Chapter 17 Earth’s Interior and Geophysical PropertiesEvidence from Seismic WavesSeismic ReflectionSeismic RefractionRay Paths and Snell’s LawSeismic Ray Paths AnimationLocating Boundaries By RefractionRadial Distribution of Ray PathsInternal Earth Structure from SeismologySeismic Moho and LVZSummary Crustal PropertiesMantle TomographyDeep Earth Velocity StructureP wave Shadow ZoneS wave Shadow ZonePallisite Meteorites - Rocks from a CMB?D ‘’ Layer in Lower MantleIsostasy and GravityIsostasyIsostatic Adjustment: Erosion AnimationIsostatic Readjustment: ErosionPost-glacial Crustal ReboundPolar Glaciers Melting AnimationPost-glacial Rebound AnimationIsostatic Uplift due to Crustal ThickeningGravitational AttractionGravity Meter’s Response to Density VariationsGravity Profile in Isostatic EquilibriumNon-isostatically Compensated ProfileNegative Gravity AnomalyChapter 17Earth’s Interior and Geophysical PropertiesPhoto credit: NASA Apollo 17Evidence from Seismic WavesEarth’s Internal Structure•The Crust•The Mantle•The CoreSeismic ReflectionControlled by:1) Snell’s Law2) Elastic properties of the layers€ Vp=K +43μρ Vs=μρwhereK ≡ bulk or compressibility modulusμ ≡ rigidity or shear modulusρ ≡ densitySeismic RefractionRay paths are“bent” acrossvelocity or seismicdiscontinuities.Bend away fromboundary normalif V2 > V1Bend towardboundary normalif V1 > V2V1V2€ sini1sini2=AC / BCBD / BC=ACBD=V1V2 sini1sini2=V1V2V2⋅sini1= V1⋅sini2Modified from: Brown & Mussett, 1981Ray Paths and Snell’s LawV2V1In this example, V2 > V1Seismic Ray Paths AnimationLocating Boundaries By RefractionRadial Distribution of Ray PathsRays curveupward becauseof generallyincreasingvelocitywith depthVp and VsInternal Earth Structure from SeismologyCrust: 0-60 kmMantle: 60-2900 kmCore: 2900-6370 kmSeismic Moho and LVZSummary Crustal PropertiesMantle TomographyBlue = Fast (cold)Red = Slow (hot)Deep Earth Velocity StructureNB that velocity is measured from earthquake waves, but density mustBe inferred from inversion of the velocity combined with elastic moduliiP wave Shadow ZoneS wave Shadow ZonePallisite Meteorites - Rocks from a CMB?OlivineFe-NiMetal AlloyPallisite meteorites are classified as stoney-irons and are thoughtto be derived from the CMB region of an asteroidal parent body.Non-seismologicalevidence of core - mantleexchange!D ‘’ Layer in Lower MantleFrom: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28 , 509-37, 2000.Isostasy and Gravity•Isostasy is the gravitational equilibrium of bodies of different density.•Isostatically compensated uplift zones are stabilized by low density roots.•Isostatic re-adjustment can be caused by many different processes.IsostasyCompensation depth is arbitrary and selected for convenience. Total column mass (or gravitational acceleration) must be equal.Low densityHigh densityIsostatic Adjustment: Erosion AnimationIsostatic Readjustment: ErosionPost-glacial Crustal ReboundQuickTime™ and aGIF decompressorare needed to see this picture.Polar Glaciers Melting AnimationFrom: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.htmlPost-glacial Rebound AnimationIsostatic Uplift due to Crustal ThickeningNB that uplift would depend on the amount and density ofaccumulated magma accreted to the base of the crust.Gravitational AttractionGravity Meter’s Response to Density VariationsLocal gravity high can be related to a high density body within the crust,while a gravity low can be related to a lower than average density.gravityhighgravitylowLow densityHigh densityGravity Profile in Isostatic EquilibriumNon-isostatically Compensated ProfileNegative Gravity