Pierce County is however vulnerable to local tsunamis created by submarine landslides, landslides from the bluffs along the waterways of Pierce County, and potentially by large crustal earthquakes on faults underlying portions of Puget Sound, such as the Seattle Fault or the hypothesized Tacoma Fault. At risk are the people and property on beaches, at low-lying areas of Pierce County, those near tidal flats, or those near the mouth of the Puyallup River.
There is concern about the stability of the major submarine deltas like those in the Commencement Bay and Elliott Bay, but the probability of a major submarine slide on either of the deltas is unknown. The Puyallup delta shows increased sedimentation and evidence of historical submarine landslides. Tides greatly impact the effects of tsunamis. A rapid fall of the tide from high to low water can trigger shoreline landslides that in turn generate tsunamis. If a tsunami were to occur off the mouth of teh Puyallup River during high tide, a tsunami could travel up the river for some distance.Loa