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

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 kilometers, 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.

High-Resolution Seismic Reflection Imaging of Growth Folding and Shallow Faults beneath the Southern Puget Lowland, Washington State

“…2a). Seismic reflection profiles across the western part of the Tacoma fault zone beneath Case Inlet show a 360-m wide kink band that appears to be a growth fold above a deeper fault (Johnson et al, 2004).…”

“…On the north edge of the Tacoma basin, geophysical data show a west-trending series of faults and folds interpreted to be a broad Tacoma fault zone (Pratt et al, 1997;Brocher et al, 2001;Johnson et al, 2004). Pratt et al (1997) interpreted the central part of the Tacoma fault zone as a southdipping monoclinal fold or ramp, later termed the Rosedale monocline ( Fig.…”

“…Pratt et al (1997) interpreted the central part of the Tacoma fault zone as a southdipping monoclinal fold or ramp, later termed the Rosedale monocline ( Fig. 2a) by Johnson et al (2004), but tomographic models to the west suggest the Tacoma structure is a northdipping reverse fault there (Brocher et al, 2001). Johnson et al (2004) interpreted the Tacoma fault zone as an easttrending fault that crosses the Lowland, intersecting the northwest-trending Rosedale monocline beneath the central and east Lowland (Fig.…”

“…The density contrast between the sediment-filled basins and the areas of uplifted Crescent Formation volcanic basement rocks cause prominent gravity and magnetic anomalies that delineate west or northwest-trending faults and folds, such as the southern Whidbey Island fault (SWIF), Kingston Arch, Seattle fault, Tacoma fault, and Olympia structure ( Fig. 1; Gower et al, 1985;Finn, 1990;Finn et al, 1991;Johnson et al, 1994;Pratt et al, 1997;Brocher et al, 2001;ten Brink et al, 2002;Blakely et al, 2002;Johnson et al, 2004;Sherrod et al, 2008;Liberty and Pratt, 2008;Kelsey et al, 2008). These structures beneath the Puget Lowland are hypothesized to be part of an interconnected system accommodating the north to northeast compression in the area (Pratt et al, 1997;ten Brink et al, 2002;Brocher et al, 2004;Johnson et al, 2004).…”

“…Northeast-directed subduction of the Juan de Fuca plate and northward motion of the Oregon Coast Range block have caused a series of faults and folds beneath the Puget Lowland (Pratt et al, 1997;Wells et al, 1998;Brocher et al, 2001;Johnson et al, 2004;Liberty and Pratt, 2008;Kelsey et al, 2008). Pre-Quaternary strata beneath the Puget Lowland are largely covered by glacial deposits, the most recent being those of the Vashon stade of the Frasier glaciation that culminated about 16,400 years ago (Thorson, 1980;Booth, 1994;Porter and Swanson, 1998;Booth et al, 2004).…”