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Further topics in tectonics: Focal mechanisms Pac NW tectonic setting

Further topics in tectonics: Focal mechanisms Pac NW tectonic setting Shape of mid-ocean ridges (depth of the ocean) Driving forces of plate tectonics. 1. Focal mechanisms. Elastic Rebound. 1. Focal mechanisms. No offset. Earthquake break. No offset. 1. Focal mechanisms.

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Further topics in tectonics: Focal mechanisms Pac NW tectonic setting

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  1. Further topics in tectonics: • Focal mechanisms • Pac NW tectonic setting • Shape of mid-ocean ridges (depth of the ocean) • Driving forces of plate tectonics

  2. 1. Focal mechanisms Elastic Rebound

  3. 1. Focal mechanisms No offset Earthquake break No offset

  4. 1. Focal mechanisms Volume decrease (compression) Volume increase (dilation) Volume increase (dilation) Volume decrease (compression)

  5. 1. Focal mechanisms Direction of P-wave first motion

  6. 1. Focal mechanisms Direction of P-wave first motion

  7. 1. Focal mechanisms Nodal Plane wave is neither in nor out – it has zero amplitude

  8. 1. Focal mechanisms Two fault planes will produce the same motion C D D C Right lateral

  9. 1. Focal mechanisms Two fault planes will produce the same motion C D C D D C D C Right lateral Left lateral

  10. 1. Focal mechanisms Two fault planes will produce the same motion C D C D D C D C Left lateral Right lateral Right and left lateral earthquake sources with perpendicular fault planes have the same first motions

  11. 1. Focal mechanisms Orientation of fault plane can be represented by beach balls Normal or ‘rift’ faulting

  12. 1. Focal mechanisms Different orientations of different faults

  13. 1. Focal mechanisms From a network of seismometers can analyze fault orientation (the more the better)

  14. 1. Focal mechanisms So can take global map of seismicity and ….

  15. 1. Focal mechanisms Analyze the fault planes…..

  16. 1. Focal mechanisms Nature of faulting depends on style of the tectonic setting

  17. 1. Focal mechanisms

  18. 1. Focal mechanisms – normal faulting

  19. 1. Focal mechanisms – thrust faulting

  20. 1. Focal mechanisms – strike slip

  21. 2. Our local setting – earthquake locations seisomometer hydrophones Quakes stop at spreading areas

  22. 2. Our local setting – focal mechanisms Offshore, it is mainly normal and strike-slip

  23. 2. Our local setting Birth of San Andreas fault: ridge got subducted Juan De Fuca plate a remanant…

  24. 2. Our local setting

  25. 2. Our local setting – Olympia Earthquake 2001 Olympia Mw6.8 From beach balls – 2 possibilities, actually was vertical…

  26. 2. Our local setting DIFFERENT KINDS OF QUAKES in the region

  27. 2. Our local setting Click here

  28. 2. Our local setting Three types of local earthquake hazard Subduction earthquakes – brittle rocks return time 500-600 yr magnitude 9+ Crustal earthquakes – crustal faults mountain building in Cascades/Olympics return time – several hundred years magnitude 7+ Deep earthquakes – mechanism poorly understood, probably due due to minearological changes return time 50-60 yr magnitude 7+

  29. 3. Shape of ocean spreading ridges (diffusion) Spreading ridges tend to have a similar profile What accounts for this?

  30. 3. Shape of ocean spreading ridges (diffusion) Can be plotted as a function of age (if spreading rates are uniform) Follows cooling curve out to 80 Ma, then flattens out - General upflow of heat

  31. 3. Shape of ocean spreading ridges (diffusion) Age of crust controls depth of ocean

  32. 3. Shape of ocean spreading ridges (diffusion) ocean crust aethenosphere As new crust is formed and moves away from spreading center, it cools and so thickens. Go to board to solve equations

  33. 4. What drives plate tectonics?

  34. 4. What drives plate tectonics? Gavitational Geoid is not uniform miliGa (1 Gal = 0.001g)

  35. 4. What drives plate tectonics? RED = DENSE (e.g., subduction zones around the Pacific Rim)

  36. Gravity Topography & Bathymetry

  37. So, Earth split up into plates, sometimes visible on the surface Iceland San Andreas

  38. Planet can be broken up into major plates

  39. What is going on? What is driving this these motions?

  40. What forces dominate the motion of the plate? Basal stress (mantle convection)? Ridge push? Slab pull?

  41. Isolate continental mantle drag..

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