Puget Lowland

Active Shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland

Tectonics, Vol. 23, TC101, doi:10,1029/2003TC001507, January 2004, 27 pages

Samuel Y. Johnson, Richard J. Blakely, William J. Stephenson, Shawn V. Dadisman, and Michael A. Fisher

Abstract:

Margin-parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strike-slip, thrust, and oblique crustal faults beneath the densely populated Puget Lowland accommodate much of this north-south compression, resulting in large crustal earthquakes. To better understand this forearc deformation and improve earthquake hazard assessment, we here use seismic-reflection surveys, coastal exposures of Pleistocene strata, potential-field data, and airborne laser swath mapping to document and interpret a significant structural boundary near the City of Tacoma.

This boundary is a complex structural zone characterized by two distinct segments. The northwest-trending, eastern segment, extending from Tacoma to Carr Inlet, is formed by the broad (~11.5 km), southwest dipping (~11°-20°) Rosedale monocline. This monocline raises Crescent Formation basement about 2.5 km, resulting in a moderate gravity gradient. We interpret the Rosedale monocline as a fault-bend fold, forming above a deep thrust fault. Within the Rosedale monocline, inferred Quaternary strata thin northward and form a growth triangle that is 4.1 to 6.6 km wide at its base, suggesting ~2-3 mm/yr of slip on the underlying thrust. The western section of the >40-km-long, north-dipping Tacoma fault, extending from Hood Canal to Carr Inlet, forms the western segment of the Tacoma basin margin. Structural relief on this portion of the basin margin may be several km, resulting in steep gravity and aeromagnetic anomalies. Quaternary structural relief along the Tacoma fault is as much as 350-400 m, indicating a minimum slip rate of about 0.2 mm/yr. The inferred eastern section of the Tacoma fault (east of Carr Inlet) crosses the southern part of the Seattle uplift, has variable geometry along strike, and diminished structural relief. The Tacoma fault is regarded as a north-dipping backthrust to the Seattle fault, so that slip on a master thrust fault at depth could result in movement on the Seattle fault, the Tacoma fault, or both.