EARTHQUAKE HISTORY OF REVERSE FAULTS AND FOLDS IN TRENCHES ACROSS ALSM-IMAGED SCARPS IN THE SEATTLE FAULT ZONE, PUGET LOWLAND, WASHINGTON STATE

The Seattle fault zone—a 70-km-long, east-west belt of young reverse faults—is one of several fault zones in the Puget Lowland that accommodates 4-7 mm/yr of north-south shortening resulting from northward migration of forearc blocks along the Cascadia convergent margin. Like crustal faults in similar densely populated basins, the faults may pose a greater earthquake hazard than do much longer but more distant plate-boundary faults. Airborne Laser Swath Mapping (ALSM, also known as LIDAR mapping) of the Puget Lowland reveals previously unknown scarps beneath the forest canopy along some young reverse faults. From complex exposures of reverse faults and folds in trenches across the scarps we reconstruct the earthquake history of the Seattle fault zone for regional hazard assessment.

Trenches display stratigraphic and 14C evidence of early Holocene as well as late Holocene earthquakes on the Seattle fault, but trench logs differ considerably in lithology and genesis of stratigraphic units, styles of faulting and folding, and numbers of surface-deformation events identified. Five trenches across 1-to-6-m-high scarps on a 2.6-km-long, north-dipping backthrust to the Seattle fault about 12 km west of Seattle on Bainbridge Island reveal folds and faults, liquefaction features, and forest A horizons buried by hanging-wall-collapse colluvium that record three, or possibly four earthquakes, between 2.5 ka and 1.0 ka. The most recent 1.0 ka earthquake is probably the same earthquake that raised marine terraces and triggered a tsunami in Puget Sound. Two of three trenches on a similar 1-to-5-m-high, south-facing, en echelon scarp 3 km to the southwest at Waterman Point display hanging walls of Oligocene sandstone and mudstone thrust over forest soils developed in drift during a large earthquake about 1.0 ka. In the third trench, a second, post-1.0-ka earthquake is recorded by a forest soil developed on slope and hanging-wall-collapse colluvium from the 1.0-ka earthquake that is also overthrust by the hanging wall. At Vasa Park on the west shore of Lake Sammamish about 16 km east of Seattle, excavations at two sites across a low scarp show evidence for an earthquake likely between 10.5 and 4.5 ka. Surface folding and faulting of about 1-2 m per earthquake, estimated from stratigraphic and surface offsets, suggest magnitudes near M7 for Holocene earthquakes.