Seismic Waves
 

Seismic Waves

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Seismic Waves Seismic Waves Document Transcript

  • Introduction to Seismology-KFUPM Introduction to Seismology Chapter 2 Seismic Waves http://faculty.kfupm.edu.sa/ES/oncel/geop204chap2.htm Chapter 3, Bullen and Bolt Ali O. Oncel oncel@kfupm.edu.sa Department of Earth Sciences KFUPM Introduction to Seismology-KFUPM Seismic Waves The wiggles on a seismogram are caused by seismic waves which are generated by the movement of the rocks 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. Introduction to Seismology-KFUPM Waves in a pond The idea is analogous to waves caused by tossing a stone in a pond. 1
  • Introduction to Seismology-KFUPM Sound Wave Analogy Seismic waves represent acoustic (sound) energy and so are analogous to speech: Speech Earthquakes 1 Vocal cords vibrate A locked fault segment fails (ruptures) 2 Sound waves propagate Sound waves propagate through atmosphere through the Earth 3 Ears record these Seismometers record vibrations these vibrations 4 Brain processes the Seismologists process recordings these recordings (seismograms) Introduction to Seismology-KFUPM What is a Wave ? A wave is a disturbance that transfers energy. Waves are common in nature: Light is a wave Sound is a wave Waves are periodic in both space and time (they have wavelengths and periods) Introduction to Seismology-KFUPM Wave Terminology 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 tsunami waves 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. 2
  • Introduction to Seismology-KFUPM Wavelength and Period Crest (High Points) Wavelength Period Amplitude Amplitude Equilibrium (Middle) Distance Time from Source Through (Low Points) • At a given instant in either time and space, the displacement 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 Introduction to Seismology-KFUPM 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 Introduction to Seismology-KFUPM Sources of Seismic Waves Earthquakes generate seismic waves, but so do many other processes: Volcanic eruptions Explosions Wind Sonic Booms (planes, shuttle, meteorites) Humans 3
  • Multiple-Frequency Signals Introduction to Seismology-KFUPM Most interesting signals are composites of waves with many different frequencies. The range of frequency is sometimes called the “band” and we speak of bandwidth. Light is usually a multiple frequency signal, and the different frequencies correspond to what we call colors. Introduction to Seismology-KFUPM Jet and Earthquake Sometimes we can use the observed frequencies to identify different sources of vibrations. Which has higher frequency content, the sonic boom or the earthquake? Introduction to Seismology-KFUPM Elastic Behavior a) The deformation of a material (strain) results from a force per unit area (stress) acting on the material. b) An elastic material returns to its original shape and volume when deforming stress is removed 4
  • Introduction to Seismology-KFUPM Style of Deformation a) During ductile deformation particles remain connected and flow b) Brittle deformation results in the development of fractures Introduction to Seismology-KFUPM 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. Introduction to Seismology-KFUPM What is Ductility? 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 concrete or masonry. Adapted from the International Handbook of Earthquake and Engineering Seismology, Aki and Lee[1] 5
  • Introduction to Seismology-KFUPM Can you read this? Seismologists “read” seismograms to learn about the earth. Introduction to Seismology-KFUPM Previous Lecture 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 6
  • Introduction to Seismology-KFUPM 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. Introduction to Seismology-KFUPM 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. See the effect of media velocity on reflection and refraction of seismic waves! Click the site.. http://www.mines.utah.edu/~ggapps/snell/snell.html Introduction to Seismology-KFUPM Seismic Velocity Seismic 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 Slower Velocities • Lower temperatures • Higher temperatures • Higher pressures • Lower pressures • Solid phases • Liquid phases 7
  • Introduction to Seismology-KFUPM Recall: Elastic Deformation Reference Introduction to Seismology-KFUPM Elastic Behavior tile Duc Br i tt tic le as El Reference Introduction to Seismology-KFUPM Elastic Constants: Bulk Modules Bulk Modulus (Incompressibility) = (∆P/Θ) … where Θ = dilatation = ∆V/V Reference and P = pressure 8
  • Introduction to Seismology-KFUPM Elastic Constants: Shear Modules shear modulus (rigidity) shear stress µ = shear strain Reference The shear modulus is the shear stress = (∆F /A) ratio of shear stress to shear strain of a material during shear strain = (∆l /L) simple shear. From the International Handbook of Earthquake and Engineering Seismology, Aki and Lee[1] Introduction to Seismology-KFUPM Elastic Constants: Poisson Ratio •Under a stress (sxx) along the x-axis, longitudinal strain ∆L εxx = L transverse strain = ∆W εyy = W Then, Reference Poisson’s ratio = υ= - (εyy / εxx) 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. Introduction to Seismology-KFUPM •Typical Values of Elastic Constants for Selected Materials Elastic Moduli and Densities of Some Common Materials From Lay & Wallace (1995) Shear Module Poisson Ratio Bulk Module Lame’s constant 9
  • Introduction to Seismology-KFUPM Introduction to Seismology-KFUPM Introduction to Seismology-KFUPM 10
  • Introduction to Seismology-KFUPM Previous Lecture Snell's Law Seismic Velocity What is Seismogram? What is Elastic Behavior? Ductile Deformation Brittle Deformation Hooke's Law Elastic Constants Bulk Modules Shear Modules Poisson Ratio Typical Values of Elastic Constants for Selected Materials Introduction to Seismology-KFUPM Recall: Stress-Strain Behavior Elastic: The stress limit for a material for its strain Hooke's Law ==> elasticity 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 tile Duc eliminated and there is no permanent deformation. Elastic Limit: The stress limit for a material for its Br strain to be recoverable. Up to the elastic limit, it ic t tle t as Hooke’s Law pertains, stress is proportional to El strain, and if the stress is eliminated the strain Reference 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. Introduction to Seismology-KFUPM Elastic Behavior: Lithosphere Elastic behavior occurs at very high strain rates, as particles are vibrated by an earthquake; the vibrations result in the passage of seismic waves Reference 11
  • Introduction to Seismology-KFUPM Plastic Behavior: Asthenosphere Inelastic (ductile) behavior results from very slow straining of the Asthenosphere; viscous flow facilitates the movement of overlying lithospheric plates Reference Introduction to Seismology-KFUPM Elastic/plastic Behavior Elastic behavior occurs at very high strain rates, as particles are vibrated by an earthquake; the vibrations result in the passage of seismic waves Reference Inelastic (ductile) behavior results from very slow straining of the Asthenosphere; viscous flow facilitates the movement of overlying lithospheric plates Introduction to Seismology-KFUPM Mechanical Waves In 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 water and 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. 12
  • Introduction to Seismology-KFUPM Recall: Types of Body Waves P-wave S-wave 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. Introduction to Seismology-KFUPM Types of Surface Waves Introduction to Seismology-KFUPM Surface Wave Dispersion Surface waves propagate along the earth’s surface. Surface waves are larger in amplitude and longer in duration than 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. 13
  • Introduction to Seismology-KFUPM Dispersion of Surface Waves Knopoff, 1972 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. Introduction to Seismology-KFUPM Particle Motions of Body Waves Introduction to Seismology-KFUPM Body/Surface wave Prorogation Reference 14
  • Introduction to Seismology-KFUPM 3D Components of Waves Introduction to Seismology-KFUPM Seismic shadow zones •P waves are refracted at the Mantle-Outer Core interface •S waves are stopped at this interface •Both create shadow zones and S wave attenuation implies a fluid outer core Introduction to Seismology-KFUPM Seismic Phases in the Earth Earthquake D ist a nce Seismic waves (shown by pP Surf ace al o ng Wav su raypaths) through the S es P, S rf ac e Earth’s interior that 36 m Pc P (k k km P and S m) 71 63 raypaths indicate structure 63 An us= m Dis gula us = k 6 (de tan r R ad i (crust, mantle, outer core, 48 PP gr e c e Radi =3 km es 16 ) Seismo- Ra dius inner core, etc.) di us = 12 Inner graph PS Core Ra Outer Core P Mantle PK Crust (thickness exaggerated) 15
  • 1 2 Write up phases of from 1 to 6? 3 6 5 4 16
  • Introduction to Seismology-KFUPM 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 seismiogram To use the program, fetch: seiswave.readme and SeismicWavesSetup.exe . http://www.geol.binghamton.edu/faculty/jones/jones.html#Seismic%20Waves 17