THE SOUTHERN WHIDBEY-ISLAND FAULT - AN ACTIVE STRUCTURE IN THE PUGET LOWLAND, WASHINGTON

Information from seismic-reflection profiles, outcrops, boreholes, and potential field surveys is used to interpret the structure and histor y of the southern Whidbey Island fault in the Puget Lowland of western Washington, This northwest-trending fault comprises a broad (as wide as 6-11 km), steep, northeast-dipping zone that includes several splay s with inferred strike-slip, reverse, and thrust displacement, Transpr essional deformation along the southern Whidbey Island fault is indica ted by along-strike variations in structural style and geometry, posit ive flower structure, local unconformities, out-of-plane displacements , and juxtaposition of correlative sedimentary units with different hi stories. The southern Whidbey Island fault represents a segment of a b oundary between two major crustal blocks, The Cascade block to the nor theast is floored by diverse assemblages of pre-Tertiary rocks; the Co ast Range block to the southwest is floored by lower Eocene marine bas altic rocks of the Crescent Formation, The fault probably originated d uring the early Eocene as a dextral strike-slip fault along the easter n side of a continental-margin rift, Bending of the fault and transpre ssional deformation began during the late middle Eocene and continues to the present, Oblique convergence and clockwise rotation along the c ontinental margin are the inferred driving forces for ongoing deformat ion. Evidence for Quaternary movement on the southern Whidbey Island f ault includes (1) offset and disrupted upper Quaternary strata imaged on seismic-reflection profiles; (2) borehole data that suggests as muc h as 420 m of structural relief on the Tertiary-Quaternary boundary in the fault zone; (3) several meters of displacement along exposed faul ts in upper Quaternary sediments; (4) late Quaternary folds with limb dips of as much as approximate to 9 degrees; (5) large-scale liquefact ion features in upper Quaternary sediments within the fault zone; and (6) minor historical seismicity, The southern Whidbey Island fault sho uld be considered capable of generating large earthquakes (M(s) greate r than or equal to 7) and represents a potential seismic hazard to res idents of the Puget Lowland.