The Seattle Fault appears to be a rupture in a slab of rock about 8 to 10 km deep, with the southern portion being thrusted over, and forcing down, the northern portion.[13] Three strands have been identified, based on aeromagnetic and gravitational data, the northernmost strand lying nearly along Interstate 90 and then under Lake Sammamish.[14] 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 Seattle Fault is believed to date from the late Eocene (~40 million years ago), and is said to overlie "a major structural boundary between Eocene marine basaltic basement rocks (Crescent Formation) to the west and a diverse suit of pre-Tertiary [much older] basement rocks to the east."[15] (That is, oceanic sediments about 40 million years old to the west; older sedimentary and volcanic rock to the east, possibly accreted seamounts.) Recent (2009) work[16] indicates that this major structural boundary is actually at the eastern end of the fault, in the vicinity of Fall City, where it terminates at the NNW-SSE Rattlesnake Mountain / South Whidbey Island Fault Zone.
Lidar-based mapping has identified other faults associated with the Seattle fault,[17][18] but most of the details of the Seattle Fault, including recurrence rate, remain to be resolved.
Other recent work[12] indicates that the Seattle Fault can generate two types of earthquakes; both pose "considerable hazard" to the Seattle metropolitan region. The A.D. 900–930 earthquake is believed to be the only instance in the past 7,000 years of the type that causes a regional uplift. The other type is more localized and shallower (and therefore more damaging); at least four such events are believed to have occurred in the past 3,000 years on the west end of the fault. (The history of the central and eastern segments is not known.)
The Seattle fault is believed to be capable of generating an earthquake of at least 7.0[2] on the Richter scale. In addition to extensive damage to unreinforced structures and structures built on fill (such as much of Seattle's Pioneer Square area, the industrial area, and the waterfront,[19][20] and landslides, modeling has shown that such earthquakes could cause a tsunami of about 3 meters (10 feet) high on Elliott Bay.[21]
Seattle is at risk of earthquakes from various sources, including from the deep subduction zone responsible for the 1700 Cascadia earthquake[22] and from various other local faults such as the lesser known but equally threatening Southern Whidbey Island Fault.[23]