Do inverse trishear models of deformed geomorphic surfaces constrain the geometry of the active strand of the blind Seattle thrust?

Juliet Crider, University of Washinton
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Abstract

The Seattle Fault Zone is an array of blind thrust faults that run beneath the Seattle urban core in Washington state. The fault zone produced a M ~7.5 earthquake about 1100 years ago. Although long recognized as major crustal structure, the geometry of the fault is poorly constrained. Secondary structures in bedrock are mostly obscured by glacial deposits, water, and dense vegetation; the fault zone is difficult to image with reflection seismology; and there is very little microseismicity in the fault zone that could reveal the structure. The earthquake in 923 CE deformed a contemporary wavecut platform, creating a broad antiform with an amplitude of about 6 m. Folding of the same amplitude is observed in a submerged shoreline, mappable in bathymetry for more than 35 km perpendicular to the fault zone. Assuming that these surfaces represent folding above a blind thrust, I explore inverse trishear modeling to evaluate plausible ranges of slip during the last major earthquake and the position, depth and dip of the active strand of the Seattle Fault.

Session

Deformation in the upper crust