Seattle Fault - Geology

Geology

The Seattle Fault is the structural boundary where 50–60 millions of years old (early Tertiary) basalt of the Crescent Formation on the south has been uplifted – the Seattle Uplift – and is tipping into the Seattle Basin, where the Tertiary bedrock is buried under at least 7 km (4.3 miles) of relatively softer, lighter sedimentary strata of the younger Blakeley and Blakely Harbor formations. This has resulted in a 4 to 7 km (2.5 to 4.3 miles) wide zone of complex faulting, with three or more main south-dipping thrust faults. Most of the faulting is "blind" (not reaching the surface), and generally difficult to locate because of the generally heavy vegetation or development. Three principal strands have been identified, their location determined by high-resolution seismic reflection and aeromagnetic surveys. The northernmost strand lies nearly along Interstate 90 and then under Lake Sammamish. The central section of the fault zone – where it crosses the apparent location of the Olympic-Wallowa Lineament – shows marked variation in the location of the strands and of the underlying structure, but the nature and significance of this is not understood.

The fault extends for approximately 70 km (43 miles) from near Fall City on the east, where it appears to be terminated by the South Whidbey Island Fault, to Hood Canal on the west (not shown on the map). where matters are as yet unclear (see discussion at Puget Sound faults#Question of western termination). It is the northern edge of the Seattle Uplift, of which the Tacoma Fault is the southern edge. One model has the Seattle and Tacoma faults converging at depth to form a wedge, which is being popped up by approximately north–south oriented compression that ultimately derives from plate tectonics. Another model (see diagram) interprets the Seattle Uplift as a sheet of rock that is being forced up a ramp. Subsequent work suggests that the structure of the Seattle Fault may vary from east to west, with both models being applicable in different sections. A later model has part of the north-thrusting sheet forming a wedge between the sedimentary formations of the Seattle Basin and the underlying bedrock.

The Seattle Fault is believed to date from about 40 million years ago (late Eocene). This is about the time that the strike-slip movement on the north-striking Straight Creek Fault to the east ceased, due to the intrusions of plutons. It appears that when the Straight Creek Fault became stuck the north–south compressive force that it had accommodated by strike-slip motion was transferred to the crust of the Puget Lowland, which subsequently folded and faulted, and the various blocks jammed over one another.

Other scarps associated with the Seattle fault have been identified by LIDAR-based mapping; trenching has generally shown the faulting to be more complex than was first realized. Many of the details of the Seattle Fault, including recurrence rate, remain to be resolved. A study of sediments in Lake Washington found evidence of seven large (M > 7) earthquakes in the last 3500 years.

Surface scarps due to faulting are rarely observed in this area (due to topography, vegetation, and urbanization); a rare exception can be seen at Mee Kwa Mooks Park south of Alki Point. The is the site of the West Seattle Fault; the prominent rise there is due to uplift on the north side of the fault.