=5http://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=6http://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=6http://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=5http://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/wiki/S-wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=4http://en.wikipedia.org/wiki/Granitehttp://en.wikipedia.org/wiki/Speed_of_soundhttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=3http://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Light_wavehttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Jason_Chervanev&action=edit&redlink=1http://en.wikipedia.org/wiki/Asteroseismologyhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=1http://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Propagation_speed


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Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are

similar to those of waves on the surface of water (note, however, that the associated particle motion

at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is

elastic, not gravitational as for water waves). The existence of these waves was predicted by John

William Strutt,Lord Rayleigh, in 1885. They are slower than body waves, roughly 90% of the velocity

of S waves for typical homogeneous elastic media.

[edit]Love waves

Main article:Love wave

Love waves (L-waves) are surface waves that cause circular shearing of the ground. They are named

afterA.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911.

They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have

the largest amplitude.

[edit]P and S waves in Earth's mantle and core

When an earthquake occurs, seismographs near theepicenterare able to record both P and Swaves, but those at a greater distance no longer detect the high frequencies of the first S wave. Since

shear waves cannot pass through liquids, this phenomenon was original evidence for the now well-

established observation that the Earth has a liquidouter core, as demonstrated byRichard Dixon

Oldham. This kind of observation has also been used to argue, by seismic testing, that theMoonhas

a solid core, although recent geodetic studies suggest the core is still molten[citation needed]

.

[edit]Notation

Earthquake wave paths

The path that a wave takes between the focus and the observation point is often drawn as a ray

diagram. An example of this is shown in a figure above. When reflections are taken into account there

are an infinite number of paths that a wave can take. Each path is denoted by a set of letters that

describe the trajectory and phase through the Earth. In general an upper case denotes a transmitted

wave and a lower case denotes a reflected wave. The two exceptions to this seem to be "g" and "n".

The notation is taken from[1]

and.[2]

c the wave reflects off the outer core
http://en.wikipedia.org/wiki/Lord_Rayleighhttp://en.wikipedia.org/wiki/Lord_Rayleighhttp://en.wikipedia.org/wiki/Lord_Rayleighhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=7http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=7http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=7http://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/A.E.H._Lovehttp://en.wikipedia.org/wiki/A.E.H._Lovehttp://en.wikipedia.org/wiki/A.E.H._Lovehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=8http://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Outer_corehttp://en.wikipedia.org/wiki/Outer_corehttp://en.wikipedia.org/wiki/Outer_corehttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Moonhttp://en.wikipedia.org/wiki/Moonhttp://en.wikipedia.org/wiki/Moonhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=9http://en.wikipedia.org/wiki/Seismic_wave#cite_note-0http://en.wikipedia.org/wiki/Seismic_wave#cite_note-0http://en.wikipedia.org/wiki/Seismic_wave#cite_note-0http://en.wikipedia.org/wiki/Seismic_wave#cite_note-1http://en.wikipedia.org/wiki/Seismic_wave#cite_note-1http://en.wikipedia.org/wiki/Seismic_wave#cite_note-1http://en.wikipedia.org/wiki/File:Earthquake_wave_paths.svghttp://en.wikipedia.org/wiki/File:Earthquake_wave_paths.svghttp://en.wikipedia.org/wiki/File:Earthquake_wave_paths.svghttp://en.wikipedia.org/wiki/File:Earthquake_wave_paths.svghttp://en.wikipedia.org/wiki/Seismic_wave#cite_note-1http://en.wikipedia.org/wiki/Seismic_wave#cite_note-0http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=9http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Moonhttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Richard_Dixon_Oldhamhttp://en.wikipedia.org/wiki/Outer_corehttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=8http://en.wikipedia.org/wiki/A.E.H._Lovehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=7http://en.wikipedia.org/wiki/Lord_Rayleigh


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d a wave that has been reflected off a discontinuity at depth d

g a wave that only travels through the crust

i a wave that reflects off the inner core

I a P-wave in the inner core

h a reflection off a discontinuity in the inner core

J an S wave in the inner core

K a P-wave in the outer core

L a Love wave sometimes called LT-Wave (Both caps, while an Lt is different)

n a wave that travels along the boundary between the crust and mantle

P a P wave in the mantle

p a P wave ascending to the surface from the focus

R a Rayleigh wave

S an S wave in the mantle

s an S wave ascending to the surface from the focus

w the wave reflects off the bottom of the ocean


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No letter is used when the wave reflects off of the surface

For example:

ScP is a wave that begins traveling towards the center of the Earth as an S wave. Upon reaching

the outer core the wave reflects as a P wave.

sPKIKP is a wave path that begins traveling towards the surface as an S-wave. At the surface it

reflects as a P-wave. The P-wave then travels through the outer core, the inner core, the outer

core, and the mantle.

[edit]Usefulness of P and S waves in locating an event

P- and S-waves sharing with the propagation

In the case of local or nearby earthquakes, the difference in thearrival timesof the P and S waves

can be used to determine the distance to the event. In the case of earthquakes that have occurred at

global distances, four or more geographically diverse observing stations (using a commonclock)

recording P-wave arrivals permits the computation of a unique time and location on the planet for the

event. Typically, dozens or even hundreds of P-wave arrivals are used to calculatehypocenters. The

misfit generated by a hypocenter calculation is known as "the residual". Residuals of 0.5 second or

less are typical for distant events, residuals of 0.1-0.2 s typical for local events, meaning most

reported P arrivals fit the computed hypocenter that well. Typically a location program will start by

assuming the event occurred at a depth of about 33 km; then it minimizes the residual by adjusting

depth. Most events occur at depths shallower than about 40 km, but some occur as deep as 700 km.

A quick way to determine the distance from a location to the origin of a seismic wave less than200 km away is to take the difference in arrival time of the P wave and the S wave insecondsand

multiply by 8 kilometers per second. Modern seismic arrays use more complicatedearthquake

locationtechniques.

At teleseismic distances, the first arriving P waves have necessarily travelled deep into the mantle,

and perhaps have even refracted into the outer core of the planet, before travelling back up to the

Earth's surface where the seismographic stations are located. The waves travel more quickly than if

they had travelled in a straight line from the earthquake. This is due to the appreciably increased

velocities within the planet, and is termedHuygens' Principle.Densityin the planet increases with

depth, which would slow the waves, but themodulusof the rock increases much more, so deeper

means faster. Therefore, a longer route can take a shorter time.
http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=10http://en.wikipedia.org/wiki/Arrival_timehttp://en.wikipedia.org/wiki/Arrival_timehttp://en.wikipedia.org/wiki/Arrival_timehttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Huygens%27_Principlehttp://en.wikipedia.org/wiki/Huygens%27_Principlehttp://en.wikipedia.org/wiki/Huygens%27_Principlehttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/File:Ondes_P_et_S_1d_30_petit.gifhttp://en.wikipedia.org/wiki/File:Ondes_P_et_S_1d_30_petit.gifhttp://en.wikipedia.org/wiki/File:Ondes_P_et_S_1d_30_petit.gifhttp://en.wikipedia.org/wiki/File:Ondes_P_et_S_1d_30_petit.gifhttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Huygens%27_Principlehttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Earthquake_locationhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Arrival_timehttp://en.wikipedia.org/w/index.php?title=Seismic_wave&action=edit&section=10


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The travel time must be calculated very accurately in order to compute a precise hypocenter. Since P

waves move at many kilometers per second, being off on travel-time calculation by even a half

second can mean an error of many kilometers in terms of distance. In practice, P arrivals from many

stations are used and the errors cancel out, so the computed epicenter likely to be quite accurate, on

the order of 1050 km or so around the world. Dense arrays of nearby sensors such as those that

exist in California can provide accuracy of roughly a kilometer, and much greater accuracy is possible

when timing is measured directly bycross-correlationofseismogramwaveforms.

[edit]See also

AdamsWilliamson equation

[edit]References

1. ^An Introduction to the Theory of Seismology, 4th ed.; K.E. Bullen and Bruce A. Bolt (1993)

2. ^International Handbook of Earthquake and Engineering Seismology, Volume 1; ed. William Han Kung

Lee; accessed through books.google.com

[edit]External links

Seismic Wave Propagation

Waves on a Seismogram

As you might expect, the difference in wave speed has a profound influence on thenature of seismograms. Since the travel time of a wave is equal to the distance the

wave has travelled, divided by the average speed the wave moved during thetransit, we expect that the fastest waves arrive at a seismometer first. Thus, if welook 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 todifferences 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, orthe travel times and the amplitudes of waves to infer the existence of featureswithin 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. therocks) are commonly observable on seismograms

Refraction

Reflection
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Dispersion

Diffraction

Attenuation

We'll examine the two simplest types of interaction refraction and reflection.

Refraction

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

When waves reach a boundary between different rock types, part ofthe 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 importantexceptions 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 Earthproduces an upward curvature to rays that pass through the mantle. A


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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

The second wave interaction with variations in rock type is reflection. I am surethat you are familiar with reflected sound waves; we call them echoes. And yourreflection in a mirror or pool of water is composed of reflected light waves. Inseismology, reflections are used to prospect for petroleum and investigate Earth's

internal structure. In some instances reflections from the boundary between themantle 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 (whichusually 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 refractedwave described above) and part is reflected back into the medium that containedthe 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 incidencewave makes with the boundary and the contrast in material properties across the

boundary. For some angles all the energy can be returned into the mediumcontaining 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 reflectedP- andS-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.


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Dispersion

I mentioned above that surface waves are dispersive - which means that different

periods travel at different velocities. The effects of dispersion become morenoticeable with increasing distance because the longer travel distance spreads theenergy 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.

P-Waves in Earth

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 appropriateapproximation 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.

Intraplate earthquakeFrom Wikipedia, the free encyclopedia


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Compared to earthquakes near plate boundaries, intraplate earthquakes are not well understood, and the

hazards associated with them may be diff icult to quantify.

[edit]Historic examples

Historic examples of intraplate earthquakes include those inMineral, Virginiain2011(estimated magnitude

5.8),New Madridin1811 and 1812(estimated magnitude as high as 8.1), theBoston (Cape Ann)

earthquake of 1755(estimated magnitude 6.0 to 6.3), earthquakes felt inNew York Cityin 1737 and 1884

(both quakes estimated at about 5.5 magnitude), and theCharleston earthquakeinSouth Carolinain 1886

(estimated magnitude 6.5 to 7.3). The Charleston quake was particularly surprising because, unlike Boston

and New York, the area had almost no history of even minor earthquakes.

In 2001,a large intraplate earthquakedevastated the region ofGujarat,India. The earthquake occurred far

from any plate boundaries, which meant the region above the epicenter was unprepared for earthquakes.

In particular, theKutchdistrict suffered tremendous damage, where the death toll was over 12,000.

[edit]Causes

Many cities live with theseismic riskof a rare, large intraplate earthquake. The cause of these earthquakes

is often uncertain. In many cases, the causative fault is deeply buried, and sometimes cannot even be

found. Under these circumstances it is difficult to calculate the exact seismic hazardfor a given city,

especially if there was only one earthquake in historical times. Some progress is being made in

understanding thefault mechanicsdriving these earthquakes.

[edit]Prediction

Scientists continue to search for the causes of these earthquakes, and especially for some indication of

how often they recur. The best success has come with detailed micro-seismic monitoring, involving dense

arrays ofseismometers. In this manner, very small earthquakes associated with a causative fault can be

located with great accuracy, and in most cases these line up in patterns consistent with

faulting.Cryoseismscan sometimes be mistaken for intraplate earthquakes.

Interplate earthquakeFrom Wikipedia, the free encyclopedia

This article does notciteanyreferences or sources. Please helpimprove this articleby adding

citations toreliable sources. Unsourced material may bechallengedandremoved.(May 2007)

An interplate earthquake is anearthquakethat occurs at the boundary between twotectonic plates. If one

plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates

to slip relative to each other. The slipping process creates an earthquake with land deformations and

resultingseismic waveswhich travel through the Earth and along the Earth's surface. Relative plate motion

can be lateral as along atransform faultboundary or vertical if along a convergentsubductionboundary or
http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=2http://en.wikipedia.org/wiki/Mineral,_Virginiahttp://en.wikipedia.org/wiki/Mineral,_Virginiahttp://en.wikipedia.org/wiki/Mineral,_Virginiahttp://en.wikipedia.org/wiki/2011_Virginia_earthquakehttp://en.wikipedia.org/wiki/2011_Virginia_earthquakehttp://en.wikipedia.org/wiki/2011_Virginia_earthquakehttp://en.wikipedia.org/wiki/New_Madrid_Seismic_Zonehttp://en.wikipedia.org/wiki/New_Madrid_Seismic_Zonehttp://en.wikipedia.org/wiki/New_Madrid_Seismic_Zonehttp://en.wikipedia.org/wiki/New_Madrid_earthquakehttp://en.wikipedia.org/wiki/New_Madrid_earthquakehttp://en.wikipedia.org/wiki/New_Madrid_earthquakehttp://en.wikipedia.org/wiki/1755_Cape_Ann_earthquakehttp://en.wikipedia.org/wiki/1755_Cape_Ann_earthquakehttp://en.wikipedia.org/wiki/1755_Cape_Ann_earthquakehttp://en.wikipedia.org/wiki/1755_Cape_Ann_earthquakehttp://en.wikipedia.org/wiki/New_York_Cityhttp://en.wikipedia.org/wiki/New_York_Cityhttp://en.wikipedia.org/wiki/New_York_Cityhttp://en.wikipedia.org/wiki/Charleston_earthquakehttp://en.wikipedia.org/wiki/Charleston_earthquakehttp://en.wikipedia.org/wiki/Charleston_earthquakehttp://en.wikipedia.org/wiki/South_Carolinahttp://en.wikipedia.org/wiki/South_Carolinahttp://en.wikipedia.org/wiki/South_Carolinahttp://en.wikipedia.org/wiki/2001_Gujarat_earthquakehttp://en.wikipedia.org/wiki/2001_Gujarat_earthquakehttp://en.wikipedia.org/wiki/2001_Gujarat_earthquakehttp://en.wikipedia.org/wiki/Gujarathttp://en.wikipedia.org/wiki/Gujarathttp://en.wikipedia.org/wiki/Gujarathttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Kutchhttp://en.wikipedia.org/wiki/Kutchhttp://en.wikipedia.org/wiki/Kutchhttp://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=3http://en.wikipedia.org/wiki/Seismic_riskhttp://en.wikipedia.org/wiki/Seismic_riskhttp://en.wikipedia.org/wiki/Seismic_riskhttp://en.wikipedia.org/wiki/Seismic_hazardhttp://en.wikipedia.org/wiki/Seismic_hazardhttp://en.wikipedia.org/wiki/Seismic_hazardhttp://en.wikipedia.org/wiki/Fault_mechanicshttp://en.wikipedia.org/wiki/Fault_mechanicshttp://en.wikipedia.org/wiki/Fault_mechanicshttp://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Intraplate_earthquake&action=edit&section=4http://en.wikipedia.org/wiki/Seismometerhttp://en.wikipedia.org/wiki/Seismometerhttp://en.wikipedia.org/wiki/Seismometerhttp://en.wikipedia.org/wiki/Cryoseismhttp://en.wikipedia.org/wiki/Cryoseismhttp://en.wikipedia.org/wiki/Cryoseismhttp://en.wikipedia.org/wiki/Wikipedia:Citing_sourceshttp://en.wikipedia.org/wiki/Wikipedia:Citing_sourceshttp://en.wikipedia.org/wiki/Wikipedia:Citing_sourceshttp://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttp://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttp://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttp://en.wikipedia.org/w/index.php?title=Interplate_earthquake&action=edithttp://en.wikipedia.org/w/index.php?title=Interplate_earthquake&action=edithttp://en.wikipedia.org/w/index.php?title=Interplate_earthquake&action=edithttp://en.wikipedia.org/wiki/Wikipedia:Identifying_reliable_sourceshttp://en.wikipedia.org/wiki/Wikipedia:Identifying_reliable_sourceshttp://en.wikipedia.org/wiki/Wikipedia:Identifying_reliable_sourceshttp://en.wikipedia.org/wiki/Template:Citation_neededhttp://en.wikipedia.org/wiki/Template:Citation_neededhttp://en.wikipedia.org/wiki/Template:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Verifiability#Burden_of_evidencehttp://en.wikipedia.org/wiki/Wikipedia:Verifiability#Burden_of_evidencehttp://en.wikipedia.org/wiki/Wikipedia:Verifiability#Burden_of_evidencehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Tectonic_platehttp://en.wikipedia.org/wiki/Tectonic_platehttp://en.wikipedia.org/wiki/Tectonic_platehttp://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia.org/wiki/Transform_faulthttp://en.wikipedia.org/wiki/Transform_faulthttp://en.wikipedia.org/wiki/Transform_faulthttp://en.wikipedia.org/wiki/Subductionhttp://en.wikipedia.org/wiki/Subductionhttp://en.wikipedia.org/wiki/Subductionhttp://en.wikipedia.org/wiki/File:Question_book-new.svghttp://en.wikipedia.org/wiki/Subductionhttp://en.wikipedia.org/wiki/Transform_faulthttp://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia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ariftat a divergent boundary. At a subduction boundary the motion is due to one plate slipping beneath the

other plate resulting in an interplate thrust ormegathrust earthquake.

Some areas of the world that are particularly prone to such events include the west coast ofNorth

America(especiallyCaliforniaandAlaska), the northeastern Mediterranean region (Greece,Italy,andTurkeyin particular),Iran,New Zealand,Indonesia,Japan, and parts ofChina.

.

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. The decrease in velocityfrom the lower mantle to the outer core casts a "shadow" on the P-waves that
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extends from about 100 to 140 distance. Other waves such as surface waves andbody 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 cannottravel through a fluid, the "S-wave shadow zone" is even larger, extending from

about 100 to 180.

Earth's Internal Structure

We have already discussed the main elements in Earth's interior, the core, themantle, and the crust. By studying the propagation characteristics (travel times,reflection amplitudes, dispersion characteristics, etc.) of seismic waves for the last

90 years we have learned much about the detailed nature of Earth's interior. Great

progress was made quickly because for the most part Earth's interior is relativelysimple, 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 usedto predict travel times of P-waves that are accurate to a few seconds for a trip allthe 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, thecenter of the planet is at 6371 km.


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Velocity and density variations within Earth based on seismic

observations. The main regions of Earth and important boundaries are

labeled. This model was developed in the early 1980's and is called

PREM for Preliminary Earth Reference Model.

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 resultof 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 ofuniform 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 themantle into two regions, upper and lower, based on the level of velocity

heterogeneity. The region from near 400 to 1000 km depth is called the transitionzone and strongly affects body waves that "turn" at this depth and arrive about 20-


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[edit]Fold terminology in three dimensions

The hinge points along an entire folded surface form a hinge line, which can be either a crest lineor

a trough line. Thetrend and plungeof a linear hinge line gives you information about the orientation of

the fold. To more completely describe the orientation of a fold, one must describe the axial surface.

The axial surface is the surface defined by connecting all the hinge lines of stacked folding surfaces. If

the axial surface is a planar surface then it is called theaxial planeand can be described by thestrike

and dipof the plane. An axial traceis the line of intersection of the axial surface with any other

surface (ground, side of mountain, geological cross-section).

Finally, folds can have, but dont necessarily have afold axis. A fold axis, is the closest

approximation to a straight line that when moved parallel to itself, generates the form of the fold.

(Davis and Reynolds, 1996 after Donath and Parker, 1964; Ramsay 1967). A fold that can be

generated by a fold axis is called a cylindrical fold. This term has been broadened to include near-

cylindrical folds. Often, the fold axis is the same as the hinge line.[3][4]

[edit]Fold shape

It is necessary to convey a sense of the shape of the fold. A fold can be shaped as achevron, with

planar limbs meeting at an angular axis, as cuspate with curved limbs, ascircularwith a curved axis,

or as elliptical with unequalwavelength.

[edit]Fold tightness

Fold tightness is defined by the angle between the fold's limbs, called theinterlimb angle. Gentle folds

have an interlimb angle of between 180 and 120 , open folds range from 120 to 70, Close folds

from 70 to 30, tight folds from 30 to 0 ,[5]

andisoclinalfolds have an interlimb angle of between 10

and zero, with essentially parallel limbs.

[edit]Fold symmetry

Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length

are termedsymmetrical, and those with highly unequal limbs areasymmetrical. Asymmetrical folds

generally have an axis at an angle to the original unfolded surface they formed on.

[edit]Deformation style classes

Folds that maintain uniform layer thickness are classed as concentricfolds. Those that do not are

called similar folds. Similar folds tend to display thinning of the limbs and thickening of the hinge zone.

Concentric folds are caused by warping from active buckling of the layers, whereas similar folds

usually form by some form of shear flow where the layers are not mechanically active. Ramsay has

proposed a classification scheme for folds that often is used to describe folds in profile based upon

curvature of the inner and outer lines of a fold, and the behavior ofdip isogons. that is, linesconnecting points of equal dip:

[6]
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9 interior of the earth

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