| Puget Sound is subject to seismic risk from several sources. Upper crustal faults are described based on gravity and magnetic anomalies, as well as high-resolution seicmic reflection profiles. The South Whidbey Island Fault zone, the Seattle Fault zone, and the Tacoma Fault zone cut sedimentary units closer to the surface than previously interpreted, breaking the surface in some cases. Massive, late Holocene, submarine landslides are located in close proximity to the three fault zones. Turbidites off the toes of slides indicate four large shaking events in the past 1100 years. Areal distribution of slides suggests at least magnitude 6, probably magnitude 7+, earthquakes occur on the three surface fault zones. The areal distribution and timing of events also suggests that large subduction zone earthquakes do not initiate slides, but may reactivate them. Submarine landslide induced tsunamis or seiches may be a significant source of seismic risk in Puget Sound. |
