Active tectonics of the Seattle fault and central Puget Sound, Washington - Implications for earthquake hazards

We use an extensive network of marine high-resolution and conventional indu stry seismic-reflection data to constrain the. location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-ang le faults in the Puget Lowland of western Washington. Analysis of seismic p rofiles extending 50 km across the Puget Lowland from Lake Washington to Ho od Canal indicates that the west-trending Seattle fault comprises a broad ( 4-6 km) zone of three or more south-dipping reverse faults. Quaternary sedi ment has been folded and faulted along all faults in the zone but is clearl y most pronounced along fault A, the northernmost fault, which forms the bo undary between the Seattle uplift and Seattle basin; Analysis of growth str ata deposited across fault A indicate minimum Quaternary slip rates of abou t 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7-1.1 mm/yr. The Seattle fault is cut into two main segments by an active, north-trendin g, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. Faults in this zone truncate and warp reflections in Terti ary and Quaternary strata and locally coincide with bathymetric lineaments, Cumulative slip rates on these faults may exceed 0.2 mm/yr, Assuming no ot her crosscutting faults, this north-trending fault zone divides the Seattle fault into 30-40-km-long western and eastern segments. Although this geome try could limit the area ruptured in some Seattle fault earthquakes, a larg e event ca. A.D. 900 appears to have involved both segments. Regional seism ic-hazard assessments must (1) incorporate new information on fault length, geometry, and displacement rates on the Seattle fault, and (2) consider th e hazard presented by the previously unrecognized, north-trending fault zon e.