Earth Whole Structure
Part I - Seismic Waves as Probes of the Interior
Earth's Internal Structure
The chemical differentiation into a core, mantle, and crust produces very strong differences in density and seismic wave velocity Our primary and highest-resolution probes of the deep interior are seismic waves
Travel time Refraction Reflection
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Seismic Methods Controlled Source: Artificially-generated seismic waves refraction reflection
Passive Source: Earthquake-generated seismic waves also reflect and refract through Earth's interior
Seismology
Seismology has been around for over a hundred years
First long-distance seismogram: Earthquake in Japan (Mag 5.8) recorded in Germany, 1889
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Seismograph Operation
Time Displacement (or velocity, acceleration)
Courtesy Guralp Instruments
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Global seismic phase travel time curve
Distance measured in degrees from epicenter to seismograph location Nearly all phases that could arrive based on PREM earth velocity model are shown
Seismic Waves and Elasticity
What are seismic waves? How are seismic waves used to learn earth properties? How are they used to learn about earthquakes? Stress and Strain
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Elastic Waves
Seismic waves of all types are one class of elastic waves. Wave propagation in turn is one form of elastic behavior. They are produced by transient stresses leading to temporary deformation or strain. Deformation
Elastic Brittle Ductile
Compressional wave (P-wave)
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Shear wave (S-wave)
Surface wave
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Fig 3-11
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Body and surface waves
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Refraction at a boundary
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Refraction at a boundary
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Refraction towards boundary when V1 < V2 Refraction away from boundary when V1 > V2
Refraction due to velocity gradients with depth causes bending of seismic rays. Reflection occurs at discrete boundaries with sharp contrast in velocity of P or S waves.
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Oversimplified Earth structure, no boundaries
Figure 7-25
Earth with both sharp gradients and boundaries
Figure 7-26
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Travel-time curves for major seismic phases (arrivals)
~1906: Deduction that the Earth has a fluid core
S-wave shadow zone beyond 103 Due to absence of shear waves in fluid outer core P-wave shadow zone 103 - 143 Due to refraction at very large velocity decrease from 13.5 km/s to 8 km/s Consistent with 2900 m depth to core-mantle boundary
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Discovery of inner core
P-wave arrivals at opposite side of suggest Earth faster velocity in center of inner core from 5100 km depth
Other (weak, controversial) wave arrivals suggest they passed through inner core as shear waves
Major Global Scale Seismic Phases (arrivals)
P is P-wave in mantle S is S-wave in mantle K is P-wave in outer core I is P-wave in inner core J is S-wave in inner core c is reflection off core surface R, L are Rayleigh and Love waves
Figure 7.27
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Crust - Mantle Boundary
1909 - A. Mohorovicic examined seismograms from across Europe for an earthquake, showed abrupt change in velocity from < ~7 km/s to ~8.1 km/s (based on refraction) Oceanic Moho at depth of ~ 10 km Continental Moho at depth of ~ 20-70 km
Lithosphere and Asthenosphere
Lithosphere has velocity of 8 km/s or greater, interpreted as rigid mantle rock of peridotitic (olivine) composition At depth of ~ 100 - 300 km, both P and S velocity drop about 6% Not a sharp boundary at top, gradual increase to 670 km depth, then sharp increase in velocity (670 km discontinuity) Smaller discontinuity at 400 km depth
Lower Mantle and CoreMantle Boundary
From 670 km down to ~2700 km, lower mantle velocity increases quite smoothly Lowermost 200 km of mantle is called D'' ("D double prime") for obscure reasons
Represents a boundary layer between core and mantle with low, laterally variable seismic velocity
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Earth's velocity structure is therefore largely spherically sy...