CHARACTERIZING THE SEATTLE FAULT THROUGH BOREHOLE ANALYSIS IN BELLEVUE, WASHINGTON

The Seattle Fault zone is an area of east-trending, north-verging thrust faults extending across the Puget Lowland. The fault has displaced Eocene basement rock northward relative to the sediment-filled Seattle basin to the north (Johnson et al., 1994). The Cenozoic tectonic history of the Puget Lowland in Washington State remains poorly understood because of thick (~9-10 km) overlying Quaternary deposits in the Seattle basin and the submergence of features underneath Puget Sound. Bedrock units are exposed at the surface in sparse areas south of Bellevue while the remaining exposures consist of Quaternary alluvium, interglacial deposits, and glacial deposits. Geophysical and aeromagnetic surveys have provided valuable information constraining the location of the fault and its strands, but the geometry and deformation of the subsurface geology is largely unknown. Interpretation of the subsurface conditions is critical for understanding crustal deformation associated with the active fault and for estimating earthquake hazards in the densely populated cities of Seattle and Bellevue. Geologic maps provide an accurate description of the surface geology, but extrapolating these features into the subsurface is a tremendous task given the complexity of folding, faulting, and glaciation in the Puget Lowland. My project utilizes existing geotechnical borehole data along the fault zone in order to characterize the subsurface geology. My analysis requires a thorough interpretation of the stratigraphy as observed in the boring logs, field work identifying outcrops in Bellevue and characterizing the exposed units, and focused surveys using ground-penetrating radar (GPR) equipment. Anticipated results include: 1) a new three-dimensional model of the structure underlying West Bellevue; and 2) several subsurface cross-sections transecting the Seattle Fault that reveal the main thrust and its strands. My new model and cross-sections will be compared with others that have been derived from geophysical surveys, in order to assist in the interpretation process and create a more robust model. The results are important for assessing regional earthquake hazards, which may be better constrained by an understanding of how the Seattle Fault has deformed unconsolidated material overlying bedrock unit.