approximate location of seattle fault zone (and other faults). section of fault zone directly under seattle corresponds red line in photo @ top. (dger)
one model of seattle uplift: cross-section (south north) along east side of central puget sound, looking west. tb = tacoma basin epz = east passage zone (maury island), sfz = seattle fault zone (alki point). grey dots hypocenters of earthquakes of magnitude 2 or more 1970–2001. (fig. 17d johnson & others 2004)
the seattle fault structural boundary 50–60 millions of years old (early tertiary) basalt of crescent formation on south has been uplifted – seattle uplift – , tipping seattle basin, tertiary bedrock buried under @ least 7 km (4.3 miles) of relatively softer, lighter sedimentary strata of younger blakeley , blakely harbor formations. has resulted in 4 7 km (2.5 4.3 miles) wide zone of complex faulting, 3 or more main south-dipping thrust faults. of faulting blind (not reaching surface), , difficult locate because of heavy vegetation or development. 3 principal strands have been identified, location determined high-resolution seismic reflection , aeromagnetic surveys. northernmost strand lies along interstate 90 , under lake sammamish. central section of fault zone – crosses apparent location of olympic-wallowa lineament – shows marked variation in location of strands , of underlying structure, nature , significance of not understood.
the fault extends approximately 70 km (43 miles) near fall city on east, appears terminated south whidbey island fault, hood canal on west (not shown on map). however, boundaries defining western termination zone unclear (see puget sound faults#question of western termination). northern edge of seattle uplift, of tacoma fault southern edge. 1 model has seattle , tacoma faults converging @ depth form wedge, being popped approximately north–south oriented compression derives plate tectonics. model (see diagram) interprets seattle uplift sheet of rock being forced ramp. subsequent work suggests structure of seattle fault may vary east west, both models being applicable in different sections. later model has part of north-thrusting sheet forming wedge between sedimentary formations of seattle basin , underlying bedrock.
the seattle fault believed date 40 million years ago (late eocene). time strike-slip movement on north-striking straight creek fault east ceased, due intrusions of plutons. appears when straight creek fault became stuck north–south compressive force had accommodated strike-slip motion transferred crust of puget lowland, subsequently folded , faulted, , various blocks jammed on 1 another.
other scarps associated seattle fault have been identified lidar-based mapping; trenching has shown faulting more complex first realized. many of details of seattle fault, including recurrence rate, remain resolved. study of sediments in lake washington found evidence of 7 large (m > 7) earthquakes in last 3500 years.
surface scarps due faulting observed in area (due topography, vegetation, , urbanization); rare exception can seen @ mee kwa mooks park south of alki point. site of west seattle fault; prominent rise there due uplift on north side of fault.
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