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Key Result

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  1. Seismic CAT Scan Key Result vs Trenching

  2. Seismic CAT Scans of Ancient Earthquakes in Utah Dave Morey, Dave Sheley and Jerry Schuster University of Utah

  3. Outline • Paleoseismology • Seismic CAT Scan Idea • Results: • Conclusions Oquirrh Fault Wasatch Fault

  4. Paleoseismology • Determine Earthquake Hazard

  5. Paleoseismology • Determine Earthquake Hazard • Trenching Studies: estimate size and recurrence intervals of ancient earthquakes from the thicknesses and separations of colluvial wedges

  6. NO SCARP: T = 0 sec

  7. SCARP FORMING: T=.01 sec

  8. RECENT EARTHQUAKE SCARP: T=.02 s

  9. Pre-Earthquake

  10. Earthquake

  11. Colluvial Wedge 1000 Years Later

  12. 1000 Years Later Colluvial Wedge Thickness = Earthquake Magnitude

  13. 2000 Years Later 2 Colluvial Wedges Separation = Recurrence Interval

  14. Trench & Wedge Trench Colluvial Wedge

  15. RECENT UTAH EARTHQUAKES 12000 ft Magnitude Elevation 6.5 4.5 4100 ft

  16. 10 Fault Segments

  17. 10 Fault Segments

  18. 10 Fault Segments

  19. } 16 earthquakes/5600 yrs 5 segments 10 Fault Segments: 300-400 Year Recurrence Intervals

  20. Trenching Problems • Trenches are: • Expensive • 2-D • Shallow Megatrench Trench

  21. Outline • Paleoseismology • Seismic CAT Scan Idea • Results: • Conclusions Oquirrh Fault Wasatch Fault

  22. Seismic ImagingofColluvial Wedges

  23. Seismic ImagingofColluvial Wedges

  24. Seismic ImagingofColluvial Wedges Fast Time

  25. Seismic ImagingofColluvial Wedges Slow Time Fast Time Colluvial Wedge

  26. Seismic ImagingofColluvial Wedges 0.5-1.0 m

  27. Discretize Earth Model into Grid of Unknown Velocities

  28. One Traveltime Equation for Each Measurement

  29. One Traveltime Equation for Each Measurement T = L /v + L /v + L /v

  30. Many Traveltime Equations for Each Shot T = L /v + L /v + L /v T = L /v+ L /v + L /v

  31. Outline • Paleoseismology • Seismic CAT Scan Idea • Results: • Conclusions Oquirrh Fault Wasatch Fault

  32. Oquirrh Fault Scarp Trench 3-D Seismic Array Oquirrh Mountains 12000 ft Elevation 4100 ft

  33. Oquirrh Mountains

  34. Oquirrh Mountains

  35. Oquirrh Mountains

  36. Oquirrh Mountains

  37. Oquirrh Mountains

  38. Oquirrh Mountains

  39. 2.5’ 140’ 5’ 40’ Oquirrh Mountains

  40. Seismic Data 100,000 Traveltimes TIME 20,000 Unknowns Multigrid SIRT OFFSET

  41. 3-D Velocity Tomogram

  42. X-Z Slice of the 3-D Tomogram 15 ft 20 ft X-Z Slice of the 3-D Tomogram at Y=10 ft

  43. 11.2 ft Comparison with Adjacent Trench

  44. Reflection Results

  45. Comparison of Tomogram and Reflection Results

  46. Outline • Paleoseismology • Seismic CAT Scan Idea • Results: • Conclusions Oquirrh Fault Wasatch Fault

  47. Wasatch Fault (Megatrench)

  48. Megatrench Survey(Sheley & Crosby) Megatrench Site 2-D Seismic Line 12000 ft Elevation 4100 ft

  49. F4 F1 MCCALPIN’S MEGATRENCH

  50. 0 3m 3-4 events 11 27 0 Trench Log F1 F4 Depth (m) Offset (m) Bonneville 12-15 ca