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Introduction to Seismology
Ali O. [email protected]
Department of Earth SciencesKFUPM
Chapter 2Seismic Waves
http://faculty.kfupm.edu.sa/ES/oncel/geop204chap2.htmChapter 3, Bullen and Bolt
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Seismic Waves
The wiggles on a seismogram are caused by seismicwaves which are generated by the movement of therocks along a fault.
The waves emanate from the “source” or earthquake, and travel:
through the body of the Earth, and
over the surface of Earth.
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Waves in a pond
The idea is analogous to waves caused by tossing astone in a pond.
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Sound Wave AnalogySeismic waves represent acoustic (sound) energy and so are analogous to speech:
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Seismologists process these recordings (seismograms)
Brain processes the recordings
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Seismometers record these vibrations
Ears record thesevibrations
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Sound waves propagate through the Earth
Sound waves propagate through atmosphere
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A locked fault segmentfails (ruptures)
Vocal cords vibrate1
EarthquakesSpeech
What is a Wave ?
A wave is a disturbance that transfers energy.
Waves are common in nature:Light is a waveSound is a wave
Waves are periodic in both space and time (theyhave wavelengths and periods)
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Wave Terminology
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Wavelength is the length of a wave. It is measured from one peak to another or from one trough to another, in the direction the wave is traveling.Period is the time between two crests in a wave train. Examples of such waves are the seismic waves traveling through Earth, or tsunamiwaves in the ocean.Frequency is the number of waves or cycles of oscillation per second. One cycle per second (cps) is one Hertz (Hz). Frequency is the inverse of the period of the wave or of the cyclic motion. If the frequencies of the seismic waves that shake a structure closely match one or more of the structure's natural frequencies (its innate tendency to vibrate at one or more periodic rates) the structure will have a large amount of dynamic response--it will shake violently.Amplitude of a wave is the height of a wave crest or depth of a trough. Amplitude can also refer to the distance an object moves(displaces) as it vibrates.
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Wavelength and Period
Ampl
itude
Distance from Source
Wavelength
• At a given instant in either time and space, thedisplacement is periodic in both space (distance) and time. • Amplitude = maximum displacement from equilibrium(ie to crest or to trough)• Wavelength or Period= crest-to-crest distance or time
Ampl
itude
Time
Period
Crest (High Points)
Through (Low Points)
Equilibrium(Middle)
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Wave Speeds
The speed that a wave propagates at is not a dynamic quantity –it is a fixed material property. (like density) No matter how big an earthquake is, the seismic waves generated by earthquake will always travel at the same speed. The seismic wave speed of a material depends mainly its upon:
Temperature
Pressure
Composition
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Sources of Seismic Waves
Earthquakes generate seismic waves, but so do manyother processes:
Volcanic eruptions
Explosions
Wind
Sonic Booms (planes, shuttle, meteorites)
Humans
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Multiple-Frequency SignalsMost interesting signals are composites of waves withmany different frequencies. The range of frequency issometimes called the “band” and we speak ofbandwidth.
Light is usually a multiple frequency signal, and the
different frequencies correspond to what we call colors.
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Sometimes we can use the
observed frequencies to
identify different sources of
vibrations.
Which has higher frequency
content, the sonic boom or
the earthquake?
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b) An elastic material returns to its original shape and volume when deforming stress is removed
Elastic Behavior
a) The deformation of a material (strain) results from a force per unit area (stress) acting on the material.
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a) During ductile deformation particles remain connected and flowb) Brittle deformation results in the development of fractures
Style of Deformation
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What is Brittle?Brittle is the opposite of ductile. Brittle can describe a material property of rock: A brittle rock fractures when it is forced to change its shape and deform only a small amount (a small amount of strain). Brittle can also describe the way rock deforms under pressure: Fracturing, including earthquake faulting, is a brittle form of deformation, and even the strongest and toughest of rocks can be deformed until they fracture. Rock, especially at depths where temperatures are high, can instead flow or deform in a ductile manner. Similarly, in structural engineering, a material or structural component that cannot deform extensively without breaking is brittle and is very undesirable for resistance to earthquakes. Plain concrete and unreinforced masonry materials are brittle, while metals such as steel or aluminum are much more ductile. As in geology, brittle can refer in structural engineering to a mode of failure rather than a material property: The sudden buckling of a column or shearing apart of a wall are brittle failures, even if the material of the column or the wall, in and of itself, is ductile.
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Ductility is toughness, the ability to deform permanently without breaking. A ductile material can stretch, compress, or distort inelastically in shear -- past the point where it returns to its original shape. Ductile rock, such as rock heated to a high temperature in the interior of Earth slowly flows rather than breaks, does not suddenly crack under load as brittle rock does. A ductile structure or structural component continues to have significant strength after it has yielded. Typically, a well-designed ductile structure or component will show, up to a point, increasing strength as its deflection increases beyond yielding, or cracking in the case of reinforced concreteor masonry.
What is Ductility?
Adapted from the International Handbook of Earthquakeand Engineering Seismology, Aki and Lee[1]In
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Can you read this?
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Seismologists “read”
seismograms to learn about the
earth.
What is Seismic Wave? Waves in a Pound Sound Wave Analogy What is Wave? Wave Terminology Wavelength and Period Wave Speeds Sources of Seismic Waves Multiple Frequency Signals
Previous Lecture
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Recall: Elastic waves
• Amplitude is the peak to trough height divided by two.
• Wavelength is the distance over which the wave goes through one complete cycle.
• Period is the time over which the wave is observed to complete a single cycle.
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Snell’s Law• Snell’s Law governs the path by which a wave would take the least amount of time to propagate between two fixed points.
• In this case, the velocity of the overlying layer is less than that of the underlying layer.
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http://www.mines.utah.edu/~ggapps/snell/snell.html
See the effect of media velocity on reflection and refraction of seismic waves! Click the site..
Seismic VelocitySeismic velocity is a material property (like density).There are two kinds of waves – Body and Surface waves.There are two kinds of body wave velocity – P and S wave velocities.P waves always travel faster than S waves.Seismic velocities depend on quantities like chemical composition, pressure, temperature, etc.
Faster Velocities
• Lower temperatures
• Higher pressures
• Solid phases
Slower Velocities
• Higher temperatures
• Lower pressures
• Liquid phases
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Recall: Elastic Deformation
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Reference
Elastic Behavior
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Elastic
DuctileBrittle
Reference
Bulk Modulus (Incompressibility) = (∆P/Θ) … where Θ = dilatation = ∆V/V
and P = pressure
Elastic Constants: Bulk Modules
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Reference
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µ = shear stressshear strain
shear modulus (rigidity)
Elastic Constants: Shear Modules
shear stress = (∆F /A)
shear strain = (∆l /L)
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From the International Handbook of Earthquake and Engineering Seismology,
Aki and Lee[1]
The shear modulus is the ratio of shear stress to shearstrain of a material during simple shear.
Reference
Poisson’s ratio = υ= - (εyy / εxx)
∆LL
Then,∆WW
εxx =
transverse strain =
•Under a stress (sxx)along the x-axis,longitudinal strain
Elastic Constants: Poisson Ratio
εyy =
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The ratio of the transverse contracting strain to the elongation strain when a rod is stretched by forces which are applied at its ends and which are parallel to the rod's axis.
Reference
From Lay & Wallace (1995)
Elastic Moduli and Densities of Some Common Materials
Poisson RatioBulk Module
Shear ModuleLame’s constant
•Typical Values of Elastic Constants for Selected Materials
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Previous Lecture
Snell's LawSeismic Velocity What is Seismogram? What is Elastic Behavior?
Ductile DeformationBrittle Deformation Hooke's LawElastic Constants Bulk Modules Shear Modules Poisson RatioTypical Values of Elastic Constants for Selected Materials
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Recall: Stress-Strain Behavior
Hooke's Law ==> elasticity
Elas
ticEl
astic
DuctileDuctile
BrittleBrittle
Reference
Elastic: The stress limit for a material for its strainto be recoverable. Up to the elastic limit, Hooke’sLaw pertains, stress is proportional to strain, and if the stress is eliminated the strain is also eliminated and there is no permanent deformation.
Elastic Limit: The stress limit for a material for its strain to be recoverable. Up to the elastic limit, Hooke’s Law pertains, stress is proportional to strain, and if the stress is eliminated the strain is also eliminated and there is no permanent deformation.
Ductility: is toughness, the ability to deform permanently without breaking. A ductile material can stretch, compress, or distort inelastically in shear -- past the point where it returns to its original shape. Brittle is the opposite of ductile. Brittle can describe a material property of rock: A brittle rock fractures when it is forced to change its shape and deform only a small amount (a small amount of strain).Strain is the change in dimensions or shape of an object or material. Strain may be the change in length, area, volume or an angle.In
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Elastic Behavior: Lithosphere
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Elastic behavior occurs at very high strain rates, as particles are vibrated by an earthquake; the vibrations result in the passage of seismic wavesElastic behavior occurs at very high strain rates, as particles are vibrated by an earthquake; the vibrations result in the passage of seismic waves
Reference
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Inelastic (ductile) behavior results from very slow straining of the Asthenosphere; viscous flow facilitates the movement of overlying lithospheric platesInelastic (ductile) behavior results from very slow straining of the Asthenosphere; viscous flow facilitates the movement of overlying lithospheric plates
Plastic Behavior: Asthenosphere
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Reference
Elastic/plastic Behavior
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Elastic behavior occurs at very high strain rates, as particles are vibrated by an earthquake; the vibrations result in the passage of seismic waves
Inelastic (ductile) behavior results from very slow straining of the Asthenosphere; viscous flow facilitates the movement of overlying lithospheric plates
Reference
Mechanical WavesIn seismology, several types of surface waves are encountered. Surface waves, in this mechanical sense, are commonly known as either Love waves or Rayleigh waves. A seismic wave is a wave that travels through the Earth, often as the result of an earthquake or explosion. Love waves have transverse motion (movement is perpendicular to the direction of travel, like light waves), whereas Rayleigh waves have both longitudinal (movement parallel to the direction of travel, like sound waves) and transverse motion. Seismic waves are studied by seismologists and measured by a seismograph or seismometer. Surface waves span a wide frequency range, and the period of waves that are most damaging is usually 10 seconds or longer. Surface waves can travel around the globe many times from the strongest earthquakes.Surface wave can describe waves propagating over an ocean, even when they are approximated by Airy functions and are more properly called creeping waves. Examples are the waves at the surface of waterand air (ocean surface waves), or ripples in the sand at the interface with water or air. Another example is internal waves, which can be transmitted along the interface of two water masses of different densities.
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Recall: Types of Body WavesP-wave
S-wave
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Body waves are seismic waves that travel through Earth's interior. Body waves are different from surface waves, which are seismic waves that travel along Earth's surface. There are two main types of body waves, P waves and S waves.
Simulation of body waves traveling through Earth’s interior.Source: Thomas Boyd, Colorado School of Mines.
Types of Surface Waves
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Surface Wave DispersionSurface waves propagate along the earth’s surface.Surface waves are larger in amplitude and longer in durationthan body waves.Surface waves propagate at a speed lower than body waves and are recorded after the P and S waves.There are two types of surface waves: Rayleigh and Love waves.Rayleigh waves are denoted by LR or R, and Love waves are denoted by LQ or Q (L for long; R for Rayleigh; Q for Querwellen, German, ‘transverse waves’).Surface wave amplitudes decays exponentially with depth.Surface waves are dispersive, which means that their velocities depend on frequency.The first surface wave energy to arrive at any seismometer is of those frequencies that have the greatest velocities.The other frequencies will arrive later according to their frequencies.
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Dispersion of Surface Waves
Knopoff, 1972
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A plot of velocity against period is called the dispersion curve.
Dispersion curves contain much information about the velocity structure of the crust and upper mantle.
Particle Motions of Body Waves
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Body/Surface wave Prorogation
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Reference
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3D Components of Waves
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Seismic Seismic shadow zonesshadow zones
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•P waves are refracted at theMantle-Outer Core interface•S waves are stopped atthis interface•Both create shadow zonesand S wave attenuationimplies a fluid outer core
Rad
i us =
637
1 km
Rad i
us =
633 6
km
R adi
us =
348
6 km
Radi
u s =
12
16 km
Outer Core
InnerCore
Mantle
Earthquake
Crust (thickness exaggerated)
P and S raypaths
Distance along surface (km)
Angular Distance(degrees) Seismo-
graph
P, SS
PP
Surface Waves
PS
pP
PcP
PKP
Seismic waves (shown byraypaths) through the Earth’s interior that indicate structure(crust, mantle, outer core, inner core, etc.)
Seismic Phases in the Earth
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1
2
36
5 4
Write up phases of from 1 to 6?
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Seismic Waves: A program for the visualization of wave propagation
Seismic Waves is a Windows program which illustrates how wave propagate from an earthquake hypocenter to seismic stations throughout the earth. One sees waves propagating out from the epicenter on a three-dimensional view of the earth at the same time one sees waves propagating through a cross-sectional view of the earth. These two wave propagation views are synchronized with actual event waveforms so that as a particular phase arrives at a station, one sees the effect on the seismiogramTo use the program, fetch: seiswave.readme andSeismicWavesSetup.exe .
http://www.geol.binghamton.edu/faculty/jones/jones.html#Seismic%20Waves
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