Geology of Seattle and the Seattle area, Washington

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

Stability of infinite slopes under transient partially saturated seepage conditions

“…Here, we consider the transient effects of infiltration on the stability of a hypothetical hillslope with a surficial cover of medium sand. A comparable real‐world setting might be the Puget Sound region where glacial deposits mechanically weather to form a sandy colluvium a few meters thick on steep coastal hillsides [ Galster and Laprade , 1991; Schulz , 2007; Troost and Booth , 2008].…”

Kinematics of shallow backthrusts in the Seattle fault zone, Washington State

“…The shallowest strata imaged by the seismic profiles are as much as 300 m of late Pleistocene to Holocene unconsolidated strata, which lie above a prominent unconformity at the top of as much as 700 m of hummocky Quaternary glacial deposits (Fig. 2;Easterbrook, 1986;Booth, 1994;Porter and Swanson, 1998;Johnson et al, 1999;Booth et al, 2003Booth et al, , 2005Troost and Booth, 2008). The base of the Quaternary deposits, and top of consolidated Tertiary strata, is interpreted at the downward change to more reflective and more-continuous strata (Johnson et al, 1999).…”

“…60 ka, the rate of horizontal contraction across the Seattle fault zone would be ~2 mm/yr, and the recurrence interval for large earthquakes would be ~5000 yr, both of which can be reconciled with paleoseismic and geodetic evidence. This age for the lower post-unconformity strata would require that the latest, Vashon glacial advance did not leave an obvious unconformity, and indeed at some locations, only a disconformity is observed between the pre-Vashon, Olympia-aged beds and the overlying strata (e.g., base of the Lawton Clay in Seattle; Booth et al, 2003Booth et al, , 2005Troost and Booth, 2008). Given this uncertainty in the age of the deformed strata, we cannot yet determine a slip rate based on the apparent uplifts above the Seattle fault strands.…”

Time-Dependent Capacity Increase of Piles Driven in the Puget Sound Lowlands

“…This study presents an analysis of a database of piles driven in the Puget Sound Lowlands, a region that has been subjected to repeated glaciation over the last 2.4 million years (Troost and Booth 2008). Based on previous research, Easterbrook (1994) suggested that glaciation occurred at least six times during the Pleistocene.…”