1. Further topics in tectonics: 1.Focal mechanisms 2.Pac NW tectonic setting 3.Shape of mid-ocean ridges (depth of the ocean) 4.Driving forces of plate tectonics

2. Elastic Rebound 1. Focal mechanisms

3. No offset Earthquake break 1. Focal mechanisms

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

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

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

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

8. CD DC Right lateral 1. Focal mechanisms Two fault planes will produce the same motion

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

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

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

12. Different orientations of different faults 1. Focal mechanisms

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

18. 1. Focal mechanisms – normal faulting

19. 1. Focal mechanisms – thrust faulting

20. 1. Focal mechanisms – strike slip

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

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

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

24. 2. Our local setting

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

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

27. Click here 2. Our local setting

28. 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+ 2. Our local setting

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

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

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

32. 3. Shape of ocean spreading ridges (diffusion) aethenosphere crust ocean 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. Gavitational Geoid is not uniform 4. What drives plate tectonics? miliGa (1 Gal = 0.001g)

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

36. Topography & Bathymetry Gravity

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

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?

47. Isolate continental mantle drag..

53. WHAT IS THE SOURCE OF THE DRIVING FORCE? Correlation of plate area, trench length, ridge length vs plate velocity. Correlates only with trench length - n.b. area ~ basal stress; ridge length ~ ridge push; trench length ~ slab pull As a % of total perimter