LANDFALL OF THE SEATTLE FAULT ZONE, WEST SEATTLE, WA

From its clearest expression on the Kitsap Peninsula and Bainbridge Island, the Seattle fault zone trends east across Puget Sound to make next landfall through the city of Seattle. With 7+ m of vertical movement only 1100 years ago, evidence of faulting in this highly modified urban environment has been obscured but is still recognizable in both the topography and the stratigraphy. Its identification here has been greatly augmented by recently completed geologic mapping in this part of the Puget Lowland, supported by an extensive database of subsurface explorations that permit unprecedented detail in characterizing the geology of an urban environment.

Based on stratigraphic and topographic evidence, three separate strands of the Seattle fault zone can be mapped in West Seattle. The northern-most strand, informally named the "North Alki deformation zone," is best expressed by the northern termination of an extensive uplifted beach terrace. This topographic expression aligns with a consistent stratigraphic offset of 20 m (north down, south up) of the bases of early last glacial deposits (<18 ka). Sheared glaciolacustrine sediments are recorded in multiple borings within this zone. The middle "Mee Kwa Mooks deformation zone" is about 1000 m wide. On the north, a 2.5-m vertical step in the uplifted beach terrace lies on trend with borings that show at least a 7-m offset in the base of a glaciolacustrine unit, extensively folded 25 ka sediments, and three possible faults suggested by offset contacts and shearing in nearby borehole logs. To the south, this zone displays a broad north-up, south-down sense of displacement and is marked by the termination of the uplifted beach terrace and stratigraphic offset of at least 7 m across multiple borings. The southernmost "Lowman Beach fault strand" displays the greatest stratigraphic offset (at least 25 m) in the base of the last-glacial deposits. That offset, which may include a component of differential erosion, is also readily visible in the pattern of outcropping older and younger units at map scale. Each of these strands also shows excellent spatial correlation with faults in the seismic-reflection data and aeromagnetic anomalies previously identified by other colleagues.