Seattle Fault Zone

The Seattle Fault is a zone of multiple shallow east-west thrust faults that cross the Puget Sound Lowland and through Seattle (in the U.S. state of Washington) in the vicinity of Interstate Highway 90. The Seattle Fault was first recognized as a significant seismic hazard in 1992, when a set of reports showed that about 1,100 years ago it was the scene of a major earthquake of about magnitude 7 – an event that entered Native American oral legend. Extensive research has since shown the Seattle Fault to be part of a regional system of faults.

First suspected from mapping of gravitational anomalies in 1965 and an uplifted marine terrace at Restoration Point (foreground in picture above), the Seattle Fault's existence and likely hazard were 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 (distant right side of picture),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.

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" called Psai-Yah-hus near the Fauntleroy ferry dock in West Seattle.

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.

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Wikipedia