SHIPS - Tacoma Fault

SHIPS helped delineate and characterize the Tacoma fault

Geophysical anomalies show that the Tacoma fault zone is associated with abrupt and large changes in seismic velocity, density, and magnetic properties. These abrupt changes correspond to large differences in the depth to the top of the volcanic Crescent Terrane to the north and south of the Tacoma fault. These anomalies increase westward along the fault zone, suggesting that the Tacoma fault becomes a larger and larger fault to the west.

Geophysical anomalies

Recent advances in our ability to map fault scarps in the Puget Lowland using high resolution laser-based topographic mapping allowed us to identify a fault scarp located along the Tacoma fault zone. This figure makes the point that although traditional air photos do not see the fault (see top figure), the new topographic (lidar) maps allow us to image and map the fault scarp. The scarp lies right along the large geophysical anomalies, including the seismic velocity model derived from SHIPS work. Once the scarp was located on the ground, a trench was excavated across it (bottom on figure), providing evidence for late Holocene ground deformation (as recently as about 900 AD).

Air photo and Lidar image

Structural engineers were able to incorporate this new information on the Tacoma fault zone into their seismic designs for a new span of the Tacoma Narrows bridge.

Tacoma bridge