Since the last ice age, the southern Whidbey Island fault zone has probably spawned several highly destructive shallow earthquakes. For several years, scientists pondered where this important regional fault zone continues southwestward from its mapped location in the Everett area. Recent geologic mapping by Washington State Department of Natural Resources (DNR) geologists reveals that this major fault zone extends through the Snoqualmie River valley in the vicinity of Carnation, Fall City, and North Bend.
A team headed by Joe Dragovich of DNR’s Division of Geology and Earth Resources, assisted by geologists from King County, Pacific Northwest Seismic Network, Colorado College, and Washington State University, has been mapping in this area for the past three years. The team determined that the Rattlesnake Mountain fault zone, originally mapped by DNR geologist Tim Walsh in the 1980s, is likely the southern continuation of the southern Whidbey Island fault, extending this fault zone from the Strait of Juan de Fuca to Rattlesnake Mountain near North Bend. In the area of North Bend, the fault zone is 4 miles wide and consists of a series of parallel faults. These fault strands follow the valley edges and control the location of the Snoqualmie River along some portions of the valley.
Over time, fault movement has created some interesting geologic features. For example, the DNR team found evidence that the rocks creating Snoqualmie Falls are much younger than previously thought. The spectacular falls flow over the remains of a newly discovered 20-million-year-old volcano, apparently formed atop the main part of the fault zone as magma rose upward along weak fault planes. Later movement on this long-term active fault zone cut the volcano. The team also found that accumulated movement on the fault has uplifted old Snoqualmie River sediments to their current position on ridges, in some places several hundreds of feet above the modern Snoqualmie valley floor. These sediments were laid down 20,000 to 60,000 years ago, before the last ice age.
Mapping along the Snoqualmie River valley also helped answer the important question of where the Seattle fault lies east of its last mapped position near Issaquah. The DNR team found that the Seattle fault is intercepted by the southern Whidbey Island fault zone in the vicinity of Fall City. Additional fault studies by state and federal geologists in the next few years will help determine the frequency and severity of earthquakes along these fault zones.
Geologic maps of 7.5-minute topographic quadrangles covering parts of the Seattle fault and southern Whidbey Island fault zone are available on the DNR website as:
Geologic Map (GM) -67 (Fall City quadrangle)
GM-73 (North Bend quadrangle)
GM-75 (Snoqualmie quadrangle)
Printed copies may be purchased through the Washington State Department of Printing.