EXPLORING SHALLOW FAULT AND FOLD DEFORMATION OF THE SEATTLE FAULT ZONE USING NEW HIGH-RESOLUTION MARINE GEOPHYSICAL DATA

Active faults that accommodate crustal shortening across the Puget Lowland represent a significant hazard to population centers in western Washington State. Paleoseismic investigations indicate that the Seattle fault zone (SFZ) sustained a ~M7 earthquake approximately 1100 years ago. Extensive areas covered by water and late Quaternary deposits limit our knowledge of the SFZ, highlighting the importance of high-resolution marine geophysical information. Questions exist about the spatial extent and timeframe of earthquake rupture along mapped fault traces and how these structures interact with the primary south-dipping SFZ at depth.

In February 2017, a new marine geophysical dataset was collected including >240 km of collocated high-resolution multichannel and chirp seismic-reflection data to characterize fault connectivity and shallow deformation above the SFZ. This dataset, combined with chirp data collected in 2011, images stratigraphic and structural relationships between bedrock and Quaternary glacial and interglacial units in the top 50-100 m. Here we present initial interpretations of these new data from between southern Bainbridge Island (Rich Passage) and downtown Seattle, and from Lake Washington. These data document folding, faulting, unconformities and local growth strata associated with fault traces, some of which coincide with visible scarps in multibeam bathymetry. Scarps near Rich Passage represent offshore continuations of steep, north-dipping backthrusts mapped on land. While imaging quality is variable along strike, the data show clear evidence of faulting close to the seafloor beneath offshore scarps. We focus initial interpretations on mapping stratigraphic units and correlating them with existing dated stratigraphy to develop an age model for shallow deformation on the SFZ. We also interpret cross-cutting relationships and relative ages of submerged marine shorelines and subaqueous landslide deposits that are visible in the bathymetry. Ultimately, these offshore interpretations will be integrated with existing geophysical and geologic data, along with onshore analyses of co-seismically uplifted shorelines and variations in local channel steepness, to constrain the extent and timing of near-surface faulting and its relationship to deeper SFZ fault structure.