Quiz answers

1: Compositional descriptionsb) Mantle

c) Crust

2: The mantle is made up primarily of:c) Olivine and Silicate minerals

3: The thickness of the continental crust is:c) 10-70 km

4: Which one (1) of the following is a liquid?d) Outer Core

5: What is the temperature at the base of the crust?b) 1000C

6: Name this ray path: pPPS

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Observations at surface

-> Whole-earth model?

Lecture 2: Intro to Seismic Waves

• Review from last time– Blackboard site! (Quiz

Monday)

• Today– Overview of structure -

target properties– Types of seismic

waves• Why are there

different types?• Broad-brush wave

equation– Springs!

lithospheric boundary layer

slowly convecting mantle:plate tectonic engine

rapidly convectingouter core:

geomagnetic dynamo

solid inner core

2000 4000 km 6000 8000 10,000

subd

uctio

n

seafloor spreading

Core-mantle boundary layer

• Goals:– Composition of Earth

– Temperature

– Depth/horizontal variations Interpretations of the above

• Observables:– Rocks brought to surface

from depth (~100 km)

– Gravity/magnetic fields

– Earthquakes and their seismic waves

• How fast? • What types?

Rock types, temperature

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USGS figure

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Courtesy Fritz Keller

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Courtesy Fritz Keller

*Earthquake

Wavefront

Ray Path

Ray Path is perpendicular to wavefront

Seismograph

Cross SectionThrough Earth

Stations forSeismograms

The Seismic Waves program

From Alan Jones, SUNY, Binghamton

http://bingweb.binghamton.edu/~ajones/

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Compressional Wave (P-Wave) Animation

• Particle motion: alternating compression/dilation in propagation direction.• No shear• Vp = speed of sound. ~5.5 km/s in crust, 8+ km/s in mantle

Animations: Larry Braille

Shear Wave (S-Wave) Animation

• Particle motion: alternating transverse motion perpendicular to propagation.• Shear, no volume change or compression• Can be in any direction, often divided into (SV; in the vertical plane) or horizontal (SH) shear motions.•Velocity ~ 60% P-wave, 0 in fluids

Wave Type (and names)

Particle Motion Other Characteristics

P, Compressional, Primary, Longitudinal

Alternating compressions (“pushes”) and dilations (“pulls”) which are directed in the same direction as the wave is propagating (along the raypath); and therefore, perpendicular to the wavefront.

P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram.  Generally smaller and higher frequency than the S and Surface-waves.  P waves in a liquid or gas are pressure waves, including sound waves.

S, Shear, Secondary, Transverse

Alternating transverse motions (perpendicular to the direction of propagation, and the raypath); commonly approximately polarized such that particle motion is in vertical or horizontal planes.

S-waves do not travel through fluids, so do not exist in Earth’s outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma).  S waves travel slower than P waves in a solid and, therefore, arrive after the P wave.

Seismic Body Waves

Rayleigh Wave (R-Wave) Animation

• Particle motion: Elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. • Amplitude decreases with depth. • Vr ~ 3-4 km/s

Vertical section

Vertical particle motion vrs time

horizontal particle motion vrs time

Love Wave (L-Wave) Animation

• Particle motion: Alternating transverse motions, perpendicular to propagation • Amplitude decreases with depth. • No vertical motion•VL ~ 3.5-4.5 km/s

Wave Type (and names)

Particle Motion Other Characteristics

L,                  Love, Surface waves, Long waves

Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface.

Love waves exist because of the Earth’s surface.  They are largest at the surface and decrease in amplitude with depth.  Love waves are dispersive, that is, the wave velocity is dependent on frequency, generally with low frequencies propagating at higher velocity.  Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.

R,            Rayleigh, Surface waves, Long waves, Ground roll

Motion is both in the direction of propagation and perpendicular (in a vertical plane), and  “phased” so that the motion is generally elliptical – either prograde or retrograde.

Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth.  Appearance and particle motion are similar to water waves. Depth of penetration of the Rayleigh waves is also dependent on frequency, with lower frequencies penetrating to greater depth. Generally, Rayleigh waves travel slightly slower than Love waves.

Seismic Surface Waves