Key ResultKey Result
TrenchingTrenching
vsvs
Seismic CAT ScanSeismic CAT Scan
Dave Morey, Dave Sheley and Dave Morey, Dave Sheley and Jerry SchusterJerry Schuster
University of UtahUniversity of Utah
Seismic CAT Scans Seismic CAT Scans of Ancient of Ancient
Earthquakes in UtahEarthquakes in Utah
OutlineOutline–PaleoseismologyPaleoseismology
–Seismic CAT Scan IdeaSeismic CAT Scan Idea
–Results:Results:
–ConclusionsConclusions
Oquirrh FaultOquirrh FaultWasatch FaultWasatch Fault
PaleoseismologyPaleoseismology• Determine Earthquake HazardDetermine Earthquake Hazard
PaleoseismologyPaleoseismology• Determine Earthquake HazardDetermine Earthquake Hazard
• Trenching Studies: estimate Trenching Studies: estimate size and size and recurrencerecurrence intervals of ancient intervals of ancient earthquakes from the thicknesses earthquakes from the thicknesses and separations of colluvial wedgesand separations of colluvial wedges
NO SCARP: T = 0 secNO SCARP: T = 0 sec
SCARP FORMING: T=.01 secSCARP FORMING: T=.01 sec
RECENT EARTHQUAKE SCARP: T=.02 sRECENT EARTHQUAKE SCARP: T=.02 s
Pre-EarthquakePre-Earthquake
EarthquakeEarthquake
1000 Years Later1000 Years Later
Colluvial WedgeColluvial Wedge
1000 Years Later1000 Years Later
Colluvial WedgeColluvial WedgeThickness = Earthquake MagnitudeThickness = Earthquake Magnitude
2000 Years Later2000 Years Later
2 Colluvial Wedges2 Colluvial Wedges
Separation = Recurrence IntervalSeparation = Recurrence Interval
Trench & WedgeTrench & Wedge
Trench Colluvial WedgeTrench Colluvial Wedge
6.56.54.54.5
4100 ft4100 ft
12000 ft12000 ft
MagnitudeMagnitude
RECENT UTAH EARTHQUAKESRECENT UTAH EARTHQUAKES
ElevationElevation
10 Fault Segments10 Fault Segments
10 Fault Segments10 Fault Segments
10 Fault Segments10 Fault Segments
10 Fault Segments: 300-400 Year Recurrence Intervals10 Fault Segments: 300-400 Year Recurrence Intervals
16 earthquakes/5600 yrs16 earthquakes/5600 yrs 5 segments5 segments}
Trenching ProblemsTrenching Problems
• Trenches are:Trenches are:– ExpensiveExpensive
– 2-D2-D
– ShallowShallow
Megatrench TrenchMegatrench Trench
OutlineOutline–PaleoseismologyPaleoseismology
–Seismic CAT Scan IdeaSeismic CAT Scan Idea
–Results:Results:
–ConclusionsConclusions
Oquirrh FaultOquirrh FaultWasatch FaultWasatch Fault
Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges
Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges
Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges
Fast TimeFast Time
Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges
Colluvial WedgeColluvial Wedge
Fast TimeFast Time
Slow TimeSlow Time
Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges
0.5-1.0 m0.5-1.0 m
Discretize Earth Model into Grid of Discretize Earth Model into Grid of Unknown VelocitiesUnknown Velocities
One Traveltime Equation for Each One Traveltime Equation for Each MeasurementMeasurement
One Traveltime Equation for Each One Traveltime Equation for Each MeasurementMeasurement
T = T = L /vL /v + + L /vL /v + + L /vL /v
Many Traveltime Equations for Each Many Traveltime Equations for Each ShotShot
T = T = L /v L /v + + L /vL /v + + L /vL /v
TT = = L /vL /v + + L /vL /v + + L /vL /v
OutlineOutline–PaleoseismologyPaleoseismology
–Seismic CAT Scan IdeaSeismic CAT Scan Idea
–Results:Results:
–ConclusionsConclusions
Oquirrh FaultOquirrh FaultWasatch FaultWasatch Fault
4100 ft4100 ft
12000 ft12000 ft
ElevationElevation
Oquirrh MountainsOquirrh Mountains
TrenchTrench
Oquirrh Fault ScarpOquirrh Fault Scarp
3-D Seismic 3-D Seismic ArrayArray
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
Oquirrh Mountains
5’5’
2.5’
2.5’
140’140’
40’40’
Seismic DataSeismic DataT
IME
TIM
E
OFFSETOFFSET
100,000 Traveltimes100,000 Traveltimes20,000 Unknowns20,000 UnknownsMultigrid SIRTMultigrid SIRT
3-D Velocity Tomogram3-D Velocity Tomogram
X-Z X-Z SliceSlice of the 3-D Tomogram of the 3-D Tomogram at Y=10 ftat Y=10 ft15 ft15 ft
X-Z X-Z SliceSlice of the 3-D Tomogram of the 3-D Tomogram20 ft20 ft
Comparison with Adjacent Comparison with Adjacent TrenchTrench
11.2 ft11.2 ft
Reflection ResultsReflection Results
Comparison of Tomogram and Comparison of Tomogram and Reflection ResultsReflection Results
OutlineOutline–PaleoseismologyPaleoseismology
–Seismic CAT Scan IdeaSeismic CAT Scan Idea
–Results:Results:
–ConclusionsConclusions
Oquirrh FaultOquirrh FaultWasatch FaultWasatch Fault
Wasatch Fault (Megatrench)Wasatch Fault (Megatrench)
4100 ft4100 ft
12000 ft12000 ft
ElevationElevation
Megatrench SurveyMegatrench Survey (Sheley & Crosby)(Sheley & Crosby)
2-D Seismic Line2-D Seismic Line
Megatrench SiteMegatrench Site
MCCALPIN’S MEGATRENCHMCCALPIN’S MEGATRENCH
F4F4
F1F1
00
111100 2727
Trench LogTrench LogD
epth
(m
)D
epth
(m
)
Offset (m)Offset (m)
3m3m
F1F1
F4F43-4 events3-4 events
BonnevilleBonneville12-15 ca12-15 ca
Seismic TrenchingSeismic Trenching
Megatrench SiteMegatrench Site
2-D Seismic Line 2-D Seismic Line 0.5 m src/rec Interval0.5 m src/rec Interval
168-chan., 40 Hz geo.168-chan., 40 Hz geo.
> 27 K traveltimes > 27 K traveltimes
Common Mid-Point GatherCommon Mid-Point Gather0 s0 s
0.2 s0.2 s
Velocity TomogramVelocity TomogramD
epth
(m
)D
epth
(m
)
Offset (m)Offset (m)
00
404012012000
Velocity (m/s)Velocity (m/s)00 18001800400400 14001400
Velocity TomogramVelocity TomogramD
epth
(m
)D
epth
(m
)
Offset (m)Offset (m)
00
404012012000
Velocity (m/s)Velocity (m/s)00 18001800400400 14001400
BonnevilleBonneville??
SummarySummary• First time colluvial wedges have been First time colluvial wedges have been
imaged with seismic methodimaged with seismic method
• Seismic methods can compliment Seismic methods can compliment trenching studies by providingtrenching studies by providing wider, wider, deeper, deeper, andand cheaper cheaper images of the fault images of the fault zone. But, zone. But, less resolutionless resolution..
• New Paleoseismic tool: Seismic tomography can be an New Paleoseismic tool: Seismic tomography can be an alternative/complementalternative/complement to the intrusive task of fault to the intrusive task of fault trenching. trenching.
SummarySummary
• Seismic Megatrench Images suggestSeismic Megatrench Images suggest
Arrythmia beyond 15,000 years ago.Arrythmia beyond 15,000 years ago.
Does lake act as a wedge eraser?Does lake act as a wedge eraser?
Was it also quiet along F1?Was it also quiet along F1?
• Limitations:Limitations:No datesNo dates
Sesmic wedge, 1 event or several?Sesmic wedge, 1 event or several?
3-D certainty > 2-D certainty3-D certainty > 2-D certainty
Future WorkFuture Work• Apply method to other faults.Apply method to other faults.
• Dig and date colluvial wedge --> Dig and date colluvial wedge --> recurrence interval as well as the recurrence interval as well as the magnitude.magnitude.
Wave equation traveltime Wave equation traveltime tomographytomography
Possible Solution
• Use 3-D refraction traveltime tomography to image colluvial wedges
TOMOGRAMTOMOGRAM
Offset (m)Offset (m) 120120
12012000
00
4040
4040
00
00
MIGRATION SECTIONMIGRATION SECTION
1.8 km/s1.8 km/s
0 km/s0 km/s
3-D Seismic Survey
0
10
20
30
40
50
1 2 3 4 5 6 7Cross-line Number
In-l
ine
Sta
tion
Num
ber
2-D Seismic Line
OutlineOutline–PaleoseismologyPaleoseismology
–Seismic CAT Scan IdeaSeismic CAT Scan Idea
–Results:Results:
–ConclusionsConclusions
Oquirrh FaultOquirrh FaultWasatch FaultWasatch Fault
3-D Traveltime Tomography
1
22( )t ti
obs
iical
Data Misfit Function
j
j
ii
i
j
ii
ij ii
j
j
ii
stt
st l l t
Nt
1
1
Model Updating Direction
2-D Reflection Processing
Seismic Data & InversionSeismic Data & InversionT
IME
TIM
E
OFFSETOFFSET
100,000 Traveltimes100,000 Traveltimes20,000 Unknowns20,000 UnknownsMultigrid SIRTMultigrid SIRT
Results
• Synthetic Results
• 3-D Traveltime Tomography Results
• 2-D Reflection Results
Synthetic Results
3-D Tomography Results
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
2 4 6 8 10 12 14 16 18
RM
S R
esid
ual (
s)
Iteration Number
GoalUse traveltime tomography to image
colluvial wedges
Reflection Results
CMP Number
Calculate Net Vertical Tectonic Displacement
T T W Tnet m a [( tan )
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 MethodReflection Method
Reflection MethodReflection Method
Reflection MethodReflection Method
Reflection MethodReflection Method
Reflection MethodReflection Method
Reflection MethodReflection 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 ResolutionTomographic ResolutionIntersection of Wavepath Fresnel ZonesIntersection of Wavepath Fresnel Zones
Intersection of Fresnel ZonesIntersection of Fresnel Zones
/2 /2 dx =dx =dz =dz = sin sin /4 /4
Wasatch Fault (Hidden Park)Wasatch Fault (Hidden Park)
0’0’
150’150’
150’150’
0’0’
200’200’0’ OFFSET0’ OFFSET
2500’/s2500’/s
0’/s0’/s
600’/s600’/s
520’/s520’/s
600’/s600’/s
520’/s520’/s0’ 35’ 0’ 35’
0’0’
35’ 35’
0’0’
35’35’
WASATCH FAULT TOMOGRAMWASATCH FAULT TOMOGRAM (Hidden Park)(Hidden Park)
0’0’
150’150’
150’150’
0’0’
200’200’0’ OFFSET0’ OFFSET
2500’/s2500’/s
0’/s0’/s
600’/s600’/s
520’/s520’/s
0’ 50’0’ 50’
0’0’
20’ 20’
WASATCH FAULT TOMOGRAMWASATCH FAULT TOMOGRAM (Hidden Park)(Hidden Park)
Colluvial Wedge Thickness