Quiz answers 1: Compositional descriptions b)Mantle c)Crust 2: The mantle is made up primarily of:...
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Transcript of Quiz answers 1: Compositional descriptions b)Mantle c)Crust 2: The mantle is made up primarily of:...
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
2001
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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