The Seattle Fault cuts across Puget Sound and into Seattle itself. Restoration Point in the foreground, Alki Point is barely seen at the right edge of the picture.The Seattle Fault cuts across Puget Sound and into Seattle itself. Restoration Point in the foreground, Alki Point is barely seen at the right edge of the picture.
The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle (in the US State of Washington) in the vicinity of Interstate Highway 90.
First suspected from mapping of gravitational anomalies in 1965 and an uplifted marine terrace at Restoration Point (foreground in picture), its existence was definitely established by a set of five reports published in Science in 1992. These reports looked at the timing of abrupt uplift and subsidence around Restoration Point and Alki Point (right side of picture, above), tsunami deposits on Puget Sound, turbidity in lake paleosediments, rock avalanches, and multiple landslides around Lake Washington, and determined that all these happened about 1100 years ago (between A.D. 900–930), and most likely due to an earthquake of magnitude 7 or greater on the Seattle Fault.
Representation of the a'yahos spirit.
Although the A.D. 900–930 earthquake was over a thousand years ago, local native legends have preserved an association of a powerful supernatural spirit – a'yahos, noted for shaking, rushes of water, and landsliding – with five locales along the trace of the Seattle Fault, including a "spirit boulder" near the Fauntleroy ferry dock in West Seattle.
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.
Approximate location of the Seattle Fault Zone (and other faults). The section of the fault zone directly under "Seattle" corresponds to the red line in the photo at the top.
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.Three strands have been identified, based on aeromagnetic and gravitational data, the northernmost strand lying 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 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."(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 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, but most of the details of the Seattle Fault, including recurrence rate, remain to be resolved.
Other recent work 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 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.
Seattle is at risk of earthquakes from various sources, including from the deep subduction zone responsible for the 1700 Cascadia earthquake and from various other local faults such as the lesser known but equally threatening Southern Whidbey Island Fault.
Source: www.wikipedia.com