2 Seismic Wave Propagation and Earth models

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2. Seismic Wave Propagation and Earth models radial directions of wave propagation. However, when the distance is large enough, the curvature of the wavefronts becomes so small that we can approximate them locally (e.g., within the aperture of a local seismic network or an array; see Chapters 8 and 9) by plane waves with parallel seismic rays. Tab. 2.1 Typical values (averages and/or approximate ranges) of elastic constants, density, Poisson's ratio and seismic wave velocities for some selected materials, unconsolidated sediments, sedimentary rocks of different geologic age and igneous/plutonic rocks. Values for granite relate to 200 MPa confining pressure, corresponding to about 8 km depth, for basalt to 600 MPa (about 20 km depth), and for Peridotite, Dunite and Pyroxenite to1000 MPa (about 30 km depth) (compiled from Hellwege, 1982; Lillie, 1999; and other sources). Material or Geologic Formation Bulk Modulus in 10 9 Pa Shear Modulus in 10 9 Pa Density in kg m -3 8 Poisson Ratio vp in km s -1 vs in km s -1 vp/vs Air 0.0001 0 1.0 0.5 0.32 0 ∞ Water 2.2 0 1000 0.5 1.5 0 ∞ Ice 3.0 4.9 920 -0.034 3.2 2.3 1.39 Clastic sedi- (1.4-5.3) mentary rocks Sandstone 24 17 2500 0.21 4.3 2.6 1.65 Salt 24 18 2200 0.17 4.6 (3.8-5.2) 2.9 1.59 Limestone 38 22 2700 0.19 4.7 (2.9-5.6) 2.9 1.62 Granite 56 34 2610 0.25 6.2 3.6 (47-69) (30-37) (2340-2670) (0.20-0.31) (5.8-6.4) (3.4-3.7) Basalt 71 38 2940 0.28 6.4 3.6 (64-80) (33-41) (2850-3050) (0.26-0.29) (6.1-6.7) (3.4-3.7) Peridotite, 128 63 3300 0.29 8.0 4.4 Dunit, Pyroxenite (113-141) (52–72) (3190-3365) (0.26-0.29) (7.5–8.4) (4.0–4.7) Metamorphic& igneous rocks (3.8-6.4) Ultramafic rocks (7.2-8.7) Cenozoic 1500-2100 0.38-
9 2.2 Elastic moduli and body waves (longitudinal polarization) but that of the S wave is perpendicular (transverse) to it, in the given case oscillating up and down in the vertical plane (SV-wave). However, S waves may also oscillate purely in the horizontal plane (SH waves) or at any angle between vertical and horizontal, depending on the source mechanism (Chapter 3), the wave propagation history, and the incidence angle io at the seismic station (see Fig. 2.27). Fig. 2.5 Displacements from a harmonic plane P wave (top) and SV wave (bottom) propagating in a homogeneous isotropic medium. Λ is the wavelength. 2A means double amplitude. The white surface on the right is a segment of the propagating plane wavefront where all particles undergo the same motion at a given instant in time, i.e., they oscillate in phase. The arrows indicate the seismic rays, defined as the normal to the wavefront, which points in the direction of propagation (modified according to Shearer, Introduction to Seismology, 1999; with permission from Cambridge University Press). The wavelength Λ is defined by the distance (in km) between neighboring wave peaks or troughs or volumes of maximum compression or dilatation (see Fig. 2.5). The wave period T is the duration of one oscillation (in s) and the frequency f is the number of oscillations per second (unit [Hz] = [s -1 ]). The wavelength is the product of wave velocity v and period T while the wavenumber is the ratio 2π/Λ. Tab. 2.2 summarizes all these various harmonic wave parameters and their mutual relationship. Tab. 2.2 Harmonic wave parameters and their mutual relationship. Name Symbol Relationships Period T T = 1/f = 2π/ω = Λ/v Frequency f f = 1/T = ω/2π = v/Λ Angular frequency ω ω = 2πf = 2π/T = v⋅k Velocity v v = Λ/T = f⋅Λ = ω/k Wavelength Λ Λ = v/f = v⋅T = 2π/k Wavenumber k k = ω/v = 2π/Λ = 2πf/v In any case, the polarization of both P and S waves, when propagating in a homogenous and isotropic medium, is linear. This is confirmed rather well by particle motion analysis of real seismic recordings, if they are broadband (or long period). But higher frequencies, which are more strongly affected by local inhomogeneities in the Earth, show a more elliptical or irregular particle motion. Fig. 2.6 shows an example. While the rectilinearity of P is almost 1
Page 1 and 2: CHAPTER 2 Seismic Wave Propagation
Page 3 and 4: 3 2.2 Elastic moduli and body waves
Page 5 and 6: 5 2.2 Elastic moduli and body waves
Page 7: 7 2.2 Elastic moduli and body waves
Page 11 and 12: 11 2.3 Surface waves S waves are al
Page 13 and 14: 2.3.2 Dispersion and polarization 1
Page 15 and 16: 15 2.3 Surface waves Strictly speak
Page 17 and 18: 17 2.3 Surface waves (Z-E) plane, w
Page 19 and 20: 2.3.4 Mantle surface waves 19 2.3 S
Page 21 and 22: 2.4 Normal modes 21 2.4 Normal mode
Page 23 and 24: 23 2.4 Normal modes First observati
Page 25 and 26: 2.5 Seismic rays, travel times, amp
Page 43 and 44: 2.6 Seismic phases and travel times
Page 53 and 54: Overlay to Fig. 2.47 2.6 Seismic ph
Page 57 and 58: 57 2.7 Global Earth models on verti
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59 2.7 Global Earth models As shown
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61 2.7 Global Earth models The mode
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2.8 Synthetic seismograms and wavef
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