Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound

A new three-dimensional (3-D) model shows seismic velocities beneath the Pu get Lowland to a depth of 11 km. The model is based on a tomographic invers ion of nearly one million first-arrival travel times recorded during the 19 98 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher-re solution mapping of subsurface structures than previously possible. The mod el allows us to refine the subsurface geometry of previously proposed fault s (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures (Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of th e Tacoma basin; we informally refer to this structure here as the Tacoma fa ult zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault z one is suggested by seismicity along it and paleoseismic evidence for abrup t uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low-velocity basins (Tacoma, Seattle, Eve rett, and Port Townsend) separated by regions of higher velocity ridges tha t are coincident with fault-bounded uplifts of Eocene Crescent Formation ba salt and pre-Tertiary basement. The shapes of the basins and uplifts are si milar to those observed in gravity data; gravity anomalies calculated from the 3-D tomography model are in close agreement with the observed anomalies . In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high- velocity uplift, locally with a velocity "overhang" that suggests a basin v ergent thrust fault boundary. Crustal fault zones grow from minor folds int o much larger structures along strike. Inferred structural relief across th e Tacoma fault zone increases by several kilometers westward along the faul t zone to Lynch Cove, where we interpret it as a zone of south vergent faul ting overthrusting Tacoma basin. In contrast, structural relief along the S eattle fault zone decreases west of Seattle, which we interpret as evidence that the N-S directed compression is being accommodated by slip transfer b etween the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east-west trendin g, pop-up structures underlying Puget Lowland from the Black Hills to the S an Juan Islands.