Faults That Cross Whidbey Island...
As you look west from Whidbey Island, the Olympic Mountains dominate the horizon. These giants are the eroded remnants of ancient seafloor sediments and lavas that were forced eastward and then buckled by the movement of two plates in the Earth’s crust. The plates, colliding along a zone about 50 miles off the coast of Washington and British Columbia, are the source of occasional large earthquakes affecting the region.
A myriad of shallower faults cross Whidbey Island but they are mostly buried by glacial sediment. In northern Whidbey, faults cross E-W from Dugualla Bay to the northern perimeter of Naval Air Station-Whidbey. Another set of faults trend SE to NW, from Oak Harbor to the southern edge of the naval base. Several of these faults cross the NAS-Whidbey golf course and are visible where they surface along the beach at Rocky Point. There is evidence for movement along these faults during the last 3000 years.
There is a lot of chatter about the dreaded, northwest-trending “South Whidbey Fault,” which is actually a fault zone buried under glacial sediment. There is no surface evidence on the island for the fault zone but it was identified by marine geophysical surveys of Holmes Harbor, Possession Sound, and Admiralty Inlet. What was seen by these surveys were older rocks that had been pushed upward against younger rocks, revealing as much as 1300 feet of vertical offset. The South Whidbey fault zone on land is mostly “blind,” in that it can best be seen by the geophysical surveys.
Vertical displacement along the South Whidbey fault zone is evident in the history of the sediments of Crockett Lake and Hancock Lake (along the western shore of Whidbey Island). The sediments are similar in composition and age but those in Crockett Lake, north of Hancock Lake, are now 3 to 6 feet higher, likely caused by vertical fault movement over the last 3000 years.