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TECTONICS, VOL. 23, TC1011, doi:10.1029/2003TC001507, 2004
Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland
Samuel Y. Johnson,1 Richard J. Blakely,2 William J. Stephenson,3 Shawn V. Dadisman,4 and Michael A. Fisher2
Received 28 January 2003; revised 17 October 2003; accepted 11 November 2003; published 31 January 2004.
[1] Margin-parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strikeslip, 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 ($1120) 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 $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 350400 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 INDEX TERMS: 8107 Tectonophysics: fault, or both.
Continental neotectonics; 8015 Structural Geology: Local crustal structure; 3025 Marine Geology and Geophysics: Marine seismics (0935); 7230 Seismology: Seismicity and seismotectonics; 8110 Tectonophysics: Continental tectonicsgeneral (0905); KEYWORDS: Seattle uplift, Tacoma fault, Rosedale monocline, margin-parallel shortening. Citation: Johnson, S. Y., R. J. Blakely, W. J. Stephenson, S. V. Dadisman, and M. A. Fisher (2004), Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland, Tectonics, 23, TC1011, doi:10.1029/2003TC001507.

1. Introduction

[2] Oblique convergence of tectonic plates at subduction zones commonly leads to strain partitioning in which deformation is resolved into two components of strain [e.g. Fitch, 1972; Jarrard, 1986; Yu et al., 1993; Chemenda et al., 2000]. One strain component, perpendicular to the subduction zone, is accommodated by margin-parallel thrust faults (margin-normal shortening). The second strain component is parallel to the subduction zone and typically results in a broad region of oblique-slip faulting in the forearc region of the upper plate. Combined movement along thrust and strike-slip faults in the forearc can lead to the simultaneous translation and rotation of large crustal blocks. The style and geometry of strain partitioning results from the interplay among many variables such as the obliquity of the plate convergence, the dip of the subducted plate, the amount of interplate coupling, the thermal state of the subduction zone, and the structural fabric and geometry of the forearc. Strain partitioning and associated forearc deformation has been described from several oblique-convergent margins, among them Sumatra [Fitch, 1972; McCaffrey et al., 2000b], New Guinea [Abers and McCaffrey, 1988], Japan [Hashimoto and Jackson, 1993; Fabbri and Fournier, 1999; Lallemand et al., 1999], the Aleutians Islands [Ave Lallemant, 1996; Geist et al., 1988], South America [Freymueller et al., 1993], and Cascadia [Wells et al., 1998; McCaffrey et al., 2000a], which is the subject of this report. [3] The Cascadia convergent margin is characterized by oblique subduction of the Juan de Fuca plate beneath North America. Paleomagnetic, geologic, and GPS data indicate that the Oregon portion of the Cascadia forearc, comprised largely of Eocene volcanic and overlying sedimentary

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U.S. U.S. U.S. 4 U.S.
Geological Geological Geological Geological
Survey, Survey, Survey, Survey,
Santa Cruz, California, USA. Menlo Park, California, USA. Denver, Colorado, USA. St. Petersburg, Florida, USA.
This paper is not subject to U.S. copyright. Published in 2004 by the American Geophysical Union.

TC1011

JOHNSON ET AL.: ACTIVE SHORTENING OF THE CASCADIA FOREARC
rocks, is rotating clockwise at about 1.5/m.y. and translating northward at a rate of 6 mm/yr. Farther north in Washington, this northward migrating forearc block abuts against a relatively stationary buttress of Mesozoic and older rocks in southwestern Canada and northwestern Washington, yielding margin-parallel shortening [Johnson et al., 1996; Wells et al., 1998]. Our challenge in this report is to document the style and geometry of margin-parallel shortening in the densely populated Puget Lowland portion of the Cascadia forearc (Figure 1) and to address the implications of this active deformation for earthquake hazard assessment. Meeting this challenge has required integrated analysis of several data sets, including conventional and high-resolution seismic reflection profiles, potential fields surveys, geologic mapping, and airborne laser swath mapping. [4] Crustal faults of the Puget Lowland were originally inferred on the basis of geophysical anomalies [Danes et al., 1965; Gower et al., 1985]). The Seattle fault, Devils Mountain fault, and southern Whidbey Island fault (Figure 1), for example, all correspond to significant linear gravity and (or) magnetic anomalies [Finn et al., 1991; Johnson et al., 1996, 2001; Blakely et al., 2002]. Documentation of these structural zones has been difficult due to minimal exposure and to extensive cover of dense vegetation and late Pleistocene glacial and interglacial deposits. [5] The Seattle fault forms the boundary between the Seattle uplift and Seattle basin, juxtaposing dense, highly magnetic, and high-velocity rocks to the south and less dense, less magnetic, and lower-velocity strata to the north. Paleoseismologic studies indicate several large, middle to late Holocene earthquakes occurred on the Seattle fault [e.g., Bucknam et al., 1992; Sherrod et al., 2000; Nelson et al., 2002, 2003a, 2003b]. Complex gravity and magnetic gradients also mark the southern end of the Seattle uplift, along the boundary with the Tacoma basin [Pratt et al., 1997; Brocher et al., 2001]. Gower et al. [1985] suggested either a fault or a monoclinal fold caused these anomalies but provided no documentation. On the basis of seismic reflection data, Pratt et al. [1997] proposed that this structural zone is a monoclinal fold that formed above a thrust fault that underlies the Seattle uplift. In contrast, seismic tomographic models led Brocher et al. [2001] to propose that the boundary between the Seattle uplift and the Tacoma basin is a steep, north dipping reverse fault, which they refer to as the Tacoma fault. Paleoseismological studies have documented late Holocene uplift and subsidence along this boundary [Bucknam et al., 1992; Sherrod et al., 2002, 2003] and accurate characterization of its geometry and history are essential to regional seismic hazard assessment. [6] This paper presents a summary of seismic reflection and relevant geologic and geophysical data across the boundary between the Seattle uplift and Tacoma basin, and concludes that parts of both of the seemingly contradictory structural models outlined above [Pratt et al., 1997; Brocher et al., 2001] are correct. Our data show that the northwestern margin of the Tacoma basin is a north dipping thrust fault, the Tacoma fault, and that the northeastern

deeper, lower-frequency, lower-resolution profiles was less than that of the high-resolution survey (Figure 3). 2.4. Geologic Mapping [15] Local geologic mapping was conducted on all of the shorelines of south central Puget Sound in the vicinity of the Gig Harbor gravity gradient, Allyn aeromagnetic gradient, Tacoma fault, and Rosedale monocline (Figures 2 and 4). The purpose of this mapping is to further confirm and clarify the style, geometry, and rates of Quaternary deformation with field observations. Given the Quaternary glacial history of the region (see below), differentiating between tectonic and glaciotectonic deformation [e.g., Van Der Meer, 1987; Croot, 1988; Aber et al., 1989; Aber, 1993], each of which can produce outcrop scale faults and folds, is obviously important and can be difficult. In this study and in our previous similar investigations in the Puget Lowland [e.g., Johnson et al., 1996, 1999, 2001], we have learned that deformed Quaternary strata in the Puget Lowland are generally concentrated along projections of faults imaged on nearby offshore seismic reflection profiles and have a structural style and geometry consistent with that of the larger-scale fault or fold imaged on the seismic reflection data. Where there is not this coincidence of seismic reflection and outcrop data, or where structural styles are inconsistent in the two data sets, as in some cases described below, glaciotectonic deformation is considered likely.
3. Stratigraphy and Seismic Stratigraphy
[16] Four stratigraphic units underlie the study area and are imaged on marine seismic reflection data [Johnson et al., 1994, 1999, 2001; Pratt et al., 1997]. These units include Eocene Crescent Formation volcanic rocks, Eocene and younger sedimentary rocks, uppermost Pliocene (?) to Pleistocene strata, and uppermost Pleistocene to Holocene postglacial strata. The older two units occur only in the subsurface of much of this portion of the Puget Lowland (Figure 4) and are not distinguished on the seismic reflection profiles included herein. Surface exposures in the region consist almost entirely of Pleistocene glacial and interglacial deposits, also widespread at the seafloor and in the shallow subsurface offshore. The youngest unit occurs primarily as the fill of alluvial valleys onshore and of glacial erosional channels offshore. For this report, the older two units are distinguished from the younger two Quaternary units on seismic reflection profiles based on stratigraphic position and seismic stratigraphic facies [e.g., Sangree and Widmier, 1977; Stoker et al., 1997]. 3.1. Crescent Formation [17] Predominantly marine basaltic rocks of the Eocene Crescent Formation form the basement below the southern Puget Lowland. These rocks tend to be dense and magnetic, and are considered to be responsible for the gravity and magnetic highs measured over the Seattle uplift [Blakely et al., 2002; Hagstrum et al., 2002] (Figures 1 and 2). The Crescent Formation crops out in the northern part of the Seattle uplift [Yount and Gower, 1991; Haeussler and

glaciofluvial scour and fill, but could also partly reflect local tectonic disruption. 3.3.2. Offshore Stratigraphy [21] Two seismic units occur above the Tertiary section in the Tacoma region. On both industry and higher resolution seismic reflection data, the lower of these two units consists of seismic facies typical of glacial deposits [Davies et al., 1997]. Characteristics include discontinuous, variable-amplitude, parallel, divergent, and hummocky reflections, with common internal truncation, onlap, and offlap of reflections. On the basis of this seismic character and on stratigraphic position, this lower unit is inferred to comprise uppermost Pliocene(?) to Pleistocene deposits. Regional physiography suggests that the presently submerged regions of the Puget Sound area formed as subglacial erosional channels [Booth, 1994] that were partly filled in with fluvial and lacustrine sediment during ice retreat. Thus, much of the Quaternary fill imaged by marine seismic reflection data in the Tacoma region is of recessional origin. We did not recognize any internal sequences within the inferred uppermost Pliocene(?) to Pleistocene section that could be traced across the region and might correlate with the multiple glacial and nonglacial intervals. We attribute this lack of internal stratigraphy to repeated irregular and large-scale glacial erosion (during ice advance) and deposition (during ice retreat). On industry seismic reflection data, the TertiaryQuaternary contact is typically imaged as a moderate- to high-amplitude, fairly continuous reflection separating higher amplitude, more continuous, commonly parallel reflections (Tertiary sedimentary rocks and Crescent Formation) from lower-amplitude, discontinuous, hummocky or irregular reflections (Quaternary strata). On the Seattle uplift, the Tertiary rocks are locally folded and the contact is locally an angular unconformity. This angular unconformity passes laterally into a disconformity to the south in the Tacoma basin. 3.3.3. Identification and Age of the Base of the Uppermost Pliocene(?) to Pleistocene Section [22] As outlined above, the base of the inferred uppermost Pliocene(?) to Pleistocene seismic unit is typically more distinct on conventional industry seismic reflection data, where it can commonly be recognized on the basis of contrasts in seismic facies [Johnson et al., 1994, 1996, 1999, 2001] and local angular unconformities. On highresolution seismic reflection profiles (Figure 3a), the contrast in seismic facies between Tertiary and mainly Quaternary strata ranges from quite distinct to irresolvable. In our interpretation of these profiles, the location of the contact is generally based on projection from nearby conventional industry profiles or on the basis of locally distinct unconformities and onlapping surfaces. Once the contact at the base of the mainly Quaternary section is identified at one or more locations on individual high-resolution profiles, it can generally be traced across the profile based on reflection continuity. Complete coverage through the central Puget Sound waterways is thus accomplished by iteratively combining the industry and high-resolution data. Figure 4 includes a contour map of depth to the base of the Quaternary section based on these data and a single bore-

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Figure 5. Mobil 34W seismic reflection profile, gravity profile, and aeromagnetic profile, Case Inlet (Figure 3b). Solid dots show the inferred base of the Quaternary section and heavy dashed lines show inferred faults. Gravity profile (solid line) and aeromagnetic profile (dashed line) extracted from Figure 2. [31] Paleoseismologic investigations based on marsh stratigraphy also indicate uplift (from 1.5 m to as much as 4 m) in the past $3000 years in North Bay (Figure 4) [Sherrod et al., 2002] north of the inferred north dipping fault. This uplift is consistent with that noted by Bucknam et al. [1992] from Lynch Cove (>2 m) and Burley (Figure 4), sites that also lie north of the Gig Harbor gravity gradient and Allyn magnetic gradient (Figure 2). This uplift significantly postdates postglacial rebound, which was largely completed by about ka [e.g., Dethier et al., 1995). [32] The contours on the base of the Quaternary in this area are based on the interpretation in Figure 6 and on the Union Hofert 1 borehole, the only known well in this area that penetrated through the Quaternary section to bedrock. 4.1.4. Discussion [33] The blind fault imaged on USGS 284 (Figure 6) coincides with the Gig Harbor gravity gradient and the Allyn aeromagnetic gradient (Figure 2), and a steep velocity gradient [Brocher et al., 2001] representing the boundary between the Seattle uplift and the Tacoma basin. We follow Brocher et al. [2001] in designating this fault the Tacoma fault. Using methods described by Schneider et al. [1996] for assessing folding above blind faults, Quaternary shortening across the kink band (Figure 6) is about 120 m, the thickness of Quaternary growth strata is about 360 m, fault dip is approximately 71, and the amount of Quaternary fault slip needed to generate the growth fold is about 380 m. [34] The tomography cross section of Brocher et al. [2001, Plate 3] at this longitude suggests the top of the Crescent Formation south of the Tacoma fault is at a depth of 6 to 7 km, yielding an estimate of the total amount of structural relief across the zone. However, the northward thickening of the Tacoma basin approaching the Tacoma fault predicted by the tomographic model is not apparent from existing seismic reflection data (Figure 5). If the basin fill does thicken northward, it must occur below the level of the stratigraphy imaged on Mobil 34W ($km, Figure 5). [35] The onset of deformation along the Tacoma fault correlates with the pregrowth-growth boundary within the kink band [Suppe et al., 1992], which occurs at the top of the Tertiary section (Figure 6). Because the Tertiary-Quaternary contact is an inferred unconformity (see above) and the age of the uppermost Tertiary strata in this area is not known, the timing of the onset of deformation cannot be precisely determined. The lack of significant thickening within Tacoma basin strata in younger Tertiary and Quaternary strata approaching the Tacoma fault (Figures 5 and 6), which would be predicted by contractional faulting, folding, and basin-margin crustal loading, is consistent with the onset of deformation being relatively recent (i.e., Quaternary). The presence of an obvious scarp above the upper fold axis in the kink band (Figure 7) indicates Holocene activity on this structure. 4.2. Carr Inlet 4.2.1. Industry Seismic Reflection Profile [36] Pratt et al. [1997, Figure 4] show an industry seismic reflection profile that extends northward through Carr Inlet (Figure 3). At its southern end, the profile images flat (dip < 1) reflectors in the Tacoma basin. These basinal reflectors are warped upward at the north end of the profile into a south

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Figure 6. U.S. Geological Survey seismic reflection profile 284, Case Inlet (Figure 3a). Solid dots show the inferred base of the Quaternary section and heavy dashed lines shows fault. The m shows water-bottom multiples. Gravity profile (solid line) and aeromagnetic profile (dashed line) extracted from Figure 2. Detailed 1:1 image of the fault zone at the northern part of the profile is shown in Figure 7b.
Figure 7. (a) Shot point map for USGS seismic reflection profile 284 (Figures 3 and 6) plotted on digital elevation model (DEM) derived from airborne laser swath mapping survey in the northern Case Inlet-North Bay area (Figure 3a). Note that a north-side-up, west trending scarp lies along strike with the upper hinge of the kink band imaged on line 284 (Figures 6 and 7b). Prominent north-northeast lineations on the landscape are of glacial origin. (b) Northern part of USGS profile 284 showing kink band.
dipping monocline, herein informally referred to as the Rosedale monocline after a local community that lies above the area of dipping strata. There is no obvious evidence of shallow (upper km) faulting on the profile. 4.2.2. U.S. Geological Survey Line 272 (Figure 8) [37] USGS Line 272 is nearly parallel to the industry profile shown by Pratt et al. [1997] (Figure 3), but extends 1050 m farther up Carr Inlet and provides significantly more detail in the upper 1 to 2 km. This profile also images flat reflectors in the Tacoma basin on the southwest that are warped upward at the northeast end of the profile into the southwest dipping Rosedale monocline. A prominent angular unconformity northeast of shot point 500 is inferred to be the base of the Quaternary section, associated with lowering of sea level and the occupation of Puget Sound by the first of several lobes of the continental ice sheet. South of shot point 540, this angular unconformity passes southward into a conformable surface within the monocline. Tertiary beds within the monocline dip approximately 16 to 19 south assuming velocities of 3000 to 3500 m/s [Pratt et al., 1997]. Above the unconformity, strata of inferred Quaternary age have a similar dip in the middle of the monocline, but dip less steeply to the north and the south. The northern termination of the Rosedale monocline within Tertiary strata (subunconformity) lies north of U.S. Geological Survey Line 272; hence the width of the deeper, Tertiary portion of the monocline is more than the 7 km shown in Figure 8. In contrast, the panel of dipping Quaternary strata is completely imaged on Line 272 and has a width of approximately 4100 m. The width of the panel of south dipping Quaternary strata decreases upward, outlining a syndepositional growth triangle [Suppe et al., 1992], for which the upper boundary (inactive axial surface) is defined
by the change from subhorizontal to south dips. There is a gentle increase in gravity along USGS Line 272 (Figure 8), corresponding to the north rise of Tertiary rocks in the Rosedale monocline. 4.2.3. Relevant Geologic Data [38] Quaternary glacial and interglacial strata are discontinuously exposed in coastal bluffs along northern Carr Inlet, adjacent to the monocline imaged in USGS Line 272. Strata in one exposure opposite the lowest part of the monocline dip 20 west-southwest (Figure 4), but there are no obvious dips in exposures farther northeast. Bucknam et al. [1992] presented evidence for coastal uplift at Burley at the northern end of Carr Inlet about 1100 years B.P., about 3 km north of the north end of Line 272 (Figure 4). 4.2.4. Discussion [39] No fault comparable to the Tacoma fault imaged in northern Case Inlet (Figure 6) was imaged on seismic reflection data in northern Carr Inlet. If the Tacoma fault extends this far to the east, it must lie north of the Carr Inlet seismic reflection profiles (Figure 3) and south of the Burley uplift site [Bucknam et al., 1992]. Alternatively, the fault must terminate below the depth ($5 km) of the industry seismic reflection profile shown by Pratt et al. [1997, Figure 4] with no obvious effect on overlying strata. The growth triangles imaged in Case Inlet and Carr Inlet are markedly different in both width (4100 m versus 360 m) and dip ($versus 35), suggesting different origins. 4.3. Narrows and Colvos Passage 4.3.1. Mobil 34E (Figure 9) [40] Mobil seismic reflection profile 34E extends northward through The Narrows, ending approximately 700 m

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Figure 8. U.S. Geological Survey seismic reflection profile 284, Carr Inlet (Figure 3a). Solid dots show the inferred base of the Quaternary section. The a at top of profile shows inferred active axial plane of folding and dashed shaded lines show boundaries of growth triangle above Rosedale monocline. Gravity profile (solid line) and aeromagnetic profile (dashed line) extracted from Figure 2. north of Point Defiance (Figure 3). The north and south ends of this profile are about 12 km and 7.5 km farther south, respectively, than those of an overlapping industry seismic reflection profile shown by Pratt et al. [1997, their Figure 5]. Mobil 34E images gently north dipping (mean dip < 1.5) strata in the northern Tacoma basin that are clearly folded upward in the south dipping Rosedale monocline (defining a gentle asymmetric syncline). Two continuous reflectors, b and c, highlight the gentle folding and indicate that there is no significant faulting in this part of the monocline. The maximum dip of reflectors imaging Tertiary strata in the monocline is about 9 to 11 assuming velocities of 3000 to 3500 m/s [Pratt et al., 1997]. Shallower monocline dips, interpreted as apparent dips, occur at the north end of the profile where the seismic profile has a more northerly azimuth and a highly oblique trend to the monocline and gravity gradient. Connecting the traces of the lower hinge of the Rosedale monocline between The Narrows and Carr Inlet reveals that this structure has a northwest trend, parallel to the Gig Harbor gravity gradient. [41] The rise in Tertiary strata in the Rosedale monocline along the profile corresponds to increases in gravity and magnetism. The short, high-amplitude magnetic anomaly shown above location 10 on Mobil 34E does not correspond to geologic structure and is probably related to a cultural feature. 4.3.2. SHIPS PS-1 and Industry 1 (Figure 10) [42] SHIPS PS-1 seismic reflection profile extends south through Colvos Passage, then bends about 90 and trends east through Dalco Passage (Figure 3b). Figure 10 also shows a short segment of Industry 1 that overlaps SHIPS PS-1 and provides a better image of shallow structure in an important area. A longer portion of Industry 1 was previously shown by Pratt et al. [1997, Figure 5] but with different processing and vertical exaggeration, and without the interpretation in this report. On PS-1, both the north and the east trending line segments are oblique ($45) to the dip of the Rosedale monocline (Figure 4). The uplift of Tertiary rocks in the Rosedale monocline on both the north and east trending parts of SHIPS PS-1 is associated with increasing gravity and magnetism. [43] The north trending portion of SHIPS PS-1 shows the upper hinge of the monocline, where south dipping (apparent dips of 11 to 13) reflectors and gently dipping reflectors are juxtaposed along a steep, north dipping reverse fault (2). Industry 1 reveals that this fault and a

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Figure 10. Multichannel SHIPS PS-1 and overlapping single-channel Industry 1 seismic reflection profiles, Colvos Passage and Dalco Passage (Figure 3b). Black dots show the inferred base of the Quaternary section; heavy lines show inferred faults; a and b show distinctive reflections used as markers to determine fault slip. Portion of single-channel Industry 1 overlaps the northern part of SHIPS PS-2. Gravity profile (solid line) and aeromagnetic profile (dashed line) extracted from Figure 2.
as a high-amplitude reflection in the central part of the line. Below the unconformity, inferred Tertiary strata dip 8 to 10 to the west and have a true dip of $11. Quaternary strata imaged in the western part of the line also dip south but at a slightly lower angle ($7, assuming 1800 m/s). The Quaternary section is essentially horizontal on the eastern part of the line. The hinge between dipping and flat Quaternary strata is analogous to the upper hinge within Quaternary strata imaged in Carr Inlet (Figure 7), and represents the top of the Quaternary part of the monocline (Figure 4). [46] There is a shallowing of dip within the Tertiary section at about SP 2723, the site where the trace of fault 1a 1b imaged on SHIPS PS-1 (Figure 10) intersects USGS Line 314. As on SHIPS PS-1, this fault appears to die out within the upper part of the Tertiary section. The uplift of Tertiary strata in the Rosedale monocline correlates with an increase in gravity. 4.3.4. Relevant Geologic Data [47] Sherrod et al. [2002] and B. L. Sherrod (oral communication, 2003) describe 1 to 2 m of late Holocene ($A.D. 980 1190) subsidence at Wollochet (Figure 4), a location that corresponds to the axis of the gentle syncline at the base of the Rosedale monocline imaged on Mobil Line
34E (Figure 9). Nearby in bluffs on the west coast of The Narrows, undated flatlying Quaternary sand and gravel are abruptly warped upward to a 10 southwest dip (Figure 12a); this dip transition represents the hinge at the base of the monocline. Farther north along the west side of The Narrows, there are at least seven sea level exposures of undated Quaternary strata with relatively gentle (<14) south to southwest dips (Figure 4). Flatlying Quaternary strata higher in the bluffs unconformably overlie these dipping beds. [48] More highly deformed Quaternary strata crop out in a few isolated tidal zone exposures at Point Evans on the west coast of The Narrows (Figure 4) and in tidal zone and low-bluff exposures along the north and southwest coastline of Point Defiance (S. Boyer and R. E. Wells, oral and written communication, 1996 2002). At Point Defiance, laminated silty mud glaciolacustrine deposits are deformed into gentle north to northeast trending ($350 to 20) folds (Figure 12b) with wavelengths of a few tens of meters, and strata overlying anticlinal axes are cut by extensional bending-moment faults (Figure 12c). [49] The Quaternary stratigraphy on the east and west flanks of Colvos Passage (north of Gig Harbor and on

tion on PS-2 is overlain by Quaternary strata along a prominent angular unconformity. This unconformity is relatively flat and is not obviously disturbed by the underlying faulting and folding. 4.4.2. U.S. Geological Survey Line 205 (Figure 14) [54] U.S. Geological Survey line 205 extends from offshore Tacoma to offshore Point Robinson (Figure 3), west of and relatively parallel to the southern part of SHIPS PS-2. A prominent pair of high-amplitude reflections that overlie an angular unconformity represents the inferred base of the Quaternary section. At the southwest end of the line, Tertiary strata dip (velocities of m/s, respectively). Tertiary strata flatten at about shot point 590, the inferred upper hinge of the Rosedale monocline. Uplift of Tertiary beds in the monocline is associated with increased gravity. [55] Farther northeast, Tertiary strata are folded into an anticline. The anticlinal axis ($shot point 290) is cut by a steep fault that can be traced to fault 3 on SHIPS PS-2 (Figure 13) using other nearby seismic reflection profiles. Fault 4 on line 205 is not well imaged; if present, it is continuous with fault 4 on SHIPS PS-2.
[56] Reflections in inferred Quaternary strata across the profile are hummocky to planar, and have variable amplitude and continuity. Several prominent erosional surfaces truncate reflectors, consistent with a Quaternary history of multiple glaciations and associated glacial and glaciofluvial scour and fill. There is no obvious structural dip (>$2) in the Quaternary section across the profile, except on the south flank of the faulted anticline where reflections in a $1300-m-wide panel have an apparent south dip of about 7. The unconformity at the base of the Quaternary section is relatively flat over most of the profile but dips south ($5) at the south end of the profile. 4.4.3. U.S. Geological Survey Line 316 (Figure 15) [57] USGS Line 316 extends north from offshore Tacoma across Dalco Passage into Quartermaster Harbor (Figure 3). The Quaternary section is characterized by discontinuous, locally hummocky, variable amplitude reflections. Numerous low-angle channels are consistent with multiple pulses of glacial and glaciofluvial erosion. The inferred unconformity at the base of the Quaternary section is a moderate to high-amplitude reflection couplet similar to that

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Figure 14. U.S. Geological Survey seismic reflection profile 205, Dalco Passage and East Passage (Figure 3b). Solid dots show the inferred base of the Quaternary section. The a reflection highlights folding in the Rosedale monocline. Heavy dashed line shows inferred fault. Gravity profile (solid line) and aeromagnetic profile (dashed line) extracted from Figure 2.
seen on lines P205 and P314 (Figures 11 and 14). Overlying Quaternary strata are flat (dip < 1). [58] Below the Quaternary unconformity, Tertiary strata are poorly imaged, perhaps due to sideswipe. We map the upper hinge of the Rosedale monocline at about shot point 520. South of this point, Tertiary strata have an overall south dip but are gently folded into an anticline and syncline with limb dips of about 5 to 7 (highlighted by reflector a). The gentle folds cannot be traced onto adjacent lines and are inferred to be minor structures with limited lateral extent. North of the projected hinge of the Rosedale monocline, Tertiary strata are flat. As along other profiles, an increase in gravity is associated with uplift of Tertiary strata in the monocline. 4.4.4. Relevant Geologic Data [59] Seismic reflection profiles in Quartermaster Harbor (Figures 3 and 15) do not clearly reveal Quaternary deformation. Hence, faults that might connect the deformation in East Passage with that in Colvos Passage and farther west must either lie north of the seismic survey or deeper than the stratigraphic level imaged in Figure 15 ($1 km). Quaternary strata exposed on the flanks of Quartermaster Harbor and East Passage provide a potential test of the hypothesis that the east trending Tacoma fault structural zone extends east across Puget Sound waterways to the mainland. [60] Outcrops along the northwestern shore of Quartermaster Harbor on Vashon Island consist of massive till or are of such poor quality that they provide no data to test the faulting hypothesis. Along the northeastern shore of the harbor (on western Maury Island), highly fractured, sheared, and destratified sandy and clayey silt (Figure 12e) occur discontinuously along the base of the coastal bluff for about 170 m (Figure 4), consistent with eastern continuation of the Tacoma fault. Fractures generally strike 95 to 110 and dip 45 to 70N. The eastern projection of this structural zone intersects the east coast of Maury Island in an area of extensive landsliding and poor exposure where there is no evidence for or against eastward continuation of the Tacoma fault. [61] The zone of deformation imaged on Figure 13 between faults 3 and 4 can be traced with seismic reflection data across East Passage, projecting onland at Saltwater State Park (Figure 4) where there is considerable evidence for deformation in Quaternary deposits exposed along the coastal bluffs and in the wave-cut bench. These strata yielded reversed magnetic polarities and are probably older than 780 ka [Hagstrum et al., 2002]. In the southern part of the park, a 200-m-wide section of distinctive alluvial sand and silt strikes 95 and dips as steeply as 16 north. A distinctly different stratigraphic section, consisting of sandgravel outwash, till, and interglacial sand, silt, and peat, occurs in the northern part of Saltwater State Park. These beds locally dip as steeply as and are cut by a subvertical southeast trending (140) fault with an estimated 8 m of slip, numerous east-southeast (100 125) fractures, sand-filled dikes, and gravel-filled extensional cracks. The contact between the strata exposed in the southern and northern parts of the park is not exposed but is a suspected fault based on the stratigraphic mismatch and on the abundant nearby deformation.

Figure 18. Schematic diagrams showing different hypotheses for structural deformation in the central Puget Lowland based on different principal shortening directions, indicated by bold arrows: (a) north-south shortening; (b) northeast-southwest shortening. such as the Seattle and Tacoma faults are characterized by north-south shortening and a lesser component of left-lateral slip. Also, deformation in the East Passage zone (Figure 13) is partitioned into thrust or reverse faults (faults 4 and 5) and a strike-slip fault (fault 3). The Dewatto basin could have formed through a combination of thrust loading on its eastern basin margin, transfer of right-lateral slip between north and northeast trending faults, and transfer of left-lateral slip from the Tacoma fault to the Seattle fault.

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[77] Geologic, geophysical, and geodetic data from the central Puget Lowland are thus consistent in that they indicate north or northeast directed crustal shortening. At this time, it is difficult to determine whether the different trends of contractional structures indicate local variations in thrust geometry and (or) the stress field, the effects of crustal heterogeneity and relict structural grain, or some combination of the above. Some specific tests of the above hypotheses could involve examination of the Seattle and Tacoma faults for evidence of left-lateral slip, and investigation of the structures on the flanks of the Dewatto basin.
6. Seismicity and Earthquake Hazards
[78] The regional seismicity catalog does not provide information sufficient to constrain tectonic models. Figure 17 shows that most crustal seismicity beneath the Puget Lowland occurs at depths of 15 to 25 km and is widely scattered both in map view and cross section. Although depth uncertainties (km) make any interpretation questionable, we infer that some of the earthquakes in this seismogenic zone occur in a band along the master decollement of Pratt et al. [1997] that links to the Seattle fault. Other lower-crustal earthquakes may occur in weaker portions of the Crescent Formation, perhaps within sedimentary or volcaniclastic horizons. Shallow portions (above $15 km) of the Seattle fault, the Seattle uplift, the Tacoma fault, and the Tacoma basin are essentially aseismic. The Seattle basin experienced a recent swarm of shallow earthquakes, initiated by the 6/23/97 M 4.9 Bremerton earthquake which we infer is related to an intrabasinal normal fault stressed by the hanging wall of the Seattle fault [Blakely et al., 2002]. [79] Brocher et al. [2001] noted the apparently synchronous uplift at $A.D. 900 along both the Seattle fault and the Tacoma fault [Bucknam et al., 1992] (Figure 4), and the contrasting trend in structural relief along the Tacoma fault (more to the west) and Seattle fault (more to the east). They interpreted these observations as linked slip between the Seattle and Tacoma faults. This hypothesis is consistent with the cross sections of Figures 17b and 17d, which suggest that movement on a master thrust could result in synchronous shallow displacement on either the Tacoma fault, the Seattle fault, or on both. [80] Pratt et al. [1997] used the width of the Rosedale monocline to estimate the total amount of slip on the master thrust fault since its inception. Of more importance is the amount of fault slip during the Quaternary, which (assuming the structural model is correct) can be inferred from the $4.1 km (Carr Inlet) to 6.6 km (The Narrows to Dalco Passage) width of the panel of dipping Quaternary strata on the monocline (Figures 4 and 16). Dividing these two widths by 2 Ma, the assumed age for the onset of Quaternary deposition, yields a slip rate of $2.1 to 3.3 mm/yr. Given the shallow dips on the monocline, the rate of shortening is only slightly less than the rate of slip, about 2 to 3 mm/yr. [81] These estimated rates indicate that a significant portion of the estimated mm/yr of northward shorten-

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Harding, T. P. (1985), Seismic characteristics and identification of negative flower structures, positive flower structures, and positive structural inversion, AAPG Bull., 69, 582 600. Hashimoto, M., and D. D. Jackson (1993), Plate tectonics and crustal deformation around the Japanese Islands, J. Geophys. Res., 98, 16,149 16,166. Jarrard, R. D. (1986), Terrane motion by strike-slip faulting of forearc slivers, Geology, 18, 780 783. Johnson, S. Y., C. J. Potter, and J. M. Armentrout (1994), Origin and evolution of the Seattle basin and Seattle fault, Geology, 22, 71 74. Johnson, S. Y., C. J. Potter, J. M. Armentrout, J. J. Miller, C. Finn, and C. S. Weaver (1996), The southern Whidbey Island fault, an active structure in the Puget Lowland, Washington, Geol. Soc. Am. Bull., 108, 334 354. Johnson, S. Y., S. V. Dadisman, J. R. Childs, and W. D. Stanley (1999), Active tectonics of the Seattle fault and central Puget Lowland: Implications for earthquake hazards, Geol. Soc. Am. Bull., 111, 1042 1053. Johnson, S. Y., S. V. Dadisman, D. C. Mosher, R. J. Blakely, and J. R. Childs (2001), Active tectonics of the Devils Mountain fault and related structures, northern Puget Lowland and eastern Strait of Juan de Fuca region, Pacific Northwest, U.S. Geol. Surv. Prof. Pap., 1643, 46 pp., 2 plates. Johnson, S. Y., et al. (2003), Maps and data from a trench investigation of the Utsalady Point fault, Whidbey Island, Washington, U.S. Geol. Surv. Misc. Invest. Map 2420, 1 plate. Jones, M. A. (1996), Thickness of unconsolidated deposits in the Puget Sound Lowland, Washington and British Columbia, U.S. Geol. Surv. Water Resour. Invest. Rep., 94-4133. Khazaradze, G., A. Qamar, and H. Dragert (1999), Tectonic deformation in western Washington from continuous GPS measurements, Geophys. Res. Lett., 26, 3153 3156. Lallemand, S., L. Char-Shine, S. Dominguez, P. Schnurle, and J. Malavieille (1999), Trenchparallel stretching and folding of forearc basins and lateral migration of the accretionary wedge in the southern Ryukyus: A case of strain partition caused by oblique convergence, Tectonics, 18, 231 247. Lees, J. M., and R. S. Crosson (1990), Tomographic imaging of local earthquake delay times for 3-D velocity variation in western Washington, J. Geophys. Res., 95, 4763 4776. Mahan, S. A., D. B. Booth, and K. G. Troost (2000), Luminescence dating of glacially derived sedimentsA case study for the Seattle mapping project, Geol. Soc. Am. Abstr. Programs, 32, A27. Mazotti, S., H. Dragert, and R. D. Hyndman (2002), N-S shortening across the Olympic Mountains and Puget Sound evidenced by GPS, Geol. Soc. Am. Abstr. Programs, 34, A90. McCaffrey, R., M. D. Long, C. Goldfinger, P. C. Zwick, J. L. Nabelek, C. K. Johnson, and C. Smith (2000a), Rotation and plate locking at the southern Cascadia subduction zone, Geophys. Res. Lett., 27, 3117 3120. McCaffrey, R., P. C. Zwick, Y. Block, L. Prawirodirdjo, J. F. Genrich, C. W. Stevens, S. S. O. Puntodewo, and C. Subarya (2000b), Strain partitioning during oblique plate convergence in northern Sumatra: Geodetic and seismologic constraints and numerical modeling, J. Geophys. Res., 105, 28,363 28,376. McClay, K. R. (1992), Glossary of thrust tectonic terms, in Thrust Tectonics, edited by K. R. McClay, pp. 419 433, Chapman and Hall, New York.

R. Blakely and M. A. Fisher, U.S. Geological J.
Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, USA. (blakely@usgs.gov; mfisher@usgs. gov) S. V. Dadisman, U.S. Geological Survey, 600 4th Street South, St. Petersburg, FL 33701, USA. (sdadisman@usgs.gov) S. Y. Johnson, U.S. Geological Survey, Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA. (sjohnson@usgs.gov) W. J. Stephenson, U.S. Geological Survey, MS 966, P.O. Box 25046, Denver, CO 80225, USA. (stephens@ usgs.gov)

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