Key Result Trenching vs Seismic CAT Scan Seismic CAT Scan

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


Colluvial WedgeColluvial WedgeThickness = Earthquake MagnitudeThickness = Earthquake Magnitude


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 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



–Results:Results:



Seismic ImagingSeismic Imaging ofof Colluvial Colluvial WedgesWedges



Fast TimeFast Time


Colluvial WedgeColluvial Wedge

Fast TimeFast Time

Slow TimeSlow Time


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



–Results:Results:



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



–Results:Results:



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

)


00

404012012000

Velocity (m/s)Velocity (m/s)00 18001800400400 14001400

Velocity TomogramVelocity TomogramD

epth

(m

)D

epth

(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



–Results:Results:



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






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

Documents

Key Result Trenching vs Seismic CAT Scan Seismic CAT Scan