Ups and downs of life in the coastal zone:
Shoreline deformation from holocene
earthquakes in Puget Sound, Washington

Brian Sherrod
University of Washington/USGS

Friday, May 15, 1998
12:00 noon—Pacific Forum

A spectacular bedrock point directly west of downtown Seattle has a striking wave-cut terrace. Evidence now suggests that this point was suddenly uplifted about seven meters during a large earthquake on the Seattle fault that occurred 1,100 years ago. A small marsh that sits astride the bedrock terrace provides me with a 7,000-year record of intertidal deposits. From this record, I have calibrated past salinity and elevation changes using fossil diatoms. The salinity and elevation reconstructions show an abrupt change from low-elevation, tideflat environments to higher elevation, freshwater marsh environments sometime after 1,500 years B.P. Several small-scale changes in elevation and salinity occurred earlier in the record, possibly the result of small earthquakes or storms.

In southern Puget Sound near Olympia, studies at several sites show that Douglas-fir forests on high-marsh soils abruptly subsided into the lower intertidal zone about 1,100 years ago, possibly from movement on a large fault nearby. Perhaps the most striking feature found in the stratigraphic record of several salt marshes is evidence that large areas of southern Puget Sound were occupied by freshwater lakes and marshes throughout most of the holocene. The first intertidal deposits appear in the stratigraphic record of present-day salt marshes in southern Puget Sound after 1,100 years B.P. This sudden change of environments is possibly related to abrupt subsidence during an earthquake, or prehistoric landslides that opened connections to Puget Sound from former freshwater environments.

Next: Marine animals and deep-sea video/data recording