The Seattle Fault is the structural boundary where 50–60 million 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 of relatively softer, lighter sedimentary strata of the younger Blakeley and Blakely Harbor formations. This has resulted in a 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 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.