Holocene Relative Sea Level Changes along the Seattle Fault at Restoration Point, Washington

Abstract

At a marsh on the hanging wall of the Seattle fault, fossil brackish water diatom and plant seed assemblages show that the marsh lay near sea level between ∼7500 and 1000 cal yr B.P. This marsh is uniquely situated for recording environmental changes associated with past earthquakes on the Seattle fault. Since 7500 cal yr B.P., changes in fossil diatoms and seeds record several rapid environmental changes. In the earliest of these, brackish conditions changed to freshwater ∼6900 cal yr B.P., possibly because of coseismic uplift or beach berm accretion. If coseismic uplift produced the freshening ∼6900 cal yr B.P., that uplift probably did not exceed 2 m. During another event about 1700 cal yr B.P., brackish plant and diatom assemblages changed rapidly to a tidal flat assemblage because of either tectonic subsidence or berm erosion. The site then remained a tideflat until the most recent event, when an abrupt shift from tideflat diatoms to freshwater taxa resulted from ∼7 m of uplift during an earthquake on the Seattle fault ∼1000 cal yr B.P. Regardless of the earlier events, no Seattle fault earthquake similar to the one ∼1000 cal yr B.P. occurred at any other time in the past 7500 years.

Modeling the spatial distribution of landslide‐prone colluvium and shallow groundwater on hillslopes of Seattle, WA

“…Coastal bluff formation in the area has been driven by sea-level rise (Downing, 1983;Booth, 1987;Terich, 1987;Shipman, 2004;Schulz, 2004Schulz, , 2005Schulz, , 2007. Sea level in the Seattle area rose rapidly following retreat of glacial ice; about 6-10 m of rise has occurred during the past 5000 years (Booth, 1987, Figure 7;Sherrod et al, 2000). Although sea level rise continues, this rise no longer results in erosion of most Seattle bluff toes due to construction of shoreline protection, but the effects of bluff-toe erosion are still apparent in the distributions of colluvium and landslides, including very recent landslides (Schulz, 2005(Schulz, , 2007.…”

Salt marshes as late Holocene tide gauges

“…microfossil species abundances against elevation), improved computing power and more widely available software (Birks, 1995;. These developments have led to numerous publications from the late 1990s of both diatom-and foraminifera-based transfer function models that use the relationship between the modern distribution of microfossils (Figure 2) to provide quantitative estimates, with error terms, of RSL based on fossil assemblages in coastal sediments ( Figure 1F and 1G) (Barlow et al, 2012;Edwards and Horton, 2000;Edwards et al, 2004a;Gehrels, 1999;Gehrels, 2000;Gehrels et al, 2012;Gehrels et al, 2006;Horton et al, 1999a;Horton et al, 2000;Kemp et al, 2009a;Long et al, 2003;Long et al, 2012;Long et al, 2010;Sawai et al, 2004a;Sawai et al, 2004b;Sherrod et al, 2000;Woodroffe and Long, 2009;Woodroffe and Long, 2010;Zong and Horton, 1999). These records have sometimes been termed 'high resolution', though high resolution is a relative term (Edwards, 2007), and rarely defined.…”

Measurements and Modeling of Gravel Transport under Wind Waves, Vessel-Generated Waves, and Tidal Currents

“…The most dramatic of these is a 5-to 7-m vertical displacement of the marine platform in central Puget Sound that has been dated to around 900 to 930 AD (Bucknam et al 1992, Atwater 1999. Other displacements on this fault and on Whidbey Island within the past 3,500 years were between 1 m and 2 m (Sherrod et al 2000, Kelsey et al 2004. There are ongoing investigations into vertical displacements in other locations.…”

“…In contrast, relative sea level in south Puget Sound has risen more or less monotonically from about 40 m below present levels (Thorson 1980). By about 5,000 cal yr BP, sea level was within 2 m to 3 m of present and has been rising at a rate of 2 mm yr -1 to 3 mm yr -1 since (Sherrod et al 2000, Kelsey et al 2004. The modern marine platform is no more than 5,000 years old throughout the Sound, representing an entirely transgressive sea-level history in the south, while in the north the current platform was previously (if briefly) under wave action 10, 000 years ago.…”