LOCATING THE SEATTLE FAULT IN BELLEVUE, WA: COMBINING GEOMORPHIC INDICATORS WITH BOREHOLE DATA

Active crustal thrust faults can be difficult to study when surficial traces are either removed, masked, or don't exist. The Seattle Fault Zone is an active east-west trending reverse fault zone that intersects both Seattle and Bellevue, two of the most populated cities in Washington. Rupture along strands of the fault poses a serious threat to infrastructure and thousands of people in the region. Precise locations of fault strands are still poorly constrained in Bellevue due to blind thrusting, urban development, and/or erosion. Seismic reflection and aeromagnetic surveys have shed light on structural geometries of the fault where it intersects bedrock. However, the fault displaces both bedrock and unconsolidated glacial deposits; seismic data can be poor indicators of the location of fault strands within unconsolidated strata. In order to conduct an accurate assessment of fault outcrop, I interpret existing subsurface data sets and analyze surficial hillslope and channel morphologies to determine where the fault intersects bedrock and/or unconsolidated sediments. Geotechnical borings are used to look for potential fault offsets, depths and locations of bedrock, and to construct cross sections of the fault with Rockworks software. Correlating strata, sheared zones, and bedrock type between drill sites, provides information about the structural geometry of major fault splays. ArcGIS and LiDAR data are used in hillslope/channel geomorphology analysis. Geomorphological analysis aims to determine if slip along the fault is recorded within stream channel morphology as knickpoints that can be identified in longitudinal profile. Working models created from borehole and geomorphological analyses are tested with field investigations to determine if traces of the fault are visible in locations of model projected fault planes. Existing aeromagnetic and seismic reflection data also aid in refining fault geometry models. This study provides a method and context for characterizing blind thrust faults using a spectrum of data analysis techniques.