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What’s seismology about?
• Seismology is the study of the generation, propagation and recording of elastic waves in the Earth (and other celestial bodies) and of the sources that produce them.
Elastic Moduli
• Young’s Modulus
• Poisson Ratio
• Bulk Modulus
• Shear Modulus
• Lamb’s constants
• Vp, Vs.
= tensile stress = (F /A)
= tensional strain = (L /L)… for uniaxial tension or compression
E =
Young’s modulus
= tensile stress = (F /A)
= tensional strain = (L /L)… for uniaxial tension or compression
E =
Young’s modulus
Poisson’s ratio = - (yy / xx)
Under a stress (xx) along the x-axis,
longitudinal strain = xx =
and transverse strain = yy =
L L
Then,
W W
Bulk Modulus = (P/)
… where = dilatation = V/V and P = pressure
A
shear stress = (F /A)
shear strain = (l /L)
L
=shear stressshear strain
shear modulus
From Lay & Wallace (1995)
Elastic Moduli and Densities of Some Common Materials
From Lay & Wallace (1995)
= k - =2 E3 ( 1 + ) ( 1 - 2 )
V = = =K + ( ) + 2
p4/3
V = =s
Where are the Lamé coefficients and is
S-wave
P-wave
dilatations
compressions
wavelengthamplitude
Body Waves
Body wave
Surface wave
Amp ~ 1/x
Amp ~ x-1/2
Late homework policy
• Late homework will not be accepted one week after the deadline.
• Late homework will be penalized: 90% (1 day late), 80% (2 day late), 60% (3 day late), 40 % (4 day late) and 20% (5-7 day late).
S-wave
P-wave
dilatations
compressions
wavelengthamplitude
Body Waves
Seismic Rays & Geometric Ray Theory
• Rays are defined as the normal to the wave fronts and thus point in the direction of propagation. Connecting the these successive directions forms a ray paths. In the case of plane wave, rays are a family of straight lines; in the case of spherical wave, rays are spokes radiating out from the seismic source. They provide a convenient means of tracking an expanding wave front.
• The seismic rays corresponding to P and S waves behave very much as light does in traveling through materials of varying indices of refraction. This leads to many parallels with optics: rays bend, focus, and defocus depending on the velocity distribution. Although geometric ray theory is an approximation, it, however, simplifies the analysis and yet gives the correct answer or a good approximation.
Fermat’s Principle
• Ray paths between two points are those for which travel time is an extremum, either a minimum or maximum, with respect to the nearby possible paths.
Snell’s Law and Fermat Principle
Fermat Principle: T(x) is either minimum or maximum
2
2
1
1 sinsin
v
i
v
i
Snell’s Law
Reynolds (1997)
R sin i
V= constant = p
Spherical Ray Parameter
What control reflection and transmission?
• Continuity of stress
• Continuity of displacement
I =
I =2
1
Reynolds (1997)