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How do we know there is a Core to the Earth? Earth Structure from Seismology

How do we know there is a Core to the Earth? Earth Structure from Seismology - Seismic Ray Paths - Earth Structure Crust Mantle Core Inner Core. Snell’s Law Fermat’s Principle. q 1. Air. Water.

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How do we know there is a Core to the Earth? Earth Structure from Seismology

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  1. How do we know there is a Core to the • Earth? • Earth Structure from Seismology • - Seismic Ray Paths • - Earth Structure • Crust • Mantle • Core • Inner Core

  2. Snell’s Law Fermat’s Principle q1 Air Water q2 sin q1 / sin q2 = n21

  3. a1 q1 q2 a２ q2 q3 a３ a1 < a2 <a3

  4. Time 1/a3 1/a2 1/a1 Distance Ray Paths in a Layered Medium a1 a2 a3

  5. Andrija Mohorovicic (1857-1936) Found seismic discontinuity at 30 km depth Kupa Valley (Croatia). Mohorovicic discontinuity ‘Moho’

  6. Forward Branch Backward Branch

  7. Forward Branch Shadow Zone Forward Branch Backward Branch

  8. PcP Forward Branch Shadow Zone 1912 Gutenberg observed shadow zone 105o to 143o 1939 Jeffreys fixed depth of core at 2898 km (using PcP) Forward Branch Backward Branch Backward Branch PKP Forward Branch Shadow Zone PcP P Forward Branch

  9. Crust 6349 ~ 6379 km Mantle 2998 ~ 6349 km Core 0 ~ 2998 km

  10. PKIKP Refractions from the Inner Core Shadow Zone

  11. Backward Branch PKP PKIKP Forward Branch Shadow Zone PcP P Forward Branch Inner Core discovered by Lehman (1935)

  12. Crust 6340 ~ 6370 km Mantle 2998 ~ 6340 km Core 1250 ~ 2998 km Inner Core 0 ~ 1250 km

  13. When velocity increasesrapidly with depth Velocity Depth Triplication Travel time prograde retrograde prograde Epicentral distance

  14. Velocity model for calculation Calculated traces Observed traces

  15. (200 km discontinuity) 440 km discontinuity (550 km discontinuity) 660 km discontinuity D’’ PKP ‘GH’ branch

  16. 440 km discontinuity 660 km discontinuity Mantle S P D’’ Outer Core Inner Core IASPEI 91 Model Mantle Outer Core Inner Core

  17. Many types of P and S phases

  18. Inner Core Outer Core Lower Mantle Bulk Sound Velocity Upper Mantle Density What is the mantle made of ? Olivine About ６０％ Also Orthopyroxene Clinopyroxene Garnet

  19. Inner Core Outer Core Lower Mantle Bulk Sound Velocity Upper Mantle Density What is the core made of? Fe plus Light element About 10-15 % Iron-Nickel alloy Light Element Candidates Sulphur Ｓ Silicon Ｓｉ Oxygen Ｏ Potassium Ｋ Hydrogen Ｈ

  20. whole mantle Vs structure （Masters et al., 2000) Data： ・Body waves ・Surface waves ・Free Oscillations ①Large heterogeneity in ranges of about 300km beneath the surface and above the core ②Similarity in spatial pattern of high and low Vs

  21. Plume beneath Africa Vs depth profile Surface CMB 660km Slow Fast

  22. Ed Garnero

  23. Conclusions Studies of how seismic waves travel through the Earth led to the discoveries of its basic structure. Strong discontinuities in seismic velocity identify the crust, mantle, outer core, and inner core. The traditional 1-D Earth structures are currently being improved into 3-D structures. Information about the Earth’s structure provides insights into the convection and other physical processes that are occurring within the Earth’s interior.

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