Uplift of Shorelines Caused by Holocene Anticlines Formed During Late Holocene Earthquakes in Puget Sound, Washington State

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems
Type: Oral
Date: 4/28/2020
Time: 03:00 PM
Room: 240
Description: 

Three reverse faults in northwestern Washington – the Seattle, Tacoma and Birch Bay faults – experienced late Holocene earthquakes. Warped intertidal platforms in the hanging wall of each fault form broad anticlines as a result of deformation during these three earthquakes. Estimates of past deformation rely on differencing raised shoreline features and corresponding modern features using lidar digital elevation models.

Previous coastal paleoseismology and fault scarp trenches document earthquakes on reverse faults in the Puget Sound region. A single M~7.5 earthquake on the Seattle fault ~1.1 ka before present (BP) lifted a broad platform cut on sedimentary rocks out of the intertidal zone. Newly revised LiDAR elevation profiles of the platform at three locations along the western end of the Seattle fault zone define an anticline 8-10 km wide (measured orthogonal to the fault) with a maximum uplift during the earthquake of ~5–8 m. Another large earthquake ~1.1 ka BP uplifted an intertidal platform along the western part of the Tacoma fault. The raised platform associated with the Tacoma fault earthquake formed an anticline ~10 km wide (orthogonal to the fault) with a maximum uplift of ~5 m. Another earthquake ~1.2 ka BP on a fault at Birch Bay fault near Bellingham, WA raised shorelines in the hanging wall above an anticline observed on seismic reflection profiles. Shoreline angles were not preserved in the northern limb of the anticline and impacted mapping the extent of the anticline. Estimated width of this anticline is ~8 km with a maximum uplift of ~2.5 m.

Using elastic half-space modeling for anticlines along the Seattle and Tacoma fault zones, I attempt to match modeled uplift with mapped anticline geometries. The best fit model for the Seattle fault required ~15 m of slip on a wedge-type blind master fault with a shallow hanging wall backthrust. The Tacoma fault required ~14 m of slip on a fault that steepened dip below a bend in the shallow subsurface. Modeling is not complete on the Birch Bay fault because of poor anticline preservation.

Presenting Author: Brian L. Sherrod

Authors