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Seismic Classification of Materials

Seismic Classification of Materials. Basis of Classification: Ability to transmit shear. Solids Liquids P-waves √ P-waves √ S-waves √ S-waves X Cup of Cement Cup of Tea Stir cement –cup spins S tir tea – cup motionless Shear force transmitted Shear force not

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Seismic Classification of Materials

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  1. Seismic Classification of Materials Basis of Classification: Ability to transmit shear. SolidsLiquids P-waves √ P-waves √ S-waves √ S-waves X Cup of Cement Cup of Tea Stir cement –cup spins Stir tea – cup motionless Shear force transmitted Shear force not to cup. transmitted to cup.

  2. P- and S-wave velocities (α and β) P-wave S-wave Brittle Solid Viscous Solid Liquid Solid 2

  3. Physical Basis of Plate Tectonics The Upper and Lower Mantle are Viscous solids. The Lithosphere is a Brittle solid. Distinction between Viscous and Brittle behavior is fundamental to understanding Plate Tectonics. 3

  4. Look here F t Classification of Earth Materials Conventional terminology Basis of Classification: Response to applied force Solids: maintain their form (rigid, hard) Liquids: deform easily (fluid, soft)

  5. Solids (maintain form) No relative displacement of internal particles

  6. Liquids (deform easily) Relative displacement of internal particles ✓

  7. Solids are hard Liquids are soft ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ x ✓ Relative displacements of internal particles indicate the internal flow properties or Rheology of a material.

  8. Rheology: flow properties of material RheologyBehaviourExample Brittle fractures glass, chalk Elastic stretches rubber band & rebounds Viscous deforms toothpaste ( or ductile)

  9. Rheological Classification • Basis of Classification: Net response to short term forces. i.e., A force is applied and then removed. Look at net response after force is removed. applied removed F Look here t 1 2

  10. Rheological Classification BrittleElastic Viscous ForceResponse Applied Fracture Displacement Displacement Removed No Change Oscillations No Change Net R.D.’s X X √

  11. Viscoelasticity • Most materials have: an Elastic rheology on a short time scale, and a Viscous rheology on a long time scale. e.g. spring with weight is elastic on time scale of a few seconds or minute, but slowly stretches on a time scale of several years.

  12. Mantle Viscoelasticity • The Mantle responds as an elastic material to seismic wave disturbances. • Time scale of < 1 second. • The Mantle responds as a viscous material to long term geologic (or tectonic) forces • Time scale of hundreds of millions of years.

  13. “Short” and “Long” are relative terms. • Forces acting for the same duration may be “short” for one material but “long” for another material with different physical properties. • Since most materials have different physical properties at different temperatures => Forcing for the same duration may result in different rheological responses from the same material when at different temperatures.

  14. “Short” and “Long” are relative terms. E.g. Block of tar – cold=> brittle, splits -warm => viscous, deforms E.g. Toffee – cold => brittle, cracks - warm => viscous Mantle Rocks: Below 100km, Warm tar Top 100 km, Cold tar

  15. Tectonic Forces Buoyancy Forces acting for millions of years => Mantle Response: • Viscous flow in the ‘warm’ interior. • Brittle fracture in the ‘cold’ outer 100 km. => The Lithosphere is a Rheological Boundary • Same material above and below, but different rheology due to temperature profile with depth:

  16. Temperature Profile in Earth Temperature (C) 0 1300 Crust T < 1300 C Lithosphere Brittle Mantle 100 km Mantle Depth T > 1300 C Viscous

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