Locating the Seattle Fault in Bellevue, WA: Combining geomorphic indicators with borehole data

Abstract

The Seattle Fault is an active east-west trending reverse fault zone that intersects both Seattle and Bellevue, two highly 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 zone in bedrock. However, the fault displaces both bedrock and unconsolidated Quaternary deposits, and seismic data are poor indicators of the locations of fault strands within the unconsolidated strata. Fortunately, evidence of past fault strand ruptures may also be recorded indirectly by fluvial processes and should also be observable in the subsurface. I analyzed hillslope and river geomorphology using LiDAR data and ArcGIS to locate surface fault traces and then compare/correlate these findings to subsurface offsets identified using borehole data. Geotechnical borings were used to locate one fault offset and provide input to a cross section of the fault constructed using Rockworks software. Knickpoints, which may correlate to fault rupture, were found upstream of this newly identified fault offset as well as upstream of a previously known fault segment.