ABSTRACT Seattle’s geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc. Older Quaternary deposits are locally exposed. Most of the city is underlain by up to 100 m of glacial drift deposited during the Vashon stade of Fraser glaciation, 18–15 cal k.y. B.P. Vashon Drift includes lacustrine clay and silt of the Lawton Clay, lacustrine and fluvial sand of the Esperance Sand, and concrete-like Vashon till. Mappable till is absent over much of the area of the Vashon Drift. Peak local ice thickness was 900 m. Isostatic response to this brief ice loading was significant. Upon deglaciation, global ice-equivalent sea level was about −100 m and local RSL (relative sea level) was 15–20 m, suggesting a total isostatic depression of ~115–120 m at Seattle. Subsequent rapid rebound outstripped global sea-level rise to result in a newly recognized marine low-stand shoreline at −50 m. The Seattle fault is a north-verging thrust or reverse fault with ~7.5 km of throw. Conglomeratic Miocene strata may record initiation of shortening. Field relations indicate that fault geometry has evolved through three phases. At present, the north-verging master fault is blind, whereas several surface-rupturing faults above the master fault are south verging. The 900–930 A.D. Restoration Point earthquake raised a 5 km × 35 km (or larger) area as much as 7 m. The marine low-stand shoreline is offset by a similar amount, thus there has been only one such earthquake in the last ~11 k.y. Geomorphology is largely glacial: an outwash plain decorated with ice-molded flutes and large, anastomosing tunnel valleys carved by water flowing beneath the ice sheet. Euro-Americans initially settled here because of landscape features formed by uplift in the Restoration Point earthquake. But steep slopes and tide flats were not conducive to commerce: starting in the 1890s and ending in the 1920s, extensive regrading removed hills, decreased slopes, and filled low areas. In steep slopes the glacial stratigraphy is prone to landslides when saturated by unusually wet winters. Seismic hazards comprise moderately large (M 7) earthquakes in the Benioff zone 50 km and more beneath the city, demi-millennial M 9 events on the subduction zone to the west, and infrequent local crustal earthquakes (M 7) that are likely to be devastating because of their proximity. Seismic shaking and consequent liquefaction are of particular concern in Pioneer Square, SoDo, and lower Duwamish neighborhoods, which are largely built on unengineered fill that was placed over estuarine mud. Debris from past Mount Rainier lahars has reached the lower Duwamish valley and a future large lahar could pose a sedimentation hazard.

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 Landslides, particularly debris flows, have long been a significant cause of damage and destruction to people and property in the Puget Sound region. Following the years of 1996 and 1997, the Federal Emergency Management Agency designated Seattle as a “Project Impact” city with the goal of encouraging the city to become more disaster resistant to landslides and other natural hazards. A major recommendation of the Project Impact council was that the city and the U.S. Geological Survey collaborate to produce a landslide hazard map. An exceptional data set archived by the city containing more than 100 yr of landslide data from severe storm events allowed comparison of actual landslide locations with those predicted by slope-stability modeling. We used an infinite-slope analysis, which models slope segments as rigid friction blocks, to estimate the susceptibility of slopes to debris flows, which are water-laden slurries that can form from shallow failures of soil and weathered bedrock and can travel at high velocities down steep slopes. Data used for the analysis consisted of a digital slope map derived from recent light detection and ranging (LiDAR) imagery of Seattle, recent digital geologic mapping of the city, and shear-strength test data for the geologic units found in the surrounding area. The combination of these data layers within a geographic information system (GIS) platform allowed us to create a shallow landslide hazard map for Seattle.

Abstract Seattle lies within the Puget Sound Lowland, an elongate structural and topographic basin bordered by the Cascade and Olympic Mountains. The geology of the Seattle area is dominated by a complex, alternating, and incomplete sequence of glacial and interglacial deposits that rest upon an irregular bedrock surface. The depth to bedrock varies from zero to several kilometers below the ground surface. Bedrock outcrops in an east-west band across the lowland at the latitude of south Seattle and also around the perimeter of the lowland. Numerous faults and folds have deformed both the bedrock and overlying Quaternary sediments across the lowland, most notably the Seattle fault. During an earthquake on the Seattle fault ca. 980 A.D., 8 m of vertical offset occurred. The Seattle area has been glaciated at least seven times during the Quaternary Period by glaciers coalescing from British Columbia. In an area where each glacial and interglacial depositional sequence looks like its predecessor, accurate stratigraphic identification requires laboratory analyses and age determinations. The modern landscape is largely a result of repeated cycles of glacial scouring and deposition, and recent processes such as landsliding and river action. The north-south ridges of the lowland are the result of glacial scouring and subglacial stream erosion. The last glacier reached the central Puget Sound region ca. 15,000 years ago and retreated past this area by 13,650 14 C yr B.P. Post-glacial sediments are poorly consolidated, as much as 300 m thick in deep alluvial valleys, and susceptible to ground failure during earthquakes.