Abrupt Uplift Within the Past 1700 Years at Southern Puget Sound, Washington

Abstract

Shorelines rose as much as 7 meters along southern Puget Sound and Hood Canal between 500 and 1700 years ago. Evidence for this uplift consists of elevated wave-cut shore platforms near Seattle and emerged, peat-covered tidal flats as much as 60 kilometers to the southwest. The uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform. The tidal flats also emerged abruptly; they changed into freshwater swamps and meadows without first becoming tidal marshes. Where uplift was greatest, it adjoined an inferred fault that crosses Puget Sound at Seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago. The uplift and probable fault slip show that the crust of the North America plate contains potential sources of damaging earthquakes in the Puget Sound region.

High-Resolution Seismic Reflection Imaging of Growth Folding and Shallow Faults beneath the Southern Puget Lowland, Washington State

“…Although an M 7 earthquake was documented to have ruptured the Seattle fault in the central Puget Lowland about 1100 years ago (Bucknam et al, 1992;Nelson et al, 2003), the Tacoma fault and Olympia structure have received less attention. On the north edge of the Tacoma basin, geophysical data show a west-trending series of faults and folds interpreted to be a broad Tacoma fault zone (Pratt et al, 1997;Brocher et al, 2001;Johnson et al, 2004).…”

“…Alternatively, the Olympia structure could be a predominantly pre-Tertiary fold that is cut by a fault south of the existing industry seismic profiles. Evidence for Holocene land-level changes along the Tacoma fault and Olympia structure suggest that there was a large earthquake about 1100 years ago in the southern Puget Lowland (Bucknam et al, 1992;Sherrod, 2001). Sherrod et al (2004) document an uplifted tidal flat (terrace) at Case Inlet and a series of east-trending, south-facing, en echelon topographic lineaments deforming the late Pleistocene to Holocene glacial deposits along the west part of the Tacoma fault zone (Fig.…”

The Catfish Lake Scarp, Allyn, Washington: Preliminary field data and implications for earthquake hazards posed by the Tacoma fault

“…To the north at Lynch Cove, tidal flats rose as much as 3 m between A.D. 870-990 (Bucknam and Biasi, 1994). Uplift 16 km to the east at Burley allowed woody shrubs to invade former tidal flats in A.D. 770-1000 (Bucknam et al, 1992). Shells in tidal flat mud at North Bay record uplift loosely dated to the past 3000 years.…”

“…These faults include the Seattle, Tacoma, Olympia, and southern Whidbey Island, and bound structural basins beneath the cities of Tacoma, Seattle, and Everett (Brocher et al, 2001). Several faults are suspected of Quaternary offset, and the Seattle fault has Holocene rupture (Bucknam et al, 1992;Nelson et al, 2002).…”

“…Uplifted terraces along the south side of the Seattle fault and a submerged intertidal marsh constrain a large earthquake on the Seattle fault zone to about 900-930 A.D. (Atwater and Moore, 1992;Bucknam et al, 1992). Surface rupture on a series of small en-echelon faults located within the Seattle fault zone suggest as many as three earthquakes in the last 2500 years (Nelson et al, 2002).…”