CHARACTERIZING STRANDS OF THE SEATTLE FAULT ZONE IN SOUTHEAST SEATTLE THROUGH BOREHOLE ANALYSIS AND SUBSURFACE MODELING

The Puget Lowland is characterized by a compressional stress regime related to the subduction of the Juan de Fuca Plate under the North American Plate. This stress is accommodated by brittle failure of the upper crust via several east-west-trending thrust fault zones, including the Seattle Fault Zone (SFZ). The SFZ is a south-dipping thrust fault that intersects Seattle, Washington. Previous geologic mapping and geophysical analysis of the Seattle area have mapped the locations of strands of the Seattle Fault, though the complete picture of the fault system is still incomplete. A distinct lineament in aeromagnetic surveys shows the approximate location of a strand of the Seattle Fault in southeast Seattle. Using ground penetrating radar (GPR) and an analysis of borehole logs in the shallow subsurface, this study will contribute to the body of knowledge about the Seattle Fault and the associated seismic hazard. I interpret lithology based on borehole log descriptions, and I use Rockworks software to interpolate a 3-dimensional model of the subsurface to more accurately locate and characterize the fault in this area. GPR transects, perpendicular to the fault trend, identify material density differences in the subsurface and were used to calibrate the model. The calibrated model shows the contact between the Eocene/Oligocene Blakeley Formation and Quaternary unconsolidated glacial deposits. This study will produce an accurate location of the surface trace of the fault, and cross sections perpendicular to the trace of the fault will show its orientation at depth. Further, I expect that the fault trace will be consistent with the previously mapped contact between the Blakeley Formation and Quaternary glacial till, and observed topographic inflection points in the shallow north-south-trending valleys represent scarps. Data collection and modeling are still underway, and more comprehensive results and interpretations will be presented at this meeting.