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Hood Canal, an elongate 75 km long, 2-5 km wide, northeast trending deep-water trough, defines the western limit of the Puget Sound estuary complex (Figure 1). This glacially carved feature is one of many channeled landforms occupying the Puget-Fraser Lowland. It is filled with thick Quaternary deposits of unconsolidated and semi-consolidated glacio-lacustrine, glacio-fluvial, and transgressive marine sediment deposited during multiple advance and retreat cycles of the Puget ice lobe (Eyles et al., 1990; Mullins et al., 1990; Booth and Hallet, 1993). Structural lows in the Puget Lowland underlying this broad 100 km wide sediment plain, such as the Seattle and Tacoma basins (Figure 2), are infilled to estimated depths of 3 to 10 km (Johnson et al., 1994; Pratt et al., 1994). This extraordinary abundance of sediment blanketing the Puget Lowland, coupled with dense vegetation and the expansive marine waters of Puget Sound, make geologic interpretations of buried crustal structures difficult without the aid of land and marine seismic methods, remote sensing technology, and drill hole data.
The enigmatic Hood Canal-Discovery Bay fault zone (Johnson et al., 1996) was inferred from gravity anomalies and first shown in detail but without a sense of displacement on Gower et al.’s (1985) 1:250,000 scale map (Figure 3). The continuous 75 km-long fault line begins north of Hoodsport (Figure 3) and follows Hood Canal north before curving north-northwest through the head of Dabob Bay (Figure 1) and continuing onshore for 18 km beneath West Valley (Figure 3). More regional-scale maps by Babcock et al. (1992) (Figure 4) and Snavely and Wells (1996) (Figure 5) have subsequently shown different fault line interpretations of this region, while Pratt et al. (1997) (Figure 6) adhere to the Gower et al. (1985) fault trace through Hood Canal. Regardless of how the Hood Canal fault zone is graphically portrayed, its inherent long, linear trend and distinct topographic relief are geomorphic features commonly associated with large-scale crustal faults.
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Figure 1: Hood Canal study area showing R. V. Thomas G. Thompson trackline layout. Line 1 starts the northbound zigzag pattern of tracklines 1-25 at southern Hood Canal’s "Great Bend". North of Quatsap Point and west of the Seattle Fault is the denser ‘checkerboard’ pattern of tracklines 26 and 40-50.
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Figure 2: Bouguer gravity map of the Puget Sound area showing major fault-bounded sedimentary basins (blue color tones) in the Puget Lowland and near-surface outcrops of the Olympic Peninsula’s Eocene Crescent Formation (red color tones). Olympic peripheral rock outcrops can be seen along the NW shoreline of Hood Canal. Further west are the lower density marine sedimentary core rocks of the Olympic Peninsula accretionary prism (modified from Finn et al., 1991).
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Figure 3: The Hood Canal fault trace is shown following the eastern boundary of a northeast-trending gravity high (not shown). The termination of the NW-trending Seattle fault is shown 8 km to the east and influenced the denser ship trackline layout shown in Figure 1. Quaternary faults are shown paralleling the western shoreline of Hood Canal north of Hoodsport. For reference, the location of photographs taken along Washington State Highway 101 (Figure 14A) and Seal Rock Park beach (Figure 14B) are shown (modified from Gower et al., 1985).
Although varying in success and scope, some efforts have been made to define faulting patterns in the Hood Canal area. Daneš et al. (1965) used gravity and earthquake data to propose that Hood Canal was a "...major active fault..." separating Puget Sound from the Olympics. Short marine seismic reflection lines from southern Dabob Bay, along with scattered drill hole logs, were used by Yount et al. (1985) to interpolate and map sediment thickness contours along sections of Hood Canal’s western shoreline. A seismic reflection line run by Harding et al. (1988a) crossed a 75 m west-facing step in the sea bottom south of Toandos Peninsula. Following preliminary post-processing, a crudely drawn seismic interpretation showed this bathymetric feature to correlate with faulted east-dipping Tertiary bedrock exhibiting about 350 m of apparent vertical offset (Harding et al., 1988b). Gower et al. (1985), Johnson et al. (1994), and others have suggested that the southernmost strand of the Seattle fault, shown terminating 8 km east of Hood Canal (Figure 3), may continue further west and be truncated by faulting in Hood Canal.
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Figure 4: The ‘Discovery Bay fault zone’ (DBfz). This is the fault trace proposed by Babcock et al. (1992) in which the Hood Canal fault splits south of the Leech River fault and trends into Dabob Bay, crossing Hood Canal’s main channel east and west of Toandos Peninsula (Figure 1). The Eocene Crescent Formation age-equivalent Blue Hills basalt ‘island’ is one of the rare outcrops of Coast Range basement(?) terrain seen in the "great Lowland fill" of Booth (1994). DMF ¾ Darrington-Devils Mountain Fault (modified from Babcock et al., 1992).
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Figure 5: Generalized geologic map of western Washington. A NE-trending, east-dipping thrust fault, mapped as unconcealed, is shown along Hood Canal and follows the trend of thrust faults in the Olympics (Tabor and Cady, 1978). In northern Dabob Bay the fault branches to the north and N-NW (see figure inset). The N-NW fault strand along the southern shoreline of the Strait of Juan de Fuca follows folds previously mapped by Tabor and Cady (1978) (modified from Snavely and Wells, 1996).
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Figure 6: Interpreted geologic map of the Puget Sound region. Major features shown include the HDF ¾ Hood Canal-Discovery Bay fault, SWIF ¾ Southern Whidbey Island fault, CRBF ¾ Coast Range Boundary fault, and DMF ¾ Darrington-Devils Mountain fault. Dots on the index map denote earthquakes > M 3.0. Note the resemblance of the HDF to that shown in Figure 3 (modified from Pratt et al., 1997).
More recently, a geologic map by Snavely and Wells (1996) shows the Hood Canal structure as a east-dipping thrust fault splaying northwest from Dabob Bay and continuing north-northwest along the northeastern perimeter of Olympic Peninsula (Figure 5). Pratt et al. (1997) depicted Hood Canal as a right-lateral strike-slip fault forming the western border of their 14 to 20 km-deep south-dipping Puget Lowland ‘thrust sheet’ model (Figure 6). A merged aeromagnetic map produced by Finn and Stanley (1997) shows lineations, interpreted to represent near-vertical faults and/or lithologic boundaries, paralleling Hood Canal and trending northwest into southern Dabob Bay. In addition, modeling of an east-west resistivity transect from the Pacific Coast to the Cascade foothills (Figure 7) shows that Hood Canal defines a sharp lateral transition between Crescent volcanics to the west and sediments filling the Seattle basin to the east (Aprea et al., 1998).
Recent studies suggest that great earthquakes along the 1200 km-long, shallow dipping Cascadia thrust fault may initiate fault displacement across broad regions of the forearc (Rogers et al., 1996; Wang et al., 1995). Moderate to large earthquakes along young crustal faults, especially those adjacent to densely populated urban centers (e.g. Seattle fault), may pose an equally damaging threat. Consequently, it is important to further define the specific type, lateral extent, orientation, and depth of faulting patterns in the Puget Lowland, particularly where fault displacement could present earthquake hazards to Puget Sound residents. The relatively unstudied and inferred Hood Canal-Discovery Bay fault zone is an area that requires such study.
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Figure 7: This resistivity model of the Olympic Mountains and Puget Lowland was created from a 250 km east-west transect (20 stations). It contrasts the high porosity, low resistivity Olympic core rocks (C1) and sedimentary deposits of the Seattle basin (C2) with lower porosity, high resistivity peripheral rocks of the Olympics (R1) and Cascade volcanics (R2) to the east. Hood Canal represents a prominent structural/resistivity boundary between the Crescent Volcanics (CV) and the western Seattle basin. C3 ¾ lower crustal-upper mantle conductor; C4 ¾ unknown conductor and possible fracture zone; R3 ¾ may be correlative with R1 (modified from Aprea et al., 1998).
The purpose of this study is to interpret the shallow (less than 0.7 km) seismic character of the Hood Canal-Discovery Bay fault zone. The dataset consists of approximately 100 km of single-channel, high-resolution airgun seismic data collected April 4-5, 1994 onboard the University of Washington’s R. V. Thomas G. Thompson. Particular emphasis is placed on defining Quaternary sediment thickness and faulting patterns in Hood Canal and southern Dabob Bay so that a quantifiable estimate of the seismic potential can be made. Additionally, recognizable fault patterns and/or deformation structures possibly related to late Holocene fault displacement along the Seattle fault zone will be presented.
