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Reinterpretation of the Seattle uplift, Washington, as a passive roof duplex

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. What’s a passive roof duplex?. One sided fold (monocline). A modified fault-bend

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Reinterpretation of the Seattle uplift, Washington, as a passive roof duplex

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  1. 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 Tacoma Seattle Folds Seattle fault Slip at depth A fault-propagation fold

  4. Hazards Posed by Old Model Seismic energy directed toward Seattle Seismic waves amplified by Seattle basin Shallow faulting M 7.6-7.7 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, brocher@usgs.gov

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