Land-level changes from a late Holocene earthquake in the northern Puget Lowland, Washington

Abstract

An earthquake, probably generated on the southern Whidbey Island fault zone, caused 1‐2 m of ground-surface uplift on central Whidbey Island ;2800‐3200 yr ago. The cause of the uplift is a fold that grew coseismically above a blind fault that was the earthquake source. Both the fault and the fold at the fault’s tip are imaged on multichannel seismic refection profiles in Puget Sound immediately east of the central Whidbey Island site. Uplift is documented through contrasting histories of relative sea level at two coastal marshes on either side of the fault. Late Holocene shallow-crustal earthquakes of Mw 5 6.5‐7 pose substantial seismic hazard to the northern Puget Lowland.