A LATE HOLOCENE EARTHQUAKE ON CENTRAL WHIDBEY ISLAND: SURFACE FOLDING ABOVE A BLIND FAULT

Approximately 3,000 years ago, a crustal earthquake in the northern Puget lowland, probably along the northwest-trending northeastern fault strand of the Southern Whidbey Island fault zone, displaced the north side of the fault up 1 to 2 m relative to the south side. Evidence for the earthquake comes from different relative sea level histories and paleo ecologic records of two coastal marshes located within 1.5 km of the inferred fault trace. The relative sea level histories are similar from 5,000 until about 3,000 years ago, when the sea level curves diverged for a 500-1,000 year duration because of relative sea level fall at the northern (Crockett Marsh) site relative to the southern (Hancock Marsh) site. LIDAR imagery of central Whidbey Island does not reveal a fault scarp between the two sites but rather an irregular northwest-trending break in topography separating an incised upland to the northeast and a relatively flat lowland to the southwest.

We infer that the structure responsible for the relative sea level change is an asymmetric fold. This fold is imaged in marine seismic-reflection lines in Holmes Harbor, four km southeast of Hancock Marsh along the strike of the southern Whidbey Island fault. Holocene strata are asymmetrically folded up to the northeast, and underlying Pleistocene and Tertiary strata are increasingly more folded and faulted. Interpretation of the seismic data supports the northeast-side-up sense of motion inferred from relative sea level and LIDAR data, and suggests that the Holocene fold occurs above the tip of a blind fault. If the fault dips at about 60 degrees, then observed surface deformation 3,000 years ago was the result of 2 to 4 m of slip from a Mw ~6.5-7.0 earthquake.