Segmentation of the Tacoma fault: evidence from seismic and magnetic data

We integrate seismic and magnetic data to characterize the Tacoma fault and northern portions of the Tacoma Basin in western Washington State. The Tacoma basin, a deep (greater than 4 km) basin filled with Eocene and younger sediments, is a major physiographic feature underlying southern Puget Sound. Its northern margin is bounded by the Tacoma fault, an east-striking, north-dipping backthrust that forms the southern margin of the Seattle uplift and accommodates north-south shortening of the Puget Lowland. Whereas the east-striking Seattle fault extends >70 km with a consistent style of deformation, we find evidence that the Tacoma fault is segmented by north-northeast-trending faults. A prominent east-striking magnetic lineament coincides with lidar-identified fault scarps and defines the western part of the active Tacoma fault. This lineament extends 15 km from Hood Canal east to Key Peninsula and is characterized in seismic profiles as a narrow, north-side-up kink-band. Land and marine seismic profiles, and a change in trend of magnetic lineations near Key Peninsula suggest that the Tacoma fault either terminates at this location or significantly changes character. Shortening east of Key Peninsula is expressed, in part, by the Rosedale monocline, a southeast-trending structure that shows evidence for active uplift. The Rosedale monocline extends ~20 km to the eastern margin of Puget Sound, where seismic profiles show a second north-northeast-trending set of faults. Land seismic profiles and magnetic data east of Puget Sound are consistent with the Tacoma Basin shallowing near the eastern boundary of Puget Sound. The apparent synchronous timing of large earthquakes approximately 900 AD on both the Tacoma and Seattle faults suggest these fault systems are kinematically linked, yet the surface and subsurface expression of the two fault systems are distinct. Based on the fault segment lengths, our findings suggest the Tacoma fault can support earthquakes < M 6.5 and that no backthrust on the Seattle fault system is evident in the upper one km on our data east of Puget Sound. Thus, if fault ruptures are restricted to individual segments, hazards along the Tacoma fault may be less than previously modeled.