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Simplest assumption: suppose the Earth!s interior was uniform... Wait! How do we know this?? We measure the travel times of seismic waves from earthquakes, and compare them with what we would expect from different layered models until we get a match •! P wave, S wave, and surface waves would arrive at all stations •! we would compute their arrival times at different seismometers assuming a gradual increase in velocity due to pressure “inversion” of the seismic travel time data for velocity structure •! this would work perfectly Here is what we actually see! we could make a plot of P, S, and surface wave arrival time with distance “ ∆ “ surface ∆= 50° time S time (minutes) If the Earth!s interior were uniform... P ∆ ∆ “Seismic Phases” • • • • • • P,S : P,S waves in the mantle, e.g. PPP, SS K : P-wave in the outer core, e.g. PKP, SKP I,J : P,S waves in the inner core c : reflection from CMB, e.g. PcP, ScS i : reflection from IOB, e.g. PKiKS SUMMARY: many phases are created by Earth’s stratification (reflections, and sometimes conversion from one wave type to another) P wave velocities drop suddenly at 2900 km depth, and S waves cannot pass through this layer Fig. 4.7 S wave shadow zone P wave shadow zone Fig. 4.8 Fig. 4.9 Why are there seismic velocity jumps inside the mantle? We get this picture by calculating arrival times for all seismic phases in a stratified model of the Earth, and making sure they match the observed arrival times at all points on the Earth • mantle has fairly uniform composition • same chemical elements arrange into different minerals at different depths • minerals that are stable at great depth are the densest • seismic wave speeds change as minerals change • Low Velocity Zone: close to melting temperature Meteorites: analogues to composition of the Earth!s interior? Engdahl and Kennett 1991 dots (phase travel times) match curves (modelpredicted travel times) very well. We can get seismic wavespeeds, but does this tell us the chemical composition of the layers? METEORITES Abundances of elements in the solar system are estimated from meteorites, solar corona, etc. Crust and mantle: too little iron (and nickel and lead etc.) relative to oxygen, silicon, etc. Missing: we need lots of iron and nickel inside the Earth + ~ = a small amount of + “Primordial” (never melted or re-processed) 4.5 BY old meteorite Likely close to average composition of the Earth Core - mantle boundary What is the Earth!s Core made of? • liquid outer core: must be a liquid at the P,T conditions deep in the Earth • magnetic field generation: it must be a metal • densities: the core must be dense – Earth’s average: 5.5 g/cm^3 – crust, mantle: 2.7-3.3 g/cm^3 (85% by volume) – therefore, core: 10+ g/cm^3 • meteorites suggest Fe core with trace O, Si, Ni, S CORE-MANTLE BOUNDARY • dramatic density and seismic velocity change • slab graveyard • mantle plume birthplace • site of “anti-crust’’ (ULVZ) and “antilithosphere’’ (D’’ layer) • is the core reacting with the mantle? or is it melting the mantle? D’’ layer and ultra low velocity zone (ULVZ): what’s going on? Rising plumes, sinking slabs... The Earth is not exactly radially symmetric Seismic tomography: let!s find Texas seismographs earthquakes suppose that the Earth is flat and that seismic waves travel unusually slowly through Texas... Global Seismic Tomography Finding Texas. on-time late late late on-time seismic velocity anomalies usually differ by less than 2% from surroundings Global Seismic Tomography upper mantle • similar to CAT scan in medical imaging • compares real travel times with travel times predicted by the radially symmetric Earth model • small differences in travel times are translated to seismic velocity variations • Blue (cold) is fast & Red (hot) is slow • snapshots of mantle convection: hot material rises and cold material sinks core - mantle boundary Van der Hilst et al., 1998 Subducting slabs: stronger and colder than their surroundings Giant superplume rising from the CMB under the Pacific Ocean Do they all sink to the core-mantle boundary or not? There!s another one of these under Africa too. Van der Hilst et al., 1998 Seismic Tomography of the Mantle a “snapshot” of current mantle velocity • primary influences: • composition • temperature Tomography at a finer scale - P wave velocity anomalies at 100 km depth Subducting slab 3-D image: • orange-red: hot (upward convection) Tomography at a finer scale - the Yellowstone hot spot they* could not resolve the bottom of this low-velocity feature small plumes associated with hot spots are too small to show up on the global tomographic images *Derek Schutt, Eugene Humphreys, Rebecca Salzer (P and S wave studies) Strong mantle under the Sierra Nevada batholith Yellowstone hot spot Maximum perturbation is 2%