physics and chemistry of the earth’s interior – global...
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Global geophysics and wave propagation
Reading: Fowler p76 – 83
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Remote sensing
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Seismology:
• Directly samples the physical properties of the Earth’s interior
• Earthquakes indicate the stress regime
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Gravity and bathymetry
• Connected through isostasy
• Tell us about density variations
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Magnetics
• Changes in the Earth’s magnetic field recoded within surface rocks tell us about earth history
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Surface heat flow
• Related to mantle temperature variations
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Seismological techniques
Global seismology• Earthquake sources• Global ray paths• Imaging 3D structure
of the Earth’s interior
Refraction & Reflection seismology• Controlled sources• Crustal and uppermost mantle ray paths• Crustal structure
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Body waves and surface waves
Point source seismic disturbance:
• Wavefront expands out from the point
• Body waves: sphere
• Surface waves: circle
• Rays: perpendicular to wavefront
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Body waves• Energy traveling
through the interior of the earth
• Rays bend and reflect due to variations in physical properties of the Earth’s interior
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Energy sources…Earthquakes
• Globally distributed
• Primarily along tectonic plate boundaries
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
P-waves• P for “primary” or “push-pull”
• Compression and rarefaction, no rotation
• Causes volume change as the wave propagates
• Similar to sound waves traveling through air
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
S-waves• S for “secondary” or “shear” and “shake”
• Shearing and rotation
• No volume change as the wave propagates
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
P and S-waves
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Waves – a reminder
v = fλ
λ
Terms:• Velocity, v• Wavelength, λ• Frequency, f• Period, T = 1/f
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Seismic waveform
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Seismometer
Use inertia of a mass to measure ground motion
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
P and S-velocities
ρµκα 3
4+== PV ρ
µβ == SVP-velocity S-velocity
Bulk modulus, κ• Ratio of increase in pressure to
associated volume change• Always positive
Shear modulus, µ• Force per unit area to change the shape of
the material • µ of a liquid = 0, therefore α = 0 in fluid
change of shape and volume change of shape only
Density, ρ = mass / volume
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Birch’s LawVelocity and density
A linear relationship between velocity and density
v = aρ + b
Crust and mantle rock
observations
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Nafe-Drake curve
Velocity and density
sediments and sedimentary rocks
igneous and metamorphic rocks
VP
VS
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Surface waves• Energy traveling
around the surface of the earth
• They sample the physical properties of the near-surface rocks
• Deep earthquakes do not excite surface waves as well as shallow ones
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Amplitude
Larger amplitude than body waves:
conservation of energy, sphere versus cone
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Love and Rayleigh Waves
Ground roll in plane of propagation direction
Ground shake in horizontal direction
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Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Velocity sensitivity
The amplitude of surface motion decreases with depth
Related to depth/wavelength
Longer wavelengths sample deeper
Oceanic lithosphere
Seismic velocity generally increases with depth.
Therefore, longer wavelengths arrive first
Physics and chemistry of the Earth’s interior – Global geophysics and wave propagation
Summary
Geophysical remote sensing of the Earth’s interior
• Seismology: direct sampling and stress regime• Gravity and bathymetry: density variations• Magnetics: rocks act as magnetic tape recording Earth history• Heat flow: show temperature gradients within the mantle
Seismic wave propagation
• Body waves: P and S; surface waves: Love and Rayleigh• Propagation paths dependent on the physical properties of rocks
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