SHIPS - Seattle and Tacoma Model

SHIPS contributes to a new model for the Seattle and Tacoma fault zones

SHIPS seismic reflection data and seismic velocity models has helped us develop new structural models for the Seattle and Tacoma fault zones. These are complicated geological structures that are poorly exposed at the surface, making it necessary to image them below the surface.

New interpretations of SHIPS and other geological and geophysical information were developed by Brocher et al. (2004). The map view of the faults in the Puget Lowland show that the mapped strands of the Seattle fault lie to the south of the tip of the main branch of the Seattle fault. The position of the shallowest part of the Seattle fault is shown as a thick red line. Similarly, the main branch of the Tacoma fault lies to the south of the strands of the Tacoma fault that come to the surface.

faults in the Puget Lowland

Seismic reflection data show that the Seattle fault is moving northward and plowing up sedimentary rocks in the Seattle basin to its north. These data show that the faults that come to the surface in the vicinity of Alki Point and Harbor Island must lie to the south of the main strand of the Seattle fault.

Seattle fault moving northward

A new model for the Seattle fault shows that fault zone as a northward moving wedge having a south-dipping fault along the base of the wedge and a north-dipping fault along the top of the wedge.. This model explains the fact that the shallow faults exposed at the surface dip to the north. We know that the main strand of the Seattle fault dips to the south because at greater depth older volcanic rocks are placed on top of younger sedimentary rocks to the north.

cross section Seattle fault dips south and roof thrust dips north

The Tacoma fault may be behaving in a similar fashion. Together, the Tacoma and Seattle faults allow a great block of material to be squeezed upward, allowing the Seattle uplift to form between Seattle and Tacoma.


Seattle uplift squeezed up between Tacoma and Seattle faults