1. Tom Brocher, Rick Blakely, and Ray Wells
Reinterpretation of the Seattle uplift, Washington, as a passive roof duplex
Tom Brocher, Rick Blakely, and Ray Wells
U.S. Geological Survey
Menlo Park, California

2. What’s a passive roof duplex?
One
sided
fold
(monocline)
A modified
fault-bend
fold, sometimes called a triangle wedge or a triangle zone
Wedge
tip

3. The Old Model Seattle fault Slip at depth Tacoma Seattle Folds
A fault-propagation fold

4. Hazards Posed by Old Model
Seismic energy directed toward Seattle
Seismic waves amplified by Seattle basin
Shallow faulting
M estimated
Seattle
Slip at depth
Seattle fault
Slip at depth

5. Problems with Old Model
North-dipping thrust faults not explained
2) Tacoma fault zone not incorporated
3) Shallow folding not convincingly explained
4) Seattle monocline not recognized as evidence for a wedge
5) Coseismic uplift data not well predicted

6. The New Model
M6.4
M6.7?
M7.2
Shallow slip, multiple sources

7. Depth in miles to the main faults
Seattle
Seattle fault
Tacoma fault

8. Seattle fault zone
You are here
Seattle
fault
zone

9. Seattle fault zone
Blake Island

10. Seattle fault zone - AD 900
Blake Island
M6.4
M7.2

11. Folding along Seattle fault - Lake Washington
Johnson et al. (1999)

12. Passive roof thrusts
Passive roof thrusts only slip when the master floor thrusts slip - they do not represent independent seismic sources. Paleo slip on them tells us about slip on the master floor thrusts.

13. Other wedges (Pakistan)
Banks and Warburton (1986)

14. Summary of New Model Seattle and Tacoma faults are blind thrusts
Both faults overlain by shallow roof thrusts
Leading edge (wedge tip) of Seattle fault moved northward to northern end of Elliott Bay
Directivity, basin amplification remain important
Tilting and folding are integral and important in a narrow band south of the wedge tip
Shallow north-dipping thrust faulting is passive
Deformation in Seattle basin

15. The End
BSSA in review,