Seattle is the largest city in the Pacific Northwest region of the United States. It is a major West Coast seaport, international business and commercial center, and the focal city for 1.5 million persons. The city lies on the eastern shore of Puget Sound, an inland sea occupying a glacially-scoured tectonic basin between the Cascade Range to the east and the Olympic Mountains (coast range) to the west.
The landscape configuration is a consequence of multiple Pleistocene glaciations resulting in a series of north-trending elongated ridges, or drift uplands, separated by deep troughs, the latter now occupied by marine waters, fresh-water lakes, or underfit streams. Holocene modification of the glaciated landscape is manifest in the development of sea cliffs, formation of cuspate forelands, erosion of the uplands by small streams, and major landscape modifications by man.
Deposits of the most recent (Vashon) glaciation, which ended about 13.5 ka in the Seattle area, consists of a mappable drift sequence of lacustrine deposits, advance outwash, till, and recessional outwash that collectively dominate the surface and shallow subsurface geology. Older glacial and nonglacial materials, mostly in the subsurface, are less well-defined. A major, partly-buried ridge composed of Paleogene sedimentary and volcanic rocks trends west-northwest through the south-central part of the metropolitan area. On the north side of this promontory the bedrock surface drops steeply to more than 3,000 ft (900 m) below sea level directly beneath the city.
The city is built largely on Pleistocene glacial and nonglacial deposits. However, many of the industrial sections of the metropolitan area are built on Holocene alluvium or filled land over former tidal areas. As a consequence, geotechnical involvement with a variety of foundation conditions ranging from soft saturated ground to very dense till is common, together with a pervasive requirement for assessment of landslide and slope stability problems. Engineering and construction methods necessitated by the geotechnical properties of the Pleistocene and Holocene deposits have resulted in the development of unusual procedures for slope support, foundation treatment, soft ground excavation, and instrumentation.
The Seattle metropolitan area obtains most of its water supply from surface sources in the Cascade Range. Some suburban communities obtain supplies from ground water in alluvial or glacial deposits. The metropolitan area is mostly served by a combined sewer system that includes primary and secondary treatment and disposal of effluent into deep water in Puget Sound. Solid waste disposal has been in engineered regional landfills for some years, although former less-engineered landfills continue to provide local geotechnical problems. More recently, some solid waste is being shipped by rail to remote engineered landfills.
Seattle is situated in a moderately active earthquake region where the Juan de Fuca plate has subducted beneath the North American plate. The largest historic earthquakes (magnitudes 6.5 and 7.1) appear to be adjustments within the subducted plate. The upper plate exhibits a north-south maximum stress field yielding more frequent events of lower magnitude. The possibility of larger, subduction-style earthquakes emanating from the historically benign Cascadia subduction zone off the ocean coast is a matter of continuing study and discussion.