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

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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

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Whats a passive roof duplex?
One sided fold (monocline)
A modified fault-bend fold, sometimes called a
triangle wedge or a triangle zone
Wedge tip
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The Old Model
Seattle
Tacoma
Folds
Seattle fault
Slip at depth
A fault-propagation fold
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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
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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

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The New Model
M6.4
M7.2
M6.7?
Shallow slip, multiple sources
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Depth in miles to the main faults
Seattle
Seattle fault
Tacoma fault
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Seattle fault zone
You are here
Seattle fault zone
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Seattle fault zone
Blake Island
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Seattle fault zone - AD 900
Blake Island
M6.4
M7.2
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Folding along Seattle fault - Lake Washington
Johnson et al. (1999)
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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.
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Other wedges (Pakistan)
Banks and Warburton (1986)
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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

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The End
BSSA in review, brocher_at_usgs.gov