Migration and Tomographic Imaging of Colluvial Wedges and Faults over the Washington Fault, Arizona

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Migration and Tomographic Imaging of Colluvial
Wedges and Faults over the Washington Fault,
Arizona
Shengdong Liu Univ. of Utah April 21, 2009
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Outline

• Motivation and Objective

• Seismic Surveys in Arizona

• Synthetic Tests

• Results and Interpretations

• Conclusions

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Motivation
Problem Earthquake histories are required for
land planning.
Possible Solutions Trenching and Seismic Surveys

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Motivation
Trenches, when available, are 2D, expensive, and
shallow.
Alternative Traveltime tomography from seismic
surveys.
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Oquirrh Fault Scarp
(Morey and Schuster, 1999)
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Objective of the Thesis

• To study earthquake hazard over the
• Washington fault zone, in southern Utah and
• Arizona, and guide to design a trenching
  survey.

How did we accomplish this?
1. 2D 3D seismic surveys around the Washington
fault zone.
2. Estimation of subsurface velocity models, and
the number and size of possible colluvial
wedges.
3. Delineation of fault structures.
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Outline

• Motivation and Objective

• Seismic Surveys in Arizona

• Synthetic Tests

• Results and Interpretations

• Conclusions

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Survey Location
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Seismic Surveys
Looking southeast
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Summary of Surveys

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2-D Sample Shot Gather
Offset
95
0
0
500 m/s

• f 50 Hz

2200 m/s

• ? Vmin/fmax
• 5 m

Time (s)
0.1
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3-D Sample Shot Gather
Receiver
480
0
0
Time (s)
0.07
Line 1
Line 4
Line 5
Line 6
Line 3
Line 2

• Total 115,200 traveltimes

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Outline

• Motivation and Objective

• Seismic Surveys in Arizona

• Synthetic Tests

• Results and Interpretations

• Conclusions

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Objective

• 2-D and 3-D Tomography Test
• Can it be used to delineate the fault
  structures?
• Stack and Migration Test
• Can it image the faults?

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Fault Model and 3-D Tomogram
2200
0
m/s
Depth (m)
0
30
0
117
Offset (m)
2200
0
m/s
Depth (m)
0
30
0
117
Offset (m)
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2-D Tomogram and Raypath Density Image
2200
0
m/s
Depth (m)
0
30
0
117
Offset (m)
2200
0
m/s
Depth (m)
0
30
0
13
117
Offset (m)
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Velocity and Velocity Gradient Profile at X 26 m
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Velocity and Velocity Gradient Profile at X 48
m
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Velocity and Velocity Gradient Profile at X 72
m
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2-D and 3-D RMS Traveltime Residual
0.0014
Residual (s)
15
0.0003
0.0
Iteration
24
0
0.0012
Residual (s)
15
0.0003
0.0
Iteration
24
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0
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Reflectivity Model and Stacked Section
0
Reflectivity Model
Depth (m)
30
117
Offset (m)
0
0
Stacked Section
Time (s)
0.12
Offset (m)
0
117
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Migration Image with True and Tomogram Velocity
0
Migration Image with True Velocity
Depth (m)
30
117
Offset (m)
0
0
Migration Image with Tomogram Velocity
Depth (m)
30
117
Offset (m)
0
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Summary Synthetic Tests

• Faults have significantly higher velocity
  gradient value.
• Faults cause focusing of raypaths along the
  fault plane.
• LVZs have lower raypath coverage.
• 3-D tomograms are more accurate and have fewer
  artifacts than 2-D tomograms.
• Faults can be delineated by migration image.

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Outline

• Motivation and Objective

• Seismic Surveys in Arizona

• Synthetic Tests

• Results and Interpretations

• Conclusions

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3-D Traveltime Tomogram
7.5 m
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3-D Tomogram Slices
Y 0 m
2200
0
m/s
Depth (m)
0
117
Offset (m)
0
Y 2 m
0
2200
m/s
Depth (m)
30
0
117
0
Offset (m)
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Y 4 m
2200
0
m/s
Depth (m)
0
30
Offset (m)
117
0
Y 6 m
0
2200
m/s
Depth (m)
30
0
117
Offset (m)
0
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2-D Tomogram and Raypath Density Image
2200
0
m/s
Depth (m)
0
30
117
Offset (m)
0
0
400
ray
Depth (m)
30
0
117
0
Offset (m)
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Comparison between Tomogram and Migration Image
2200
0
Depth (m)
m/s
30
0
0
93
Offset (m)
0
1
Depth (m)
Washington fault zone
-1
30
Offset (m)
93
0
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2-D 3-D RMS Traveltime Residual
0.012
Residual (s)
15
15
0.0024
0.0
Iteration
24
0
0.06
Residual (s)
15
0.003
0.0
Iteration
24
0
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Final Interpretation
0
Depth (m)
30
Offset (m)
0
117
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Fault Interpretation
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LVZ Interpretation

• LVZ1 and LVZ2 are possibly the colluvial
  wedges
• Fault slip rate is estimated to be 0.003-0.0012
  mm/yr by Earth Sciences Associates (1982), and
  the age of the fault can be speculatively
  estimated to be younger than 16 kyr.

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Outline

• Motivation and Objective

• Seismic Surveys in Arizona

• Synthetic Tests

• Results and Interpretations

• Conclusions

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Conclusions

• Three LVZs (LVZ1, LVZ2 and LVZ3) are imaged with
  both 2-D and 3-D traveltime tomography.
• F3 is likely to be the main fault and F4 is the
  possible antithetic fault.
• The depth of the bedrock is about 15 m with
  velocity of larger than 2200 m/s.
• The four faults have an apparent dip of
  approximately 70-80 degrees.

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• LVZ1 and LVZ2 are possibly the colluvial
  wedges The thickness of the LVZ1 and LVZ2 is
  about 5 m, and the thickness of LVZ3 is about 2
  m.
• If we assume that the thickness of LVZ is
  approximately identical to the fault slip, then
  the age of the fault is estimated to be younger
  than 16 kyr.

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Future Work
A future task is to compare the tomogram with
the trench log (soon to be recorded by UGS in
late spring, 2009), and analyze the accuracy of
my interpretation.
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Acknowledgment
I would like to thank

• Dr. Gerard Schuster and my committee members
• Dr. Richard D. Jarrard and Dr. Sherif Hanafy
  for their
• advice and constructive criticism.

• Bill Lund, Tyler Knudsen and UGS for the
  geological information on the Washington fault
  zone and the future trench results.

• UTAM friends

- Shuqian Dong, Naoshi Aoki, and Ge Zhan for
their help on my thesis research -
Sherif Hanafy, Wei Dai, Xin Wang, Qiong Wu, Simin
Huang, Joost Van Der Neut for their help in
field work - All UTAM members for their
support in my life and work.