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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