Predicting the Position of Fault Strands through Borehole Correlation of the Top of Tertiary Bedrock in the Seattle Fault Zone, Seattle, Washington

Abstract

Only a few strands of the Seattle Fault Zone have been identified on land in Seattle, Washington due to heavy manipulation of the landscape by repeat glaciations, geomorphic reworking, and the interference of the growing metropolitan community concealing potentially dangerous active faults below the surface. Previous difficulty locating faults on the ground surface led to the idea of using geotechnical boring logs to ìsee intoî the subsurface structure. Can the depth to the contact between Quaternary deposits and Tertiary bedrock, obtained from existing geotechnical boring logs, be used to predict the locations of fault strands in the Seattle Fault Zone in Seattle? The University of Washington Pacific Northwest Center for Geologic Mapping Studies (GeoMapNW) database contains thousands of geotechnical boring logs readily available. Over 18,000 boring logs were processed using the computer programming language Python to select boring logs containing bedrock-related terms describing Blakely Harbor Formation, Blakeley Formation, Tukwila Formation, and any other bedrock in Seattle. This process isolated 1700 boreholes that potentially intersected bedrock. Each of the 1700 boring logs were individually categorized to determine if the Python code was valid. This process reduced the queried boring logs to 809 boreholes containing bedrock within the Seattle Fault Zone. The boreholes containing bedrock were used to construct preliminary contours of the bedrock surface in AutoCAD Civil 3D. Both boreholes containing bedrock and boreholes not containing bedrock were used to generate 108 geologic cross sections using the geographic information system (GIS) Geologic Cross Section Toolbox developed in 2018. The variation in depth to the contact between Quaternary deposits and Tertiary bedrock was evaluated in the cross sections highlighting 66 anomalies in southeast Seattle. The anomalies were checked against the triangular irregular network (TIN) surface and contours generated in AutoCAD Civil 3D to better visualize the Seattle Fault Zone structure. The anomalies ranged from potential faults and geomorphic alterations to possible human alterations such as hillslope grading along Interstate 5. Fifty percent of the anomalies were identified as potential faults based on the continuation of offset patterns in neighboring cross section. Published fault locations from multiple reliable sources supplied by the U.S. Geological Survey (USGS) and Washington State Department of Natural Resources (DNR) were checked to assess the connection of anomalies to mapped faults. Some of the anomalies found through borehole correlation suggest revisions and continuations of existing published fault locations through the City of Seattle. Identifying potential fault anomalies by correlating detailed boring logs proved successful and could be applied to complex fault zones worldwide. The next step is to investigate the anomalies identified as potential faults to determine if they are representations of faults strands within the Seattle Fault Zone.