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Seismic CAT Scan. Key Result. vs. Trenching. 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: Conclusions. Oquirrh Fault Wasatch Fault. Paleoseismology.

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seismic cat scans of ancient earthquakes in utah

Seismic CAT Scans of Ancient Earthquakes in Utah

Dave Morey, Dave Sheley and Jerry Schuster

University of Utah

outline
Outline
  • Paleoseismology
  • Seismic CAT Scan Idea
  • Results:
  • Conclusions

Oquirrh Fault

Wasatch Fault

paleoseismology
Paleoseismology
  • Determine Earthquake Hazard
paleoseismology5
Paleoseismology
  • Determine Earthquake Hazard
  • Trenching Studies: estimate size and recurrence intervals of ancient earthquakes from the thicknesses and separations of colluvial wedges
1000 years later12
1000 Years Later

Colluvial Wedge

Thickness = Earthquake Magnitude

2000 years later
2000 Years Later

2 Colluvial Wedges

Separation = Recurrence Interval

trench wedge

Trench & Wedge

Trench Colluvial Wedge

slide15

RECENT UTAH EARTHQUAKES

12000 ft

Magnitude

Elevation

6.5

4.5

4100 ft

slide19

}

16 earthquakes/5600 yrs

5 segments

10 Fault Segments: 300-400 Year Recurrence Intervals

trenching problems
Trenching Problems
  • Trenches are:
    • Expensive
    • 2-D
    • Shallow

Megatrench Trench

outline21
Outline
  • Paleoseismology
  • Seismic CAT Scan Idea
  • Results:
  • Conclusions

Oquirrh Fault

Wasatch Fault

seismic imaging of colluvial wedges25
Seismic ImagingofColluvial Wedges

Slow Time

Fast Time

Colluvial Wedge

many traveltime equations for each shot
Many Traveltime Equations for Each Shot

T = L /v + L /v + L /v

T = L /v+ L /v + L /v

outline31
Outline
  • Paleoseismology
  • Seismic CAT Scan Idea
  • Results:
  • Conclusions

Oquirrh Fault

Wasatch Fault

slide32

Oquirrh Fault Scarp

Trench

3-D Seismic

Array

Oquirrh Mountains

12000 ft

Elevation

4100 ft

slide39

2.5’

140’

5’

40’

Oquirrh Mountains

seismic data
Seismic Data

100,000 Traveltimes

TIME

20,000 Unknowns

Multigrid SIRT

OFFSET

outline46
Outline
  • Paleoseismology
  • Seismic CAT Scan Idea
  • Results:
  • Conclusions

Oquirrh Fault

Wasatch Fault

slide48

Megatrench Survey(Sheley & Crosby)

Megatrench Site

2-D Seismic Line

12000 ft

Elevation

4100 ft

slide49

F4

F1

MCCALPIN’S MEGATRENCH

trench log

0

3m

3-4 events

11

27

0

Trench Log

F1

F4

Depth (m)

Offset (m)

Bonneville

12-15 ca

seismic trenching

Megatrench Site

Seismic Trenching

2-D Seismic Line

0.5 m src/rec Interval

168-chan., 40 Hz geo.

> 27 K traveltimes

velocity tomogram

0

Offset (m)

40

0

120

Velocity Tomogram

Depth (m)

0

400

1400

1800

Velocity (m/s)

velocity tomogram54

Bonneville

0

Offset (m)

40

0

120

Velocity Tomogram

?

Depth (m)

0

400

1400

1800

Velocity (m/s)

summary

Seismic methods can compliment trenching studies by providingwider, 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
  • First time colluvial wedges have been imaged with seismic method
summary56

Limitations:

  • Seismic Megatrench Images suggest

Arrythmia beyond 15,000 years ago.

Does lake act as a wedge eraser?

Was it also quiet along F1?

Summary

No dates

Sesmic wedge, 1 event or several?

3-D certainty > 2-D certainty

future work
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
Possible Solution
  • Use 3-D refraction traveltime tomography to image colluvial wedges
slide60

TOMOGRAM

1.8 km/s

0 km/s

0

40

0

120

MIGRATION SECTION

0

40

0

120

Offset (m)

3 d seismic survey
3-D Seismic Survey

50

40

30

In-line Station Number

20

10

0

1

2

3

4

5

6

7

Cross-line Number

outline64
Outline
  • Paleoseismology
  • Seismic CAT Scan Idea
  • Results:
  • Conclusions

Oquirrh Fault

Wasatch Fault

3 d traveltime tomography
3-D Traveltime Tomography

Data Misfit Function

Model Updating Direction

seismic data inversion
Seismic Data & Inversion

100,000 Traveltimes

TIME

20,000 Unknowns

Multigrid SIRT

OFFSET

results
Results
  • Synthetic Results
  • 3-D Traveltime Tomography Results
  • 2-D Reflection Results
3 d tomography results
3-D Tomography Results

0.045

0.04

0.035

0.03

RMS Residual (s)

0.025

0.02

0.015

0.01

2

4

6

8

10

12

14

16

18

Iteration Number

slide71

Goal

Use traveltime tomography to image colluvial wedges

parameters used to calculate net vertical tectonic displacement
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
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
Limitations
  • Less resolution 2-3 ft vs < 1ft
  • More uncertainty than trench studies (will I get a good image??)
  • No direct dating
slide85

dx =

/2

Intersection of Fresnel Zones

sin /4

dz =

Tomographic Resolution

Intersection of Wavepath Fresnel Zones

slide87

WASATCH FAULT TOMOGRAM

(Hidden Park)

0’

35’

0’

35’

600’/s

600’/s

0’ 35’

520’/s

520’/s

0’

2500’/s

150’

0’/s

0’

150’

0’ OFFSET

200’

slide88

WASATCH FAULT TOMOGRAM

(Hidden Park)

600’/s

0’

20’

520’/s

0’ 50’

0’

2500’/s

150’

0’/s

0’

150’

0’ OFFSET

200’

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