Key Result
vs
Trenching
Seismic CAT Scan
Seismic CAT Scans
of Ancient
Earthquakes in Utah
Dave Morey, Dave Sheley and
Jerry Schuster
University of Utah
Outline
–Paleoseismology
–Seismic CAT Scan Idea
–Results: Oquirrh Fault
Wasatch Fault
–Conclusions
Paleoseismology
• Determine Earthquake Hazard
Paleoseismology
• Determine Earthquake Hazard
• Trenching Studies: estimate size and
recurrence intervals of ancient
earthquakes from the thicknesses
and separations of colluvial wedges
NO SCARP: T = 0 sec
SCARP FORMING: T=.01 sec
RECENT EARTHQUAKE SCARP: T=.02 s
Pre-Earthquake
Earthquake
1000 Years Later
Colluvial Wedge
1000 Years Later
Colluvial Wedge
Thickness = Earthquake Magnitude
2000 Years Later
2 Colluvial Wedges
Separation = Recurrence Interval
Trench & Wedge
Trench
Colluvial Wedge
RECENT UTAH EARTHQUAKES
12000 ft
Magnitude
6.5
4.5
Elevation
4100 ft
10 Fault Segments
10 Fault Segments
10 Fault Segments
10 Fault Segments: 300-400 Year Recurrence Intervals
16 earthquakes/5600 yrs
5 segments
Trenching Problems
• Trenches are:
– Expensive
– 2-D
– Shallow
Megatrench Trench
Outline
–Paleoseismology
–Seismic CAT Scan Idea
–Results: Oquirrh Fault
Wasatch Fault
–Conclusions
Seismic Imaging of Colluvial Wedges
Seismic Imaging of Colluvial Wedges
Seismic Imaging of Colluvial Wedges
Fast Time
Seismic Imaging of Colluvial Wedges
Slow Time
Fast Time
Colluvial Wedge
Seismic Imaging of Colluvial Wedges
0.5-1.0 m
Discretize Earth Model into Grid of
Unknown Velocities
One Traveltime Equation for Each
Measurement
One Traveltime Equation for Each
Measurement
T = L /v + L /v + L /v
Many Traveltime Equations for Each
Shot
T = L /v + L /v + L /v
T = L /v + L /v + L /v
Outline
–Paleoseismology
–Seismic CAT Scan Idea
–Results: Oquirrh Fault
Wasatch Fault
–Conclusions
Oquirrh Mountains
12000 ft
Oquirrh Fault Scarp
Trench
Elevation
3-D Seism
4100 ft Array
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
2.5’
Oquirrh Mountains
5’
40’
140’
Seismic Data
TIME
100,000 Traveltimes
20,000 Unknowns
Multigrid SIRT
OFFSET
3-D Velocity Tomogram
X-Z
X-Z Slice
Slice of
of the
the 3-D
3-D Tomogram
Tomogram
at Y=10
15ftft
20
Comparison with Adjacent
Trench
11.2 ft
Reflection Results
Comparison of Tomogram and
Reflection Results
Outline
–Paleoseismology
–Seismic CAT Scan Idea
–Results: Oquirrh Fault
Wasatch Fault
–Conclusions
Wasatch Fault (Megatrench)
Megatrench Survey
(Sheley & Crosby)
12000 ft
Megatrench Site
Elevation
4100 ft
2-D Seismic Line
MCCALPIN’S MEGATRENCH
F1
F4
Trench Log
F1
0
F4
3-4 events
11
0
27
Bonneville
12-15 ca
Seismic Trenching
Megatrench Site
2-D Seismic Line
0.5 m src/rec Interval
168-chan., 40 Hz geo.
> 27 K traveltimes
Common Mid-Point Gather
0s
0.2 s
Velocity Tomogram
0
40
0
0
120
400
Velocity (m/s)
1400
1800
Velocity Tomogram
0
?
Bonneville
40
0
0
120
400
Velocity (m/s)
1400
1800
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.
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?
Future Work
• Apply method to other faults.
• Dig and date colluvial wedge -->
recurrence interval as well as the
magnitude.
Wave equation traveltime
tomography
Possible Solution
• Use 3-D refraction traveltime tomography
to image colluvial wedges
1.8 km/s
TOMOGRAM
0
40
0 km/s
0
120
MIGRATION SECTION
0
40
0
120
3-D Seismic Survey
In-line Station Number
50
40
30
20
10
0
1
2
3
4
5
Cross-line Number
6
7
2-D Seismic Line
Outline
–Paleoseismology
–Seismic CAT Scan Idea
–Results: Oquirrh Fault
Wasatch Fault
–Conclusions
3-D Traveltime Tomography
Data Misfit Function
1
   (tiobs  tical )2
2 i
Model Updating Direction

ti
j
  ti
  ti lij  l ti
sj
sj
i
i
i
1
j
 ti
N j i 1
2-D Reflection Processing
Seismic Data & Inversion
TIME
100,000 Traveltimes
20,000 Unknowns
Multigrid SIRT
OFFSET
Results
• Synthetic Results
• 3-D Traveltime Tomography Results
• 2-D Reflection Results
Synthetic Results
3-D Tomography Results
0.045
RMS Residual (s)
0.04
0.035
0.03
0.025
0.02
0.015
0.01
2
4
6
8
10
12
Iteration Number
14
16
18
Goal
Use traveltime tomography to image
colluvial wedges
Reflection Results
CMP Number
Calculate Net Vertical Tectonic
Displacement
Tnet  Tm  [(W tan  )  Ta
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
Calculation of PaleoEarthquake
Magnitude
Regression Equation
M = 6.61 + 0.71 * log(Tnet)
Paleoearthquake Moment Magnitude = 6.8
Compared to moment magnitude of 7.0
calculated from the trenching study
Limitations
• Less resolution 2-3 ft vs < 1ft
• More uncertainty than trench studies (will I
get a good image??)
• No direct dating
Reflection Method
Reflection Method
Reflection Method
Reflection Method
Reflection Method
Reflection Method
X-Z Slice of the 3-D Tomogram
at Y=15 ft
X-Z Slice of the 3-D Tomogram
at Y=20 ft
Tomographic Resolution
Intersection of Wavepath Fresnel Zones
dx = /2
dz = sin /4
Intersection of Fresnel Zones
Wasatch Fault (Hidden Park)
0’
WASATCH FAULT TOMOGRAM
(Hidden Park)
0’
0’
150’
0’/s
0’
35’
35’
150’
0’
2500’/s
600’/s
600’/s
0’
OFFSET
35’
200’
520’/s
520’/s
0’
WASATCH FAULT TOMOGRAM
(Hidden Park)
150’
2500’/s
0’/s
0’
600’/s
150’
0’
20’
520’/s
0’
OFFSET
200’
Colluvial Wedge Thickness