Puget Sound faults

The Puget Sound faults under the heavily populated Puget Sound region (Puget Lowland) of Washington state form a regional complex of interrelated seismogenic (earthquake-causing) geologic faults. These include (from north to south, see map) the:

The Puget Sound region (Puget Lowland) of western Washington contains the bulk of the population and economic assets of the state, and carries seven percent of the international trade of the United States. All this is at risk of earthquakes from three sources:

While the great subduction events release much energy (around magnitude 9), that energy is spread over a large area, and largely centered near the coast. The energy of the somewhat smaller Benioff earthquakes is likewise diluted over a relatively large area. The largest intra-crustal earthquakes have about the same total energy (which is about one-hundredth of a subduction event), but since they re closer to the surface they will cause more powerful shaking, and, therefore, more damage.

One study of seismic vulnerability of bridges in the Seattle – Tacoma area estimated that an M 7 earthquake on the Seattle or Tacoma faults would cause nearly as much damage as a M 9 subduction earthquake. Because the Seattle and Tacoma faults run directly under the biggest concentration of population and development in the region, more damage would be expected, but all the faults reviewed here may be capable of causing severe damage locally, and disrupting the regional transportation infrastructure, including highways, railways, and pipelines. (Links with more information on various hazards can be found at Seattle Fault.)

The Puget Sound region is not just potentially seismic, it is actively seismic. Mapping from the Pacific Northwest Seismic Network shows that the bulk of the earthquakes in western Washington are concentrated in four places: in two narrow zones under Mt. Saint Helens and Mt. Rainier, along the DDMFZ, and under Puget Sound between Olympia and approximately the Southern Whidbey Island Fault. The southern limit nearly matches the southern limit of the glaciation; possibly the seismicity reflects rebound of the upper crust after being stressed by the weight of the glacial ice.

Thick glacial and other deposits, heavy vegetation, urban development, and a topography of sharp relief and rapid erosion obscures the surface expression of faults in this region, and has hindered their discovery. The first definite indications of most of these faults came from gravitational mapping in 1965, and their likely existence noted on mapping in 1980 and 1985. As of 1985 only the Saddle Mountain Faults had been shown to have Holocene activity (since the last ice age, about 12,000 years ago). Not until 1992 was the first of the lowland faults, the Seattle Fault, confirmed to be an actual fault with Holocene activity, and the barest minimum of its history established.

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