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

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

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

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

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

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

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

Seattle fault zone You are here Seattle fault zone

Seattle fault zone Blake Island

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

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

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.

Other wedges (Pakistan) Banks and Warburton (1986)

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

The End BSSA in review, brocher@usgs.gov