1. Key Result
Seismic CAT Scan 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. 1000 Years Later
Colluvial Wedge

12. 1000 Years Later
Colluvial Wedge
Thickness = Earthquake Magnitude

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

14. Trench Colluvial Wedge
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. } 10 Fault Segments: 300-400 Year Recurrence Intervals
16 earthquakes/5600 yrs
5 segments }

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 Imaging of Colluvial Wedges

23. Seismic Imaging of Colluvial Wedges

24. Seismic Imaging of Colluvial Wedges
Fast Time

25. Seismic Imaging of Colluvial Wedges
Slow Time
Fast Time
Colluvial Wedge

26. Seismic Imaging of Colluvial Wedges

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

33. Oquirrh Mountains

34. Oquirrh Mountains

35. Oquirrh Mountains

36. Oquirrh Mountains

37. Oquirrh Mountains

38. Oquirrh Mountains

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

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

41. 3-D Velocity Tomogram

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

43. Comparison with Adjacent Trench
11.2 ft

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)
12000 ft
Megatrench Site
2-D Seismic Line
Elevation
4100 ft

49. MCCALPIN’S MEGATRENCHF1 F4

50. Trench Log F1 F4 Depth (m) 3m 11 27 Offset (m) 3-4 events Bonneville11 27 F1 3m F4
Depth (m)
3-4 events
Offset (m)
Bonneville
12-15 ca

51. Seismic Trenching Megatrench Site 2-D Seismic Line
0.5 m src/rec Interval
168-chan., 40 Hz geo.
> 27 K traveltimes

52. Common Mid-Point Gather
0 s
0.2 s

53. Velocity Tomogram Depth (m) Offset (m) Velocity (m/s) 40 120 400 140040
120
Depth (m)
400
1400
1800
Velocity (m/s)

54. Velocity Tomogram ? Depth (m) Offset (m) Velocity (m/s) 40 120 40040
120
Bonneville ?
Depth (m)
400
1400
1800
Velocity (m/s)

55. Summary
First time colluvial wedges have been imaged with seismic method
Seismic methods can compliment trenching studies by providing wider, deeper, and cheaper images of the fault zone. But, less resolution.
New Paleoseismic tool: Seismic tomography can be an alternative/complement to the intrusive task of fault trenching.

56. Summary Limitations: No dates Sesmic wedge, 1 event or several?
3-D certainty > 2-D certainty
Seismic Megatrench Images suggest
Arrythmia beyond 15,000 years ago.
Does lake act as a wedge eraser?
Was it also quiet along F1?

57. Future Work Apply method to other faults.
Dig and date colluvial wedge --> recurrence interval as well as the magnitude.
Wave equation traveltime tomography

59. Possible Solution
Use 3-D refraction traveltime tomography to image colluvial wedges

60. TOMOGRAM MIGRATION SECTION
1.8 km/s
0 km/s 40
120
MIGRATION SECTION 40
Offset (m)
120

62. 3-D Seismic Survey 50 40 30 In-line Station Number 20 10 1 2 3 4 5 6 71 2 3 4 5 6 7
Cross-line Number

63. 2-D Seismic Line

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

65. 3-D Traveltime Tomography
Data Misfit Function
Model Updating Direction

66. 2-D Reflection Processing

67. Seismic Data & Inversion
100,000 Traveltimes
TIME
20,000 Unknowns
Multigrid SIRT
OFFSET

68. Results Synthetic Results 3-D Traveltime Tomography Results
2-D Reflection Results

69. Synthetic Results

70. 3-D Tomography Results 0.045 0.04 0.035 0.03 RMS Residual (s) 0.025
0.015
0.01 2 4 6 8 10 12 14 16 18
Iteration Number

71. Use traveltime tomography to image colluvial wedges
Goal
Use traveltime tomography to image colluvial wedges

72. Reflection Results
CMP Number

73. Calculate Net Vertical Tectonic Displacement

74. Parameters Used to Calculate Net Vertical Tectonic Displacement
Tm=22.8 ft (Tomogram)
W=169 ft (Migrated Seismic Section)
=4 degrees (Migrated Seismic Section)
Ta=4.3 ft (Migrated Seismic Section)
Tnet=6.7 ft

75. Calculation of PaleoEarthquake Magnitude
Regression Equation
M = * log(Tnet)
Paleoearthquake Moment Magnitude = 6.8
Compared to moment magnitude of 7.0
calculated from the trenching study

76. Limitations Less resolution 2-3 ft vs < 1ft
More uncertainty than trench studies (will I get a good image??)
No direct dating

77. Reflection Method

78. Reflection Method

79. Reflection Method

80. Reflection Method

81. Reflection Method

82. Reflection Method

83. X-Z Slice of the 3-D Tomogram at Y=15 ft

84. X-Z Slice of the 3-D Tomogram at Y=20 ft

85. Tomographic Resolution Intersection of Wavepath Fresnel Zones
Intersection of Fresnel Zones
/2
dx =
dz =
sin /4

86. Wasatch Fault (Hidden Park)

87. 0’ 35’ 0’ 35’ WASATCH FAULT TOMOGRAM (Hidden Park) 0’ 2500’/s 600’/s
0’ ’ 0’
35’
150’
0’/s 0’
600’/s
520’/s
150’
0’ OFFSET
200’

88. 0’ 20’ 0’ 50’ WASATCH FAULT TOMOGRAM (Hidden Park) 0’ 2500’/s 150’
0’ ’ 0’
20’ 0’
2500’/s
150’
0’/s 0’
600’/s
520’/s
150’
0’ OFFSET
200’

89. Colluvial Wedge Thickness