Seismic Waves, Earth’s interior and GeophysicsQuestions to ponderPlate tectonics & SeismicityPowerPoint PresentationSlide 5Earth’s interiorThe behavior of seismic body wavesThe behavior of seismic body wavesSlide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Earth’s InteriorSlide 17Slide 18Earth’s “Parts”Moho: Crust-Mantle boundarySlide 21MohoCrustSlide 24The MantleSlide 26Slide 27Slide 28Slide 29Slide 30The coreThe CoreSlide 33Slide 34Slide 35Seismic tomographySeismic TomographyOther uses for seismic wavesSlide 39Subsurface Structures as observed by seismic wavesSlide 41Oil ExplorationSlide 43Environmental GeophysicsSeismic Waves,Seismic Waves,EarthEarth’’s interior and s interior and GeophysicsGeophysicsLecture 24Lecture 24Questions to ponder•How do seismic waves tell us about earth’s interior? How do each of the Body waves allow this insight?•What are the internal “parts” of earth, and how far down are these?•What are P and S wave “shadow zones” and what causes each?•How do seismic waves help us find resources, address pollution and determine other properties of our crust?Plate tectonics & SeismicityEarth’s interior•Already know that seismic activity ceases about 450 miles (~ 700 km) in depth•Therefore, we know also, that the rocks must become partially molten around this point•Seismic waves also provide information about the material they travel throughThe behavior of seismic body wavesFaster than S-waves Slower than P-wavesP wavesS wavesTravel through solids, Travel only through solids liquids, and gases (faster through solids, slower through liquids)The behavior of seismic body wavesWaves can Refract (Bend)when travellingThrough different layersWaves can Reflect when they encounter another layer of materialVelocity of Seismic waves increases with increasing densityVelocityDensityVelocity also Decreases when materials increase in temperatureThe behavior of seismic body wavesP Wave Velocity changes; S Waves cease at ~ 3000 KmThe behavior of seismic body wavesS Wave Shadow ZoneAbout 103 ˚ from epicenterThe Earth’s Interior•An S wave shadow exists beyond 103O from the epicenter of the earthquake–The S wave shadow is caused by the liquid core•S waves cannot pass through a liquid•The radius of the outer core has been determined by the size of the S wave shadowThe behavior of seismic body wavesP Wave Shadow ZoneFrom about 103 - 143˚From epicenterThe Earth’s Interior•P waves are strongly reflected and refracted by the liquid outer core–P wave shadow zone is found between 103O and 143O from the epicenter–Weak P waves may be detected in the shadow zone•Evidence for a solid inner core–P wave velocities give us good estimates of the density of the crust and mantleEarth’s Interior•Because we know:–The Size of the Planet–The “average density of earth + the density of crustal rocks–The velocity of seismic waves as the travel through different material•We can then determine the composition of the internal earthEarth’s total radius from core to crust = 6370 kmEarth’s “Parts”•Crust – varies in thickness (5 – 50 km)–Continental crust less dense than oceanic crust–Continental crust is thicker•Mantle–Upper Mantle – includes plastic Asthenosphere (Low velocity)–Lower mantle – peridotite (higher velocity)•Core–Outer – liquid outer core; seismic velocities decrease; s-waves absent–Inner – dense material (Fe-Ni)Moho: Crust-Mantle boundary•Mohorovicic Discontinuity (Moho)–First discovered by Andrija Mohorovicic–Occurs between 5 and 70 km deep–Represents the base of the crust–Compositional change from feldspar rich to olivine rich causes change in seismic velocities (increased velocities in the mantle)Moho: Crust-Mantle boundaryMohoCrustIsostacy: the tendency of the crust to float at an elevation consistent with its density and thickness (relative to the underlying material)The MantleAsthenosphere is partially moltenSeismic Structure of the Earth•Low-velocity zone–Layer from ~100 to 250 km deep–Seismic velocities usually increase with depth–Decrease by ~ 6% in low velocity zone–Caused by partially molten mantle that slows seismic wavesSeismic Structure of the Earth•Other discontinuities exist– A sharp rise in velocities occurs at ~400 km deep•Likely transition of olivine to magnesium spinel•Increase in density results in higher seismic velocitiesUpper mantle: from the Mohorovicic Discontinuity to the base of the asthenosphereLower mantle: from the base of theasthenosphere to the point where S-waves disappear and P-waves decrease in velocitySeismic Velocities Generally increase with increasing density of materialsThe coreThe CoreLiquid outer coreNo S wavesSolid Inner coreVery Dense, Iron richInner and outer cores rotate, but not at same rateRotating Metallic material = Magnetic fieldConvection in the Earth•In the core–Seismic waves show composition & structure of core–3-D models may show flowing molten iron•In the mantle–Investigations show a complex convection system occurring in the entire mantle systemConvection in the EarthSeismic tomography•Seismic wave velocity also dependent upon temperature of materials•Observe possible plate tectonic interactions•Observe possible heat convectionSeismic TomographyOther uses for seismic waves•Oil exploration•Assessing volcanoes•Locating various rock layers, pollution, resourcesSeismic Waves Reflect and Refract from each new rock layerSubsurface Structures as observed by seismic wavesUndersea Faults near SeattleMagma ascent Atlantic MOROil ExplorationEnvironmental