Johnson and others (1994) and Pratt and others (1997) show conventional industry seismic-reflection data across the Seattle fault and Seattle basin in Puget Sound. We collected a series of high-resolution profiles across the fault in order to provide additional constraints on fault structure, geometry, location, history, and slip rates. Our interpretations are based on analyses of the high-resolution and industry data. Velocities used to estimate unit thickness, dips, and vertical exaggeration are: latest Pleistocene and Holocene strata - 1600 m/s; Pleistocene strata - 2000 m/s; Neogene only - 2800 m/s. Faults are recognized on the basis of truncated reflections and abrupt changes in reflection dip or seismic facies (e.g., amplitude, frequency, continuity, geometry). Significant faults within the Seattle fault zone are designated "A", "B", and "C". "A" and "C" bound the zone on the north and south, respectively.
Seismic-reflection profiles were collected in Lake Washington across the Seattle fault zone and the southern part of the Seattle basin (Fig. 2A), with a data gap beneath the Mercer Island Bridge. Unlike Puget Sound, there are no industry data from Lake Washington. The profile on the west side of Mercer Island (Fig. 5A) shows a ~5-km-wide zone of deformation and uplift bounded on the north and south by Quaternary depocenters. The Tertiary-Quaternary contact at the north end of Figure 5A is based on correlation of a prominent angular unconformity (at 0.2 s TWT below "a") from south of the Mercer Island Bridge across the data gap to an inferred parallel to angular unconformity north of the data gap. This correlation is based mainly on a high-resolution profile collected by Harding and others (1988) and reprocessed by T.L. Pratt (unpublished data) that extends beneath the Mercer Island Bridge and clearly shows the transition from angular to parallel unconformity in the undeformed section. Using this pick, the Quaternary section in the Seattle basin beneath Lake Washington is about 500 m thick (Fig. 6) and there is about 380 m of structural relief on the base of the Quaternary across the boundary between the Seattle fault zone and the Seattle basin. The latest Pleistocene to Holocene section (placed above the highest unconformity in the Quaternary section) appears very thin (< 40 m), consistent with results from coring (e.g., Hedges et al., 1982).
Immediately south of the data gap, inferred upper Tertiary strata dip > 25° to the north. The lower part of the Quaternary section that unconformably overlies these rocks dips north more gently (about 7-8°, below "a") indicating that the fold has been active in the Quaternary. Although partly obscured by a narrow panel of strong water-bottom multiples, we infer a fault at "A" where reflections in inferred Tertiary strata appear truncated and there is an abrupt change from steeper dips on the north to shallower dips on the south.
To the south, faults "B1" and "B2" are placed where reflections truncate, dips change, and there are contrasts in reflection amplitude and frequency. The region between faults "B2" and "C" is characterized by highly variable dips and numerous diffractions, consistent with a location due east of structurally complex Tertiary rock outcrops that have north dips ranging from 15° to 88° (Fig. 6; Yount and Gower, 1991). Beneath Lake Washington, these Tertiary rocks appear to be overlain by a relatively thin (< 100 m) cover of flat-lying Quaternary sediment.
Fault "C" truncates north-dipping beds on the north limb of a large open fold in Tertiary rocks (south of the fault), juxtaposing them with more steeply dipping beds (north of the fault). The open fold in the hanging wall does not involve the thick (as much as ~600 m) overlying Quaternary beds, indicating that this folding is pre-Quaternary. Johnson and others (1994) also noted an increase in thickness of Quaternary strata south of the southern margin of the Seattle fault zone and suggested it could result from Quaternary reactivation of an older thrust fault as a normal fault.
Six north-south, high-resolution seismic profiles were collected in Elliot Bay (Fig. 2A). Previous investigations (e.g., Gower et al., 1985; Yount and Gower, 1991; Johnson et al., 1994) have inferred that the northern portion of the Seattle fault zone extended through Elliot Bay, based on extension of structures recognized on industry seismic-reflection data in Puget Sound and on gravity anomalies (e.g., Finn et al., 1991). Figure 5B shows line P37, the profile which extends farthest south in Elliot Bay. This line and the other Elliot Bay profiles show (1) relatively flat-lying Tertiary strata below about 1 s TWT, (2) a complexly stratified Pleistocene section characterized by discontinuous and hummocky reflections, numerous internal unconformities, and considerable relief at its top, and (3) a latest Pleistocene to Holocene section characterized by discontinuous flat to low-angle reflections that onlap and infill the relict Pleistocene relief. None of these profiles show the significant faulting or tilting of beds that typifies the northern front of the Seattle fault zone elsewhere in our survey area (Figs. 5A, 5C, 5D, 5E, 5F, 5G).
Figure 5C is a composite of two north-south high-resolution seismic-reflection profiles that cross the Seattle fault zone in eastern Puget Sound (Fig. 2A). This composite profile crosses from north to south: (1) the eastern margin of the central Puget Sound trough at the mouth of Elliot Bay, (2) relatively shallow hummocky terrane offshore of Alki Point, and (3) the flat-bottomed southern continuation of the central Puget Sound trough south of Alki Point (Figs. 2A, 4).
Quaternary strata in the Seattle basin on this line include a thick (~800 m) Pleistocene section and a thin (> 80 m) latest Pleistocene to Holocene section (Fig. 5C). Tertiary and Pleistocene strata in the Seattle basin are steeply tilted adjacent to fault "A" of the Seattle fault zone. Although this part of the profile displays both diffractions and water-bottom multiples, fault "A" can be identified from an abrupt dip change and truncated reflections. Structural relief on the base of the Quaternary across fault "A" is at least 900 m. The area between faults "A" and "B1" is mainly characterized by multiples and diffractions and lies offshore of Alki Point (Fig. 2A), where outcrops of mapped Oligocene strata strike west and have steep (83°) overturned dips to the south (Yount and Gower, 1991). Faults "B1" and "B2" bound a narrow (~ 600 m) syncline in which probable Quaternary strata (0.35 s) are gently folded. Fault "B2" truncates and forms the northern boundary of a panel of south-dipping Tertiary beds. Fault "C" cuts a zone of north-dipping reflections (best viewed at 0.5 to 1.0 s TWT). The basin south of fault "C" appears to have at least a partial erosional origin because it is continuous with the central Puget Sound trough (Fig. 4), and is filled by as much as 300 m of latest Pleistocene to Holocene strata.
Figure 5D shows part of a north-trending high-resolution seismic-reflection profile that extends down the central Puget Sound trough and across the Seattle fault zone. Adjacent industry data (Fig. 2B) clearly show the Tertiary-Quaternary contact in the Seattle basin (Johnson et al., 1994; Pratt et al., 1997). The high-resolution data show that this portion of the trough contains as much as 500 m of Pleistocene strata and 300 m of latest Pleistocene to Holocene strata.
Figure 5D and two nearby parallel profiles show that the boundary between the Seattle basin and the Seattle fault zone is characterized by an asymmetric anticline with a faulted axis (fault "A") marked by dip reversal and truncated reflections. Folding north of the anticlinal axis involves Tertiary rocks and the Quaternary section, indicating structural growth has continued into the Holocene. Structural relief on the base of the Pleistocene across the fold is about 640 m.
Reflections in the south-dipping limb of the asymmetric anticline are cut by fault "B" which extends upward into the lower Pleistocene section. Faults "B" and "C1" bound a broad (2 km) gentle syncline. Faults "C1" and "C2" form the northern and southern truncational boundaries of a zone of north-dipping reflections in Tertiary strata. Neither "C1" or "C2" appears to significantly disrupt the lower Pleistocene section. Reflections in inferred Tertiary strata south of fault "C2" dip south. The location of fault "C2" matches that of "fault 4" in the Seattle fault zone shown on industry seismic data by Johnson and others (1994, Fig. 2D). Other nearby industry data image this structure as an unbroken anticline axis.
Fault "A" in the Seattle fault zone ruptures and folds the lower Quaternary section but its effects on the upper Quaternary section are not clear (Fig. 5E). The area between faults "A" and "C" is characterized by abundant diffractions and water-bottom multiples, which probably reflects the combined effects of irregular bathymetric relief and the presence at shallow depths of steeply dipping Tertiary beds continuous with those mapped nearby on Bainbridge Island (Yount and Gower, 1991). If present, Quaternary strata between "A" and "C" are very thin (< 100 m); thus there is more than 825 m of relief on the base of the Quaternary across fault "A".
Because of the abundant diffractions and multiples, the location of fault "B" is located largely on the basis of parallel industry profiles and the adjacent onland juxtaposition of Tertiary units (Johnson et al., 1994, Fig. 2D). Fault "C" truncates beds and a small fold (at ~0.75 s TWT) in inferred Tertiary strata and also juxtaposes beds with different dips. About a kilometer south of fault "C" at ~ 0.8 to 1.0 s TWT, there is an asymmetric syncline in Tertiary beds (~1.0 s TWT) overlain by an angular unconformity (0.75 s TWT).
Figure 5F shows a seismic profile that extends south through Port Orchard on the west side of Bainbridge Island (Fig. 2A), then bends southwest into the mouth of Sinclair inlet south of Bremerton. The line crosses the southern part of the Seattle basin and four splays of the Seattle fault zone. Fault "A" bounds the Seattle basin and is identified by truncated reflections and dip changes. North of and adjacent to fault "A", Tertiary strata in the Seattle basin are tilted upward. Overlying Quaternary strata are tilted more gently and unconformably onlap the Tertiary section. The fault cuts reflections in the Quaternary section within 130 m of the seafloor. Overlying strata are folded above the fault within about 40 m of the seafloor. There is about 280 m of structural relief on the base of the Quaternary section across fault "A".
Fault "B1" cuts and warps inferred Quaternary strata up to within about 165 m of the seafloor. Farther south, the locations of faults "B2" and "C" are determined on the basis of significant truncations and dip changes in Tertiary strata. Fault "B2" does not obviously cut unconformably overlying Quaternary strata. Fault "C" cuts and folds reflections at the top of the Tertiary section and also appears to disrupt the lower Quaternary section. There is not significant structural relief on the base of the Quaternary across faults "B1" or "B2", but there may be some south-side-down displacement of either erosional or tectonic origin across "C" based on the position of the Tertiary-Quaternary unconformity.
We collected three short north-south seismic profiles and one east-west profile through Dyes Inlet, a shallow embayment north of Bremerton (Fig. 2A). Figure 5G displays our longest north-south profile, illustrating the northern part of the Seattle fault zone and the southern Seattle basin. The Tertiary-Quaternary contact north of fault "A" in the Seattle basin is inferred on the basis of an angular unconformity between steeply dipping beds of presumed Tertiary age and overlying flat or gently dipping Quaternary beds. The Quaternary section has a maximum thickness of about 320 m and there is no obvious contact between Pleistocene and latest Pleistocene to Holocene beds. Only a few small streams drain into Dyes Inlet so the Holocene section may be quite thin (< 20 m). Near the north end of the profile, there is a significant north-to-south change in reflection amplitude and continuity in the Quaternary section, which downlaps at its base onto the underlying Tertiary strata. These features indicate that Pleistocene strata at this location were deposited in a south-prograding delta, with topset, foreset, and bottomset beds yielding different seismic characteristics. Our east-west seismic profile through Dyes Inlet indicates a gentle (<10°) west dip to Seattle basin strata.
The three faults at the south end of the profile occupy a zone about 750 m wide. The zone lies on strike with and about 1 km west of exposures of steeply dipping to overturned Tertiary rocks mapped by Yount and Gower (1991). Based on truncated reflections and dip changes, fault "A" has cut the Tertiary section. Above fault "A", Pleistocene beds have not been noticeably broken but are gently folded. Within the fault zone south of fault "A", the Tertiary-Quaternary contact is placed above a local angular unconformity that coincides with a contrast in seismic facies (higher reflection amplitude and continuity in the inferred Quaternary). Two southern faults (labeled "B") appear to cut the lower part of the Quaternary section, and there is a small anticline that folds the lower Quaternary section in the hanging wall of the southernmost fault.
Our seismic-reflection profiles through Hood Canal (Fig. 2A) reveal considerable deformation and faulting in Tertiary strata and complex depositional patterns in Quaternary deposits that have been affected by faulting. However these profiles do not reveal a north-verging fault zone bounded on the north by a large sedimentary basin, such as that observed on seismic profiles to the east. Johnson and others (1994) proposed that the Seattle fault does not extend west as far as Hood Canal, a hypothesis supported by our data.