The southern Whidbey Island fault

The information below is derived from this published report:

The southern Whidbey Island fault, an active structure in the Puget Lowland, Washington

by Johnson, S.Y., Potter, C.J., Armentrout, J.M., Miller, J.J., Finn, C., and Weaver, C.S., 1996, Geological Society of America Bulletin, v. 108, p. 334-354 and oversize insert.

Abstract:

Information from seismic-reflection profiles, outcrops, boreholes, and potential field surveys is used to interpret the structure and history of the southern Whidbey Island fault in the Puget Lowland of western Washington. This northwest-trending fault comprises a broad (as wide as 6 to 11 km), steep, northeast-dipping zone that includes several splays with inferred strike-slip, reverse, and thrust displacement. Transpressional deformation along the southern Whidbey Island fault is indicated by along-strike variations in structural style and geometry, positive flower structure, local unconformities, out-of-plane displacements, and juxtaposition of correlative sedimentary units with different histories.

Schematic geologic map of the Whidbey Island area. A-A', B-B', and C-C' are lines of cross section shown in next figure. "SWIF" = southern Whidbey Island fault zone; DMF = Devils Mountain fault. Stereonet plots are lower-hemisphere projections showing locations and fault-plane solutions of earthquakes since 1970 along the trend of the southern Whidbey Island fault that had magnitudes > 3.5.

The southern Whidbey Island fault represents a segment of a boundary between two major crustal blocks. The Cascade block to the northeast is floored by diverse assemblages of pre-Tertiary rock; the Coast Range block to the southwest is floored by lower Eocene marine basaltic rocks of the Crescent Formation. The fault probably originated in the early Eocene as a dextral strike-slip fault along the eastern side of a continental-margin rift. Bending of the fault and transpressional deformation began in the late middle Eocene and continues to the present. Oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation.

Geologic cross sections across the Whidbey Island area. Lines of section shown in adjacent map. Many faults (heavy solid lines) are inferred to have undergone both vertical offset (indicated by arrows) and lateral offset. T = lateral offset toward viewer; A = lateral offset away from viewer. Boreholes projected onto cross sections from various distances. Map units are the same as on map except that darker green units are lower to middle Eocene sedimentary rocks, whereas lighter green units are upper Eocene to Oligocene sedimentary rocks.

Evidence for Quaternary movement on the southern Whidbey Island fault includes (1) offset and disrupted upper Quaternary strata imaged on seismic-reflection profiles; (2) borehole data that suggests as much as 420 m of structural relief on the Tertiary-Quaternary boundary in the fault zone; (3) several meters of displacement along exposed faults in upper Quaternary sediments; (4) late Quaternary folds with limb dips of as much as ~9°; (5) large-scale liquefaction features in upper Quaternary sediments within the fault zone; and (6) minor historical seismicity. The southern Whidbey Island fault should be considered capable of generating large earthquakes (Ms > 7) and represents a potential seismic hazard to residents of the Puget Lowland.