Abstract The city of Seattle, Washington State, lies within the Puget Sound Lowland, an elongate structural and topographic basin between the Cascade Range and Olympic Mountains. The area has been impacted by repeated glaciation in the past 2.4 m.y. and crustal deformation related to the Cascadia subduction zone. The present landscape largely results from those repeated cycles of glacial scouring and deposition and tectonic activity, subsequently modified by landsliding, stream erosion and deposition, and human activity. The last glacier to override the area, the Vashon-age glacier of the Fraser glaciation, reached the Seattle area ca. 14,500 14 C yr B.P. (17,400 cal yr B.P.) and had retreated from the area by ca. 13,650 14 C yr B.P. (16,400 cal yr B.P.). The Seattle area sits atop a complex and incomplete succession of glacial and nonglacial deposits that extends below sea level and overlies an irregular bedrock surface. These subsurface materials show spatial lithologic variability, are truncated by many unconformities, and are deformed by gentle folds and faults. Sediments that predate the last glacial–interglacial cycle are exposed where erosion has sliced into the upland, notably along the shorelines of Puget Sound and Lake Washington, along the Duwamish River valley, and along Holocene streams. The city of Seattle straddles the Seattle uplift, the Seattle fault zone, and the Seattle basin, three major bedrock structures that reflect north-south crustal shortening in the Puget Lowland. Tertiary bedrock is exposed in isolated locations in south Seattle on the Seattle uplift, and then it drops to 550 m below ground under the north half of the city in the Seattle basin. The 6-km-wide Seattle fault zone runs west to east across the south part of the city. A young strand of the Seattle fault last moved ~1100 yr ago. Seattle has also been shaken by subduction-zone earthquakes on the Cascadia subduction zone and deep earthquakes within the subducting plate. Certain postglacial deposits in Seattle are prone to liquefaction from earthquakes of sufficient size and duration. The landforms and near-surface deposits that cover much of the Seattle area record a brief period in the geologic history of the region. Upland till plains in many areas are cut by recessional meltwater channels and modern river channels. Till plains display north-south drumlins with long axes oriented in the ice-flow direction. Glacially overridden deposits underlie the drumlins and most of the uplands, whereas loosely consolidated postglacial deposits fill deep valleys and recessional meltwater channels. Ice-contact deposits are found in isolated locations across the uplands and along the margins of the uplands, and outwash deposits line upland recessional channels. Soft organic-rich deposits fill former lakes and bogs. A preliminary geologic map of Seattle was published in 1962 that is only now being replaced by a detailed geologic map. The new map utilizes a data set of 35,000 geotechnical boreholes, geomorphic analyses of light detection and ranging (LIDAR), new field mapping, excavation observations, geochronology, and integration with other geologic and geophysical information. Findings of the new mapping and recent research include recognition of Possession- and Whidbey-age deposits in Seattle, recognition that ~50% of the large drumlins are cored with pre-Vashon deposits and 50% with Vashon deposits, and that numerous unconformities are present in the subsurface. Paleotopographic surfaces display 500 m (1600 feet) of relief. The surficial deposits of Seattle can be grouped into the following categories to exemplify the distribution of geologic materials across the city: postglacial deposits 16%, late glacial deposits 12%, Vashon glacial deposits 60%, pre-Vashon deposits 9%, and bedrock 3%. of these, 49% are considered fine-grained deposits, 19% are considered intermediate or interbedded deposits, and 32% are considered coarse-grained deposits. These percentages include only the primary geologic units and not the overlying fill and colluvial deposits.

Abstract Seattle, Washington, has one of the most progressive landslide ordinances and mitigation programs in the United States. The necessity for this is driven by concentrated winter precipitation, steep slopes, and glacial soils that are susceptible to instability. Early in the European development, engineers recognized the vulnerability of Seattle hillsides to landslides, particularly when the hillsides were disturbed. In this paper, we trace the history of mapping of landslide-prone ground in Seattle, started by Miller ( 1973 ), detailed by Tubbs ( 1974, 1975 ), and most recently catalogued by Shannon & Wilson ( 2000, 2003 ). Owing to the relative homogeneity of geology (glacial and other Pleistocene nonglacial soils), landsliding consists mainly of debris landslides, debris avalanches, and slumps. Debris avalanches that engage the shallow colluvial soils are by far the most common. Deep-seated slumps are not as common but can encompass a large area and affect many property owners. Several studies agree that ~80% of the landslides include one or more human influences. To limit the amount of damage that landslides cause to private properties and City of Seattle infrastructure, rules were promulgated by the city in 1984. These rules have been revised three times, and they now serve as an example for the rest of the state of Washington. Following destructive, widespread slope instability in 1997 and the Shannon & Wilson studies in 2000 and 2003 , Seattle Public Utilities instituted a state-of-the-industry landslide mitigation program that has already paid dividends for the city.

Abstract In Seattle, Washington, deep-seated landslides on bluffs along Puget Sound have historically caused extensive damage to land and structures. These large failures are controlled by three-dimensional (3-D) variations in strength and pore-water pressures. We assess the slope stability of part of southwestern Seattle using a 3-D limit-equilibrium analysis coupled with a 3-D groundwater flow model. Our analyses use a high-resolution digital elevation model (DEM) combined with assignment of strength and hydraulic properties based on geologic units. The hydrogeology of the Seattle area consists of a layer of permeable glacial outwash sand that overlies less permeable glacial lacustrine silty clay. Using a 3-D groundwater model, MODFLOW-2000, we simulate a water table above the less permeable units and calibrate the model to observed conditions. The simulated pore-pressure distribution is then used in a 3-D slope-stability analysis, SCOOPS, to quantify the stability of the coastal bluffs. For wet winter conditions, our analyses predict that the least stable areas are steep hillslopes above Puget Sound, where pore pressures are elevated in the outwash sand. Groundwater flow converges in coastal reentrants, resulting in elevated pore pressures and destabilization of slopes. Regions predicted to be least stable include the areas in or adjacent to three mapped historically active deep-seated landslides. The results of our 3-D analyses differ significantly from a slope map or results from one-dimensional (1-D) analyses.

Abstract In September 1998, a geotechnical investigation was initiated by the Washington State Department of Transportation (WSDOT) to assess stability of a slope in the vicinity of Ross Point, located along the State Route 166 (SR 166) corridor. The roadway lies at the base of a marine bluff composed of Pleistocene glacial and interglacial deposits, and it has a history of roadway closures associated with landslides and embankment failures. As part of the investigation, geotechnical test borings were drilled to define subsurface conditions, and inclinometer casing and open-standpipe piezometers were installed to monitor slope movement and groundwater levels. All drilling was completed and inclinometer and piezometer installations initialized by 30 December 1998. The Ross Point landslide occurred on 29 January 1999. This landslide is unique in that the subsurface conditions had been thoroughly characterized in a geotechnical investigation prior to failure, and rainfall and groundwater levels were being frequently monitored at the time of the failure. Although slope failures are common in the landslide sequence of Esperance Sand overlying Lawton Clay, monitoring of rainfall and groundwater levels prior to and during landslide failure is unique. The groundwater levels monitored within the Ross Point landslide mass showed a rapid response to intense rainfall events, whereas the groundwater response measured in boreholes located outside of the failure was subdued. Excessive pore-water pressures rapidly developed within the landslide mass following significant rainfall events. This rapid increase in pore-water pressure, coupled with high antecedent groundwater levels, is considered to be the critical factor in triggering the Ross Point landslide. Wood sampled from the lowest nonglacial sediments exposed at Ross Point yielded a 14 C date of 40,570 ± 700 yr B.P. and are considered to be the Olympia beds.