Active Tectonics of the Seattle Fault and Central Puget Sound, Washington— Implications for Earthquake Hazards

Active tectonics of the Seattle fault and central Puget Sound, Washington— Implications for earthquake hazards

Samuel Y. Johnson* U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225 Shawn V. Dadisman } Jonathan R. Childs U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025 William D. Stanley U.S. Geological Survey, M.S. 966, Box 25046, Denver Federal Center, Denver, Colorado 80225

ABSTRACT segments. Regional seismic-hazard assess- SEATTLE FAULT ments must (1) incorporate new information We use an extensive network of marine on fault length, geometry, and displacement Danes et al. (1965) first suggested the presence high-resolution and conventional industry rates on the Seattle fault, and (2) consider of a significant west-trending fault in the Puget seismic-reflection data to constrain the loca- the hazard presented by the previously un- Lowland through Seattle on the basis of gravity tion, shallow structure, and displacement recognized, north-trending fault zone. data. They inferred a steeply north dipping zone rates of the Seattle fault zone and crosscut- consisting of two parallel normal faults with ting high-angle faults in the Puget Lowland INTRODUCTION about 11 km of vertical slip. Gower et al. (1985) of western Washington. Analysis of seismic briefly outlined geologic relationships across the profiles extending 50 km across the Puget The Seattle fault is a zone of thrust or reverse Seattle fault, and Yount and Holmes (1992) sug- Lowland from Lake Washington to Hood faults that strikes through downtown Seattle in gested that the fault dipped to the south and had Canal indicates that the west-trending Seat- the densely populated Puget Lowland of west- reverse displacement. Johnson et al. (1994) used tle fault comprises a broad (4–6 km) zone of ern Washington (Fig. 1). The fault coincides industry seismic-reflection data from Puget three or more south-dipping reverse faults. with large gravity and magnetic anomalies Sound to show that the Seattle fault is a broad Quaternary sediment has been folded and (Danes et al., 1965; Finn et al., 1991) and forms zone comprising south-dipping thrust or reverse faulted along all faults in the zone but is the boundary between an uplift of Tertiary faults. Johnson et al. also inferred that the Seattle clearly most pronounced along fault A, the rocks to the south and the Seattle basin to the fault has been active from about 40 Ma to the northernmost fault, which forms the bound- north (Johnson et al., 1994). The Seattle fault is present, and linked north-vergent thrust faulting ary between the Seattle uplift and Seattle considered to be active (e.g., Gower et al., to flexural subsidence in the adjacent Seattle basin. Analysis of growth strata deposited 1985; Bucknam et al., 1992); however, its pre- basin (Fig. 1). They suggested that the Seattle across fault A indicate minimum Quater- cise location, lateral geometry, displacement fault represents a restraining transfer zone be- nary slip rates of about 0.6 mm/yr. Slip rates history, and slip rates are poorly defined. We tween right-lateral shear zones near Hood Canal across the entire zone are estimated to be collected an extensive network of marine high- and the southwest Washington Cascade foothills 0.7–1.1 mm/yr. resolution seismic-reflection profiles across the (Fig. 1; Gower et al., 1985). Pratt et al. (1997) The Seattle fault is cut into two main seg- Seattle fault to better define these uncertainties proposed a complementary model in which the ments by an active, north-trending, high- and provide constraints for earthquake hazard Seattle fault is one structural component of a angle, strike-slip fault zone with cumulative assessments. Our purpose in this paper is to north-directed thrust sheet that underlies the cen- dextral displacement of about 2.4 km. Faults present the results of these marine geophysical tral Puget Lowland from the Black Hills on the in this zone truncate and warp reflections in surveys and local complementary onland inves- southwest to the southern Whidbey Island fault Tertiary and Quaternary strata and locally tigations. Previous interpretations of the Seattle (Johnson et al., 1996) on the north. coincide with bathymetric lineaments. Cu- fault were derived from conventional industry Gower et al. (1985) suggested that the large mulative slip rates on these faults may ex- seismic-reflection data (Johnson et al., 1994; thickness of Quaternary strata in the Seattle ceed 0.2 mm/yr. Assuming no other crosscut- Pratt et al., 1997) collected from Puget Sound, basin indicates possible large Quaternary offsets, ting faults, this north-trending fault zone and these data were also incorporated in our and noted an uplifted Holocene marine terrace divides the Seattle fault into 30–40-km-long analysis. Because much of the geologic frame- within the Seattle fault zone at Restoration Point western and eastern segments. Although this work of the Puget Lowland is obscured by (Fig. 2A, see insert). Bucknam et al. (1992) doc- geometry could limit the area ruptured in Quaternary deposits, dense vegetation, Puget umented as much as 7 m of uplift at Restoration some Seattle fault earthquakes, a large event Sound waterways, and urban sprawl, marine Point and inferred that it occurred during a large ca. A.D. 900 appears to have involved both seismic surveys provide critical information for (M > 7) earthquake on the Seattle fault ca. A.D. understanding the structure and evolution of 900. This earthquake was accompanied by a *E-mail: [email protected]. the region. tsunami in Puget Sound (Atwater and Moore,

GSA Bulletin; July 1999; v. 111; no. 7; p. 1042–1053; 8 figures.

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124°W 122°W Vancouver Island ▼

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▼ T Cascade Range 50 km Puget Lowland

BH O Surficial deposits ▼ (Quaternary) MR

Sedimentary rocks (Paleogene to Neogene) Cascade igneous rocks Coast Range DF (Oligocene and younger) and Columbia river Basalt Group (Miocene) SHZ

Crescent Formation and

▼ other Eocene volcanic rocks ▼ MSH MA Basement rocks (pre-Tertiary)

Figure 1. Schematic geologic map of northwestern Washington showing the Puget Lowland and flanking Cascade Mountains, Coast Range, and Olympic Mountains. Abbreviations for cities: O—Olympia; S—Seattle; T—Tacoma; VI—Victoria. Abbreviations for faults (heavy lines), modern Cascade volcanoes (triangles), and other geologic features: BH—Black Hills; CBF—Coast Range Boundary fault; DAF—Darrington fault; DF—Doty fault; DMF—Devils Mountain fault: GP—Glacier Peak; HC—Hood Canal; LRF—Leech River fault; MA—Mount Adams; MB—Mount Baker; MR—Mount Rainier; MSH—Mount Saint Helens; SB—Seattle basin; SCF—Straight Creek fault; SF—Seattle fault; SHZ—Saint Helens zone; SJ—San Juan Islands; SJF—San Juan fault; SWF—southern Whidbey Island fault. Geology from maps and com- pilations of Tabor and Cady (1978), Washington Public Power Supply System (1981), Gower et al. (1985), Walsh et al. (1987), Whetten et al. (1988), Yount and Gower (1991), Tabor et al. (1993), and Tabor (1994).

1992), landslides in Lake Washington (Fig. 2, A along the Seattle fault in the past 16 k.y., which evant recurrence intervals could be shorter or and B; Jacoby et al., 1992; Karlin and Abella, suggests that most postglacial uplift occurred longer. After assuming a thrust-sheet model of 1992, 1996) and rock avalanches in the Olympic during the ca. A.D. 900 event and that recur- deformation, Pratt et al. (1997) used fault- Mountains (Fig. 1; Schuster et al., 1992). rence intervals for such large events must be on segment lengths and fold geometries to deduce Rates of displacement and earthquake recur- the order of several thousand years. However, an average slip rate of 0.25 mm/yr for the Seat- rence intervals for the Seattle fault are essen- Thorson (1996) also speculated that motion on tle fault over the past 40 m.y. However, other tially unknown. Thorson (1993) used elevations the Seattle fault over the past 15 k.y. may be models of Puget Sound structure are possible of glacial deltas to infer about 9 m of uplift anomalous because of deglaciation and that rel- and there is no basis for assuming that this in-

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ferred long-term Tertiary rate applies to Quater- STRATIGRAPHY AND SEISMIC nary time. Pratt et al. also calculated the total STRATIGRAPHY Stratigraphic unit Age surface area of the Seattle fault from their model Vashon Till 15 ka and concluded that earthquakes of magnitude Tertiary Rocks 7.6–7.7 were possible. Esperance Sand Seismogenic depths below Puget Sound are Tertiary rock units imaged on seismic- Lawton Clay typically about 15–25 km (Ludwin et al., 1991). reflection data in the vicinity of the Seattle fault Drift Vashon Since 1970, when the regional seismic network include (1) mainly basaltic rocks of the lower Sediments of the became operational, the largest two earthquakes Eocene Crescent Formation, (2) unnamed nonglacial 22 ka associated with the Seattle fault include a M 5.0 Eocene marine strata, (3) turbidite sandstone and Olympia interval event that occurred at a depth of about 17 km siltstone of the upper Eocene to Oligocene Blake- beneath Point Robinson on 29 January 1995 ley Formation, and (4) nonmarine sedimentary Possession Drift 80 ka (Dewberry and Crosson, 1996), and a M 4.9 rocks of the Miocene Blakely Harbor Formation Whidbey Formation 100 ka event that occurred at a depth of 7 km beneath (Johnson et al., 1994). These rock units can be (interglacial) Point White on southwestern Bainbridge Island delineated on conventional industry data on the on 23 June 1997 (Fig. 2). basis of seismic facies characteristics (Sangree Double Bluff Drift 250 ka and Widmier, 1977) and borehole data, as de- MARINE SEISMIC-REFLECTION scribed in Johnson et al. (1994, 1996) and Pratt Unknown SURVEYS et al. (1997). On industry seismic data, the Ter- tiary-Quaternary contact is typically imaged as a Salmon Springs Drift 1 Ma High-Resolution Seismic-Reflection Data strong, fairly continuous reflection separating Puyallup Formation higher amplitude, more continuous, commonly 1.6 Ma (interglacial) Our investigation relies mainly on a network parallel reflections (Tertiary rocks) and lower of multichannel, high-resolution seismic- amplitude, discontinuous, hummocky or irregu- Stuck Drift reflection data collected in 1995 and 1997 by lar reflections (Quaternary strata). In uplifted ar- Alderton Formation 1.6 Ma the U.S. Geological Survey. About 320 km of eas, the Tertiary rocks are commonly folded and (interglacial) data were collected in the vicinity of the Seattle the contact is an angular unconformity. This an- fault (Fig. 2A) at typical survey speeds of 5.5– gular unconformity passes laterally into a discon- Orting Drift 2 Ma 9 km/hr. For the 1995 survey, the seismic formity in the Seattle basin. source consisted of two 655 cm3 airguns fired at 12.5 m intervals. Data were digitally Pleistocene Strata Figure 3. Composite stratigraphic chart recorded for 2 s using a 24-channel (6.25 m showing onland Quaternary stratigraphy group interval; 150 m active length) streamer. Pleistocene deposits of the Puget Lowland from the central and southern Puget Sound Resulting common midpoint (CMP) stacked compose a stratigraphically complex basin fill of area (based on Blunt et al., 1987; Easterbrook, data are 6 fold and have a 3.125 m CMP spac- glacial and interglacial deposits that are locally as 1994a). Light shading shows six recognized ing. For the 1997 survey, the seismic source thick as 1100 m (Yount et al., 1985; Jones, 1996). glaciogenic units. Many units are only locally consisted of a two-chambered, 1147 cm3 airgun Easterbrook (1994a, 1994b) described six dis- present or exposed. Only one time interval is fired at 20 m intervals, and data were digitally tinct glacial drift units, and there are significant shown for which there is “unknown” deposi- recorded for 2 s using a 24-channel (10 m group time gaps in the Pleistocene stratigraphic record tion and/or preservation, but several others interval; 240 m active length) streamer. CMP in which additional glaciations may have oc- are likely. stacked data are 6 fold and have a 5 m CMP curred (Fig. 3). Glacial and interglacial strata spacing. Both 1995 and 1997 data were decon- consist mainly of till, fluvial channel and flood- volved and filtered before and after stack, then plain deposits, and fine-grained lacustrine depos- quent filling (e.g., Booth, 1994) make correlation time migrated using a smoothed velocity func- its. On seismic-reflection profiles, Pleistocene with adjacent dated units on land unreliable. tion. The data quality is typically good in the strata (excluding the latest Pleistocene) form a upper 1 s and degrades significantly between distinct seismic unit, bounded below by Tertiary Latest Pleistocene and Holocene Strata 1 and 2 s. rocks and above by typically flat-lying latest Pleistocene to Holocene deposits that commonly On seismic-reflection data in Puget Sound and Industry Conventional fill in erosional relief. On both conventional and associated waterways, latest Pleistocene and Seismic-Reflection Data high-resolution seismic-reflection data, Pleis- Holocene deposits are accumulating in basins tocene strata display highly variable amplitudes, bounded by Pleistocene bathymetric highs. Proprietary industry seismic-reflection data are discontinuous, and have parallel, divergent, These basins are filled with flat-lying Holocene (3–5 s records) in the vicinity of the Seattle and hummocky reflections. Internal truncation, deposits and show up clearly on digital elevation fault (Fig. 2B) were displayed and described by onlap, and offlap of reflections are all common. images (Fig. 4) as flat surfaces. The continuous, Johnson et al. (1994, 1996) and Pratt et al. Determining the age of the Pleistocene deposits elongate, north-trending Holocene basin in cen- (1997). Most of these data were collected in the imaged on seismic-reflection data in Puget Low- tral Puget Sound is here termed the central Puget late 1960s or early 1970s when the area was land waterways is problematic but important for Sound trough. In the Seattle area, this trough has considered a frontier for petroleum exploration. understanding rates of deformation. No bore- a steep linear west flank and a more irregular east In this paper we show a few east-west lines near holes have penetrated the submerged section, and flank. Holocene basin fill typically yields vari- the Seattle fault. multiple pulses of subglacial scour and subse- able-amplitude, parallel, and continuous reflec-

1044 Geological Society of America Bulletin, July 1999

122°45′ 122°25′ 122°05′ 47° 50′

Figure 4. Digital elevation model (30 m pixel size) of the central Puget Lowland. Illumination is from N15°E at 45° altitude. White lines show coastlines. Quality digital bathymetric data are not available in black areas (e.g., Lake Washington). B—Bre- merton; BI—Bainbridge Island; K—Kingston; LW—Lake Wash- ington; S—Seattle; VI—Vashon Island.

47˚ 20′ 0 5 10 15 km

tions. Holocene sediment samples from Puget strata—1600 m/s; Pleistocene strata—2000 m/s; wide zone of deformation and uplift bounded on Sound are typically clay and silt (Wang, 1955). Neogene only—2800 m/s. Faults are recognized the north and south by Quaternary depocenters. on the basis of truncated reflections and abrupt The Tertiary-Quaternary contact at the north end NORTH-SOUTH SEISMIC-REFLECTION changes in reflection dip or seismic facies (e.g., of Figure 5A (see insert) is based on correlation of PROFILES ACROSS THE SEATTLE amplitude, frequency, continuity, geometry). Sig- a prominent angular unconformity (at 0.2 s two- FAULT nificant faults within the Seattle fault zone are way traveltime [TWT] below A) from south of the designated A, B, and C. A and C bound the zone Mercer Island Bridge across the data gap to an in- Johnson et al. (1994) and Pratt et al. (1997) on the north and south, respectively. ferred parallel to angular unconformity north of showed conventional industry seismic-reflection the data gap. This correlation is based mainly on a data across the Seattle fault and Seattle basin in Lake Washington high-resolution profile collected by Harding et al. Puget Sound. We collected a series of high- (1988) and reprocessed by T. L. Pratt (1995, per- resolution profiles across the fault in order to pro- Seismic-reflection profiles were collected in sonal commun.) that extends beneath the Mercer vide additional constraints on fault structure, Lake Washington across the Seattle fault zone and Island Bridge and clearly shows the transition geometry, location, history, and slip rates. Our in- the southern part of the Seattle basin (Fig. 2A), from angular to parallel unconformity in the un- terpretations are based on analyses of the high- with a data gap beneath the Mercer Island Bridge. deformed section. Using this pick, the Quaternary resolution and industry data. Velocities used to Unlike Puget Sound, there are no industry data section in the Seattle basin beneath Lake Wash- estimate unit thickness, dips, and vertical exag- from Lake Washington. The profile on the west ington is about 500 m thick (Fig. 6) and there is geration are: latest Pleistocene and Holocene side of Mercer Island (Fig. 5A) shows an ~5-km- about 380 m of structural relief on the base of the

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122°30′ 122°15′ Puget Sound K 200 km Kitsap anticline Peninsula 400 0 5

? 400 inferred fault 4 200 structure contour (m) ° ′ 47 45 8

600 400 ? ?

800 PO LW 7 Bainbridge Island 800 1 600 DI 400

A EB ? B 100 5 S 400

? R ? 200 ? AP A ? C B ? ? ? 200 DR 100 200 400 C ?

6 100 47°30′ 400 2 600 100 ? ? 400 ? 200 Vashon 3 Island

Figure 6. Map showing location of structures in the Seattle fault zone (bold lines marked A, B, and C) and crosscutting north-trending fault zone (marked 1–8). Land is entirely underlain by Quaternary deposits (light shading) except small areas underlain by Tertiary rocks (dark shading). Abbreviations as follows: AP—Alki Point; DI—Dyes Inlet; DR—Duwamish River valley; EB—Elliot Bay; K—Kingston; LW—Lake Wash- ington; PO—Port Orchard; R—Restoration Point; S—Seattle. Structure contour is on base of Quaternary strata.

Quaternary across the boundary between the The region between faults B2 and C is character- Elliot Bay Seattle fault zone and the Seattle basin. The latest ized by highly variable dips and numerous dif- Pleistocene to Holocene section (placed above the fractions, consistent with a location due east of Six north-south, high-resolution seismic pro- highest unconformity in the Quaternary section) structurally complex Tertiary rock outcrops that files were collected in Elliot Bay (Fig. 2A). Pre- appears to be very thin (< 40 m), consistent with have north dips ranging from 15° to 88° (Fig. 6; vious investigations (e.g., Gower et al., 1985; results from coring (e.g., Hedges et al., 1982). Yount and Gower, 1991). Beneath Lake Wash- Yount and Gower, 1991; Johnson et al., 1994) Immediately south of the data gap, inferred ington, these Tertiary rocks appear to be overlain have inferred that the northern portion of the upper Tertiary strata dip >25° to the north. The by a relatively thin (<100 m) cover of flat-lying Seattle fault zone extended through Elliot Bay, lower part of the Quaternary section that uncon- Quaternary sediment. based on extension of structures recognized on formably overlies these rocks dips north more Fault C truncates north-dipping beds on the industry seismic-reflection data in Puget Sound gently (about 7°–8°, below A) indicating that the north limb of a large open fold in Tertiary rocks and on gravity anomalies (e.g., Finn et al., 1991). fold has been active in Quaternary time. Al- (south of the fault), juxtaposing them with more Figure 5B shows line P37, the profile that extends though partly obscured by a narrow panel of steeply dipping beds (north of the fault). The open farthest south in Elliot Bay. This line and the strong water-bottom multiples, we infer a fault at fold in the hanging wall does not involve the thick other Elliot Bay profiles show (1) relatively flat- A where reflections in inferred Tertiary strata ap- (as much as ~600 m) overlying Quaternary beds, lying Tertiary strata below about 1 s TWT, (2) a pear to be truncated and there is an abrupt change indicating that this folding is older than Quater- complexly stratified Pleistocene section charac- from steeper dips on the north to shallower dips nary age. Johnson et al. (1994) also noted an in- terized by discontinuous and hummocky reflec- on the south. crease in thickness of Quaternary strata south of tions, numerous internal unconformities, and

To the south, faults B1 and B2 are placed where the southern margin of the Seattle fault zone and considerable relief at its top, and (3) a latest Pleis- reflections truncate, dips change, and there are suggested that it could result from Quaternary re- tocene to Holocene section characterized by dis- contrasts in reflection amplitude and frequency. activation of an older thrust fault as a normal fault. continuous flat to low-angle reflections that on-

1046 Geological Society of America Bulletin, July 1999

lap and infill the relict Pleistocene relief. None of and the Seattle fault zone is characterized by an zone. Fault A bounds the Seattle basin and is iden- these profiles show the significant faulting or tilt- asymmetric anticline with a faulted axis (fault A) tified by truncated reflections and dip changes. ing of beds that typifies the northern front of the marked by dip reversal and truncated reflections. North of and adjacent to fault A, Tertiary strata in Seattle fault zone elsewhere in our survey area Folding north of the anticlinal axis involves Ter- the Seattle basin are tilted upward. Overlying (Fig. 5, A and C–G). tiary rocks and the Quaternary section, indicating Quaternary strata are tilted more gently and un- that structural growth has continued into Holo- conformably onlap the Tertiary section. The fault Eastern Puget Sound cene time. Structural relief on the base of the cuts reflections in the Quaternary section within Pleistocene strata across the fold is ~640 m. 130 m of the sea floor. Overlying strata are folded Figure 5C is a composite of two north-south Reflections in the south-dipping limb of the above the fault within about 40 m of the sea floor. high-resolution seismic-reflection profiles that asymmetric anticline are cut by fault B, which There is about 280 m of structural relief on the cross the Seattle fault zone in eastern Puget extends upward into the lower Pleistocene sec- base of the Quaternary section across fault A. Sound (Fig. 2A). This composite profile crosses, tion. Faults B and C1 bound a broad (2 km) gen- Fault B1 cuts and warps inferred Quaternary from north to south, (1) the eastern margin of the tle syncline. Faults C1 and C2 form the northern strata to within about 165 m of the sea floor. Far- central Puget Sound trough at the mouth of Elliot and southern truncational boundaries of a zone of ther south, the locations of faults B2 and C are de- Bay, (2) relatively shallow hummocky terrane north-dipping reflections in Tertiary strata. Nei- termined on the basis of significant truncations offshore of Alki Point, and (3) the flat-bottomed ther C1 or C2 appears to significantly disrupt the and dip changes in Tertiary strata. Fault B2 does southern continuation of the central Puget Sound lower Pleistocene section. Reflections in inferred not obviously cut unconformably overlying Qua- trough south of Alki Point (Figs. 2A and 4). Tertiary strata south of fault C2 dip south. The lo- ternary strata. Fault C cuts and folds reflections at Quaternary strata in the Seattle basin on this cation of fault C2 matches that of fault 4 in the the top of the Tertiary section and also appears to line include a thick (~800 m) Pleistocene section Seattle fault zone shown on industry seismic data disrupt the lower Quaternary section. There is not and a thin (>80 m) latest Pleistocene to Holocene by Johnson et al. (1994, Fig. 2D). Other nearby significant structural relief on the base of the Qua- section (Fig. 5C). Tertiary and Pleistocene strata industry data image this structure as an unbroken ternary across faults B1 or B2, but there may be in the Seattle basin are steeply tilted adjacent to anticline axis. some south-side-down displacement of either ero- fault A of the Seattle fault zone. Although this part sional or tectonic origin across C based on the po- of the profile displays both diffractions and water- Western Puget Sound sition of the Tertiary-Quaternary unconformity. bottom multiples, fault A can be identified from an abrupt dip change and truncated reflections. Fault A in the Seattle fault zone ruptures and Dyes Inlet Structural relief on the base of the Quaternary folds the lower Quaternary section but its effects across fault A is at least 900 m. The area between on the upper Quaternary section are not clear We collected three short north-south seismic faults A and B1 is mainly characterized by multi- (Fig. 5E). The area between faults A and C is profiles and one east-west profile through Dyes ples and diffractions and is offshore of Alki Point characterized by abundant diffractions and water- Inlet, a shallow embayment north of Bremerton (Fig. 2A), where outcrops of mapped Oligocene bottom multiples, probably reflecting the com- (Fig. 2A). Figure 5G displays our longest north- strata strike west and have steep (83°) overturned bined effects of irregular bathymetric relief and south profile, illustrating the northern part of the dips to the south (Yount and Gower, 1991). Faults the presence at shallow depths of steeply dipping Seattle fault zone and the southern Seattle basin. B1 and B2 bound a narrow (~600 m) syncline in Tertiary beds continuous with those mapped The Tertiary-Quaternary contact north of fault A which probable Quaternary strata (0.35 s) are gen- nearby on Bainbridge Island (Yount and Gower, in the Seattle basin is inferred on the basis of an tly folded. Fault B2 truncates and forms the north- 1991). If present, Quaternary strata between A angular unconformity between steeply dipping ern boundary of a panel of south-dipping Tertiary and C are very thin (<100 m); thus there is more beds of presumed Tertiary age and overlying flat beds. Fault C cuts a zone of north-dipping reflec- than 825 m of relief on the base of the Quaternary or gently dipping Quaternary beds. The Quater- tions (best viewed at 0.5–1.0 s TWT). The basin across fault A. nary section has a maximum thickness of about south of fault C appears to have at least a partial Because of the abundant diffractions and mul- 320 m and there is no obvious contact between erosional origin because it is continuous with the tiples, the location of fault B is located largely on Pleistocene and latest Pleistocene to Holocene central Puget Sound trough (Fig. 4), and is filled the basis of parallel industry profiles and the ad- beds. Only a few small streams drain into Dyes by as much as 300 m of latest Pleistocene to Holo- jacent onland juxtaposition of Tertiary units Inlet, so the Holocene section may be quite thin cene strata. (Johnson et al., 1994, Fig. 2D). Fault C truncates (<20 m). Near the north end of the profile, there beds and a small fold (at ~0.75 s TWT) in in- is a significant north to south change in reflection Central Puget Sound ferred Tertiary strata and also juxtaposes beds amplitude and continuity in the Quaternary sec- with different dips. About 1 km south of fault C tion, which downlaps at its base onto the under- Figure 5D shows part of a north-trending high- at ~0.8 to 1.0 s TWT, there is an asymmetric syn- lying Tertiary strata. These features indicate that resolution seismic-reflection profile that extends cline in Tertiary beds (~1.0 s TWT) overlain by Pleistocene strata at this location were deposited down the central Puget Sound trough and across an angular unconformity (0.75 s TWT). in a south-prograding delta, the topset, foreset, the Seattle fault zone. Adjacent industry data and bottomset beds yielding different seismic (Fig. 2B) clearly show the Tertiary-Quaternary Port Orchard characteristics. Our east-west seismic profile contact in the Seattle basin (Johnson et al., 1994; through Dyes Inlet indicates a gentle (<10°) west Pratt et al., 1997). The high-resolution data show Figure 5F shows a seismic profile that extends dip to Seattle basin strata. that this portion of the trough contains as much as south through Port Orchard on the west side of The three faults at the south end of the profile 500 m of Pleistocene strata and 300 m of latest Bainbridge Island (Fig. 2A), then bends south- occupy a zone about 750 m wide. The zone lies Pleistocene to Holocene strata. west into the mouth of Sinclair Inlet south of Bre- on strike with and about 1 km west of exposures Figure 5D and two nearby parallel profiles merton. The line crosses the southern part of the of steeply dipping to overturned Tertiary rocks show that the boundary between the Seattle basin Seattle basin and four splays of the Seattle fault mapped by Yount and Gower (1991). On the ba-

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sis of truncated reflections and dip changes, fault mapping, based on the high-resolution seismic- The abrupt northward displacements in the A has cut the Tertiary section. Above fault A, reflection survey, reveals a more complex, seg- Seattle fault zone in Puget Sound could result Pleistocene beds have not been noticeably broken mented zone (Fig. 6). Starting on the east, the from the occurrence of lateral ramps or tear faults but are gently folded. Within the fault zone south faults (and the syncline at the northern edge of the in the thrust system, or from crosscutting dextral of fault A, the Tertiary-Quaternary contact is fault zone) imaged on Lake Washington profiles strike-slip faults. This distinction is important be- placed above a local angular unconformity that are inferred to extend along the strike of the local cause ramps and tears would tend to slip at the coincides with a contrast in seismic facies (higher gravity anomaly to eastern Puget Sound. Fault A same time as other elements of the Seattle fault reflection amplitude and continuity in the in- and the adjacent asymmetric syncline on the mar- zone, whereas crosscutting strike-slip faults ferred Quaternary strata). Two southern faults (la- gin of the Seattle basin are clearly the same basin- could move independently and represent an addi- beled B) appear to cut the lower part of the Qua- margin structures in both areas (Fig. 5, A and C). tional potential earthquake source. These possi- ternary section, and there is a small anticline that The Seattle fault therefore does not extend bilities are tested by examination of east-west folds the lower Quaternary section in the hanging through Elliot Bay (Figs. 5B and 6) as previously seismic profiles in Puget Sound. wall of the southernmost fault. interpreted, but rather lies on its southern margin. The location of the fault at Alki Point coincides NORTH-TRENDING FAULT ZONE IN Hood Canal with the northern margin of the terrace uplifted in PUGET SOUND the ca. A.D. 900 earthquake (R. C. Bucknam, Our seismic-reflection profiles through Hood 1997, written commun.). Fault C on each line A mix of east-west–oriented industry and high- Canal (Fig. 2A) reveal considerable deformation forms the northern margin of a Quaternary sedi- resolution seismic-reflection profiles extending and faulting in Tertiary strata and complex depo- mentary basin. Yount and Gower (1991) sug- from Kingston to offshore of northern Vashon Is- sitional patterns in Quaternary deposits that have gested that, east of Lake Washington, fault A ex- land in Puget Sound were examined to evaluate been affected by faulting. However, these profiles tends through southern Lake Samammish, the abrupt displacements in the location of the do not reveal a north-verging fault zone bounded consistent with seismic-reflection profiling of Seattle fault zone (Figs. 2, 6, and 7). Most crosson the north by a large sedimentary basin, such as Prunier et al. (1996). ings display evidence of high-angle faults. Eight that observed on seismic profiles to the east. On central Puget Sound seismic profiles (e.g., discontinuous fault strands were mapped and are Johnson et al. (1994) proposed that the Seattle Fig. 5D), the locations of faults A and B and the inferred to represent components of a complex fault does not extend west as far as Hood Canal, a adjacent fold in the Seattle basin are ~1200 m dextral shear zone (Fig. 6). Unless noted, each of hypothesis supported by our data. north of their position in eastern Puget Sound these mapped structures is recognized on all (Fig. 6). This displacement also corresponds to crossing seismic profiles. The expression of these MAPPING THE SEATTLE FAULT ZONE truncation of a high-resolution aeromagnetic faults (Fig. 7, see insert) on some profiles is more anomaly in the Seattle fault zone (R. J. Blakely, subtle than for the faults in the Seattle fault zone High-resolution seismic-reflection data show 1998, personal commun.). A second abrupt (Fig. 5, see insert) because the predominant mo- that the Seattle fault comprises a zone of at least northward displacement of the leading edge of tion on these structures may be largely lateral, and three or four splays that extends across the Puget the fault zone, also about 1200 m, occurs be- displacements (potentially as much as 1200 m) Lowland (Fig. 6). The zone is bounded on the tween seismic profiles in central and western are relatively small. The best images of these north by fault A and the syncline on the southern Puget Sound (Figs. 5, D and E, and 6). This offset faults are on profiles where (1) the lateral dis- margin of the Seattle basin, which have a similar is perplexing in that it also appears to dextrally placement has juxtaposed markedly different seis- and distinctive geometry for about 40 km from displace faults B and C and structure contours in mic facies or beds; (2) the lateral displacement has Lake Washington to Dyes Inlet. Quaternary defor- the Seattle basin, but does not significantly dis- juxtaposed parts of folds with different dips; (3) mation appears concentrated on fault A, consistent place (>200 m?) either a prominent aeromagnetic the lateral displacement involves a component of with northward migration of the thrust front anomaly in the Seattle fault zone (R. J. Blakely, transpressional deformation so that beds adjacent (Johnson et al., 1994). Fault A coincides with an 1998, personal commun.) or the syncline axis at to the fault or between fault splays are warped or anticlinal axis (steeply dipping synclinal beds are the southern margin of the Seattle basin. arched; and (4) vertical displacement is large in the footwall), and thus represents an “anticlinal Fault A in western Puget Sound coincides with enough to truncate continuous reflections across breakthrough” (Suppe and Medwedeff, 1990). the northern limit of late Holocene uplift recog- the fault. Where lateral displacement may have South of fault A, the network of structures labeled nized by Bucknam et al. (1992) to the west on juxtaposed similar seismic facies and vertical up- B and C are shown to be relatively continuous Bainbridge Island. The fault zone can be traced lift is minimal, faults will be difficult to recognize across the Puget Lowland (Fig. 6), but it is also westward from western Puget Sound to Port Or- on seismic data. possible that these structures are a set of anasto- chard with no apparent displacement of fault mosing, discontinuous faults. Seismic-reflection splays (Figs. 5, E and F, and 6). The width of the Fault 1 images of the B faults typically show minor trun- Seattle fault zone thus varies from about 4 km in cation and warping of Quaternary strata. Fault C at eastern Puget Sound and Lake Washington to 5.8 Fault 1 coincides with the eastern displacement the south end of the zone is locally imaged as both km in Port Orchard and Sinclair Inlet (Fig. 6). Far- in the location of fault A of the Seattle fault zone. a fault and an unbroken fold axis, and is inferred to ther west, we match and connect fault A with the Line P330 (Figs. 2A and 7F) extends west across be a pre-Quaternary structure with some local and northern fault on the Dyes Inlet profile (Fig. 5G), this fault in the southern part of the Seattle basin. minor Quaternary reactivation. which similarly forms the southern margin of a This structure truncates and warps prominent re- Largely from gravity surveys (e.g., Finn et al., sedimentary basin. Fault B is tentatively matched flections in the Pleistocene section and propagates 1991), previous workers (e.g., Gower et al., 1985; with the southern fault on this profile, suggesting up to the upper part of the latest Pleistocene to Yount and Gower, 1991; Johnson et al., 1994) in- westward narrowing of the zone. As described Holocene basin fill. A prominent basin-fill reflec- terpreted the Seattle fault as a continuous band above, the fault zone does not extend west to our tion within 40 m of the sea floor appears to be ver- across Puget Sound and the Puget Lowland. Our Hood Canal profiles. tically displaced 10 to 15 m. On an industry pro-

1048 Geological Society of America Bulletin, July 1999

file a few kilometers to the north (Fig. 7D), fault 1 ern splay similarly cuts and warps reflections displacement (>200 m?) of a continuous east- is characterized by a zone of truncated and along its trace and appears to displace the basin trending aeromagnetic anomaly (R. J. Blakely, warped reflections and local dip change that ex- floor about 40 m. Because the basin floor at this 1998, personal commun.) along the projection of tends well upward into the Quaternary section. locality probably consists of Tertiary strata, the fault 5. North of line P330, dextral displacement Farther north, fault 1 could not be identified on an timing of basin-floor displacement could be pre- of the axis of the syncline at the southern margin east-trending high-resolution seismic profile east Quaternary (i.e., relict relief filled in by latest of the Seattle basin also does not appear to exceed of northernmost Bainbridge Island (Fig. 2A). Pleistocene to Holocene deposits). However, re- 200 m, although this feature is difficult to map Figure 7H shows high-resolution seismic pro- flections in the lower part of the basin fill on the precisely. Thus, fault 5 appears to be largely con- file P326 that crosses fault 1 a few kilometers west flank of the basin-floor uplift converge to- fined to the northernmost part of the hanging wall south of the Seattle fault zone (Fig. 2A). The fault ward and over the uplift, indicating that at least of the Seattle fault, and should be regarded as a truncates, warps, and arches reflections on the some faulting and uplift coincided with latest tear fault within the Seattle fault zone. This struc- west limb of an anticline that underlies the central Pleistocene to Holocene deposition. Fault 3 was ture has a length of about 1 to 1.5 km. Puget Sound trough, and extends upward to at not identified on line P324 (Figs. 2A and 7I) and least the middle of the Pleistocene section before its southern extent has not been mapped. Thus, it Fault 6 being obscured by multiples. On a parallel indus- has a minimum length of 2–3 km. try profile (Fig. 7G), the fault is characterized by Evidence from Seismic-Reflection Data. a dip change in the inferred uppermost Tertiary Fault 4 Fault 6 extends south from the southern part of beds and by truncated reflections in the lower the Seattle fault zone in western Puget Sound part of the Pleistocene section. Farther south on Figure 7A shows an industry seismic profile (Fig. 6). On the industry profile shown in Figure the east end of line P324 east of northern Vashon that crosses fault 4 east of Kingston (Fig. 2B), 7G, the fault is marked by an abrupt change in Island (Figs. 2A and 7I), fault 1 cuts east-dipping about 20 km north of the Seattle fault zone. The dip in Tertiary beds (steeper west dips on the reflections in Tertiary strata but multiples obscure fault is imaged as a high-angle, upward-diverging west, shallower east dips on the east) as well as the Pleistocene section. On lines P324 and P326, structure that truncates and warps reflections. truncations along the inferred fault plane. Reflec- the latest Pleistocene to Holocene basin fill on the Strata east of fault 4 form a gentle, north-trending tions in the Pleistocene section are cut and west flank of the central Puget Sound trough dips (based on analysis of additional industry profiles) warped within about 100–150 m of the sea floor. east as much as 3°–5°, indicating east-west Holo- anticline, further evidence for east-west contrac- On high-resolution seismic-reflection profiles cene contraction that could be associated with re- tion in the Puget Lowland. Line P338 (Figs. 2A (e.g., line P326, Fig. 7H), fault 6 is characterized gional deformation or with more local transpres- and 7B) shows fault 4 as two splays that truncate by two splays that truncate and warp reflections sion. South of P324, fault 1 strikes onto Vashon and warp reflections and similarly cut the west in the lower part of the Quaternary section. Island in an area of extensive landsliding (Booth, limb of a gentle anticline. There is a significant Arched reflections between the two splays are 1991). Fault 1 therefore appears to have a mini- contrast in reflection amplitude and density for juxtaposed with reflections of contrasting geom- mum length of about 24 km. the strata between the two splays and strata im- etry and density. To the north, fault 6 appears to aged to the east and west of the two splays. Fault extend into the southern part of the Seattle fault Faults 2 and 3 4 occurs along the same trend as fault 1; however, zone. To the south, the fault projects onto north- we could not identify a fault that would link these ern Vashon Island. Fault 2 was only recognized on line P324 structures on a high-resolution profile between Investigation of Land Exposure. Fault 6 pro- (Figs. 2A and 7I) east of Vashon Island and thus their mapped traces (Figs. 2A and 6). Fault 4 ex- jects south along the ~3 km, north-trending west- can be no longer than about 2 km. This structure tends for at least 6 km north of Kingston, based ern shoreline of northern Vashon Island, inter- appears to displace the floor of the latest Pleis- on analysis of additional seismic profiles, and secting the coastline at Fern Cove (Figs. 6 and 8). tocene to Holocene basin between 15 and 30 m, thus has a minimum length of about 13 km. The projected trace then continues south across depending on how truncated reflections are the island and passes offshore through the mouth matched. Although it is conceivable that this off- Fault 5 of Quartermaster Harbor. With the exception of a set results from relict glaciofluvial topography, few gravel pits, virtually all of the outcrops on similar relief is rare elsewhere at the base of the Fault 5 coincides with the western abrupt dis- Vashon Island occur in discontinuous bluffs central Puget Sound trough, and this alternative placement in the location of fault A of the Seattle along shorelines. West of the projected trace of hypothesis is considered less likely. Slightly fault zone (Fig. 6) and with an ~200-m-high bathy- fault 6, Booth (1991) correlated virtually all lower in the seismic profile (~0.45 s TWT), the metric lineament that forms the western boundary shoreline exposures with pre ca. 28 ka glacial and inferred Tertiary-Quaternary contact appears to of the central Puget Sound trough (Fig. 4). Figure interglacial units (Fig. 8). East of the projected be displaced about 20–25 m in an opposite sense, 7F shows an east-trending profile that extends fault trace, Booth correlated virtually all shore- up to the east. Lower in the profile, the fault trun- from the northern part of the Seattle uplift (hang- line exposures with post ca. 28 ka glacial and in- cates and warps reflections from inferred Tertiary ing wall of the Seattle fault) on the west across terglacial units. strata, including a gentle anticline east of the fault fault 5 and into the southernmost Seattle basin On the north end of Vashon Island, there is no (best seen at 0.5–1.0 s TWT). (Figs. 2A and 6). The fault is not well imaged on exposure along the projected fault trace, but the Fault 3 comprises two splays (Figs. 2A and this profile because of the multiples and diffrac- nearest shoreline outcrops on either side of the 7J). The western splay truncates a zone of east- tions associated with steeply dipping bedrock of projection contain evidence of late Quaternary dipping reflections in inferred Tertiary strata and the Seattle uplift. However, across this fault there deformation. Quaternary strata in coastal bluffs at juxtaposes it with flat-lying beds east of the splay. is more than 500 m of vertical relief on the base of Vashon Heights (Fig. 8) on the north coast of the At 0.5 s, reflections in Tertiary strata at the base the Quaternary. Island about 600 m east of the projected fault of the latest Pleistocene to Holocene basin are South of line P330 (Fig. 7F) but within the consist of ~10–15 m of glaciolacustrine deposits markedly downwarped along the fault. The east- Seattle fault zone, there is no significant dextral of the ca. 20 ka Lawton Clay overlain by >50 m

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of sandy outwash of the ca. 15–18 ka Esperance profile, however, clearly show displacement and 122°30′ Sand (Fig. 3; Booth, 1991; Easterbrook, 1994a). warping of Quaternary strata on two structures 1 6 VH Exposures are restricted to the lower ~3–4 m of considered splays of fault 7 (e.g., Fig. 7E). The 48°30′ 2 the bluffs and a few landslide scarps. Exposed eastern splay juxtaposes and cuts reflections of FC

beds are warped (dips of ~4°) into a gentle north- contrasting amplitude and density in the Pleis- ?

trending anticline and the westernmost exposures tocene section, appears to displace the floor of ? are cut by high-angle faults and fractures with the latest Pleistocene to Holocene basin, gently 3

vertical displacements of as much as 40 cm. warps Holocene basin reflections, and projects ?

About 1 m of Quaternary strata are exposed upward to a notch in the sea floor. The western ? during the lowest tides along a 45-m-wide beach splay is harder to interpret because of multiples, Vashon Puget Colvos Passage west of Fern Cove, about 500 m west of the pro- but also cuts Pleistocene reflections and juxta- Island Sound jection of inferred fault 6 (Fig. 8). These strata poses disparate seismic facies. No Quaternary consist of glaciolacustrine silt and clay with com- deformation along the fault 7 trend can be identi-

mon dropstones, and Booth (1991) considered fied on the next two high-resolution seismic pro- ? them part of a pre-28 ka sequence of glacial drift. files to the north (Fig. 2A), and evidence for Qua- QH Beds have a general westerly strike (~260°), dip ternary deformation on the next high-resolution Maury Is. about 20° to the south, and are cut by several dis- profiles to the south is ambiguous. Thus, fault 7 continuous, north-trending, high-angle shears has an inferred minimum length of about 3 km.

bounded by meter-scale drag folds. ? The geologic relationships outlined here are Fault 8

consistent with a projection of fault 6 across ? 0 4 km Vashon Island (Figs. 6 and 8). Stratigraphic units Evidence from Seismic-Reflection Data. and/or facies exposed along shorelines are appar- Fault 8 extends northward in western Puget ently juxtaposed across the projected trace on Sound from offshore of northern Bainbridge Is- Landslides with length along coastline both a local and island scale. The east-west dis- land to the Kitsap Peninsula (Fig. 6). On an in- >500 m tribution of stratigraphic units suggests west- dustry profile (Fig. 7C), fault 8 is characterized Strata younger than Possession drift (<~28 000 ka) side-up displacement. The amount of possible by truncated and warped reflections, local dip vertical displacement cannot be easily deter- changes, and an upward divergence of splays, Strata older than interglacial Olympia beds (>~28 000 ka) mined from these data because of probable strati- and propagates up into the lower part of the Qua- graphic complexity (e.g., large-scale glacial ternary section. On high-resolution seismic-re- scour and fill). However, Booth (1991) noted flection profiles (e.g., line P338, Fig. 7B), fault 8 Figure 8. Map of Vashon Island (Fig. 1) about 100 m of relief from east to west on the is characterized by two splays that cut and warp showing inferred north-trending fault zone base of the ca. 15–18 ka Esperance Sand along Quaternary reflections and cause juxtaposition of (faults 1, 2, 3, and 6; Fig. 6) and distribution of contours subparallel to the projected fault trend. reflections with contrasting reflection geometry geologic units exposed along the shoreline or The gentle folding and small-scale structural dis- and amplitude. Fault 8 must end north of the lo- at the base of coastal bluffs based on Booth ruption (faults and fractures) noted in strata adja- cation of two high-resolution seismic profiles off- (1991, Fig. 3). Estimated ages based on Blunt cent to the projected fault trace are uncommon shore of northeastern Bainbridge Island, on et al. (1987). FC—Fern Cove; QH—Quarter- within Quaternary exposures along Puget Sound which the fault was not recognized. master Harbor; VH—Vashon Heights. and suggest that shearing and deformation asso- Investigation of Land Exposures. North of ciated with the fault may be distributed across the seismic profiles shown in Figure 7 (B and C), several hundred meters. fault 8 projects onshore into the mouth of a 500- stratigraphic contrasts across this covered area Alternatively, the east-west contrast in relative m-wide, alluvium-filled, north-trending valley on are consistent with but do not demand the pres- ages of shoreline exposures could reflect large- the Kitsap Peninsula (Fig. 6). The fault trace is ence of a fault at this locality. Fault 8 thus has a scale scour and fill associated with repeated covered, and strata exposed in shoreline bluffs in minimum length of ~4–5 km. Puget Sound glaciations (Fig. 3), and some of the the undated, pre-Vashon (Fig. 3) Quaternary sec- anomalous structural deformation could have a tion on the eastern and western flanks of the val- DISCUSSION glaciotectonic origin (e.g., Aber et al., 1989). ley are different (Yount et al., 1993). The ~15-m- Thus, geologic data on Vashon Island are consis- thick section on the east flank of the valley Seismic-reflection data indicate that a zone of tent with, but do not absolutely demand, continu- consists of an ~3 m-thick-lower unit of flat-bed- en echelon, north-trending, high-angle faults is ation of fault 6 to the south. ded sandy and silty glaciolacustrine deposits and present in central Puget Sound. These faults trun- an overlying unit (~12 m thick) of massive to lo- cate and warp reflections, commonly juxtapose Fault 7 cally crudely stratified pebbly till. The lower unit strata with different dips and contrasting seismic has a variable west dip that is typically <5° and facies, and display some evidence of dip reversal In Figure 7D, a northwest-trending industry the contact between the two units is locally an an- (Fig. 7I). These characteristics typify oblique profile (Fig. 2B), fault 7 is characterized by trun- gular unconformity. On the west flank of the val- strike-slip faults (e.g., Harding et al., 1983), con- cations and abrupt dip change (steeper east dips ley, gently west-dipping (<5°), ~20-m-thick Qua- sistent with the dextral displacement of piercing east of fault, flat dips west of fault) in Tertiary ternary strata exposed in coastal bluffs consist of points in the Seattle fault zone. strata, but does not obviously penetrate the Qua- a lower unit (5–8 m thick) of flat- and cross-strat- Faults 1 and 6 are inferred to be the longest ternary section. Two high-resolution seismic pro- ified pebbly outwash and an upper unit (12–15 m and most significant structures. An extensional files from the same area (Fig. 2A) as this industry thick) of sandy outwash. The gentle folding and stepover (Aydin and Nur, 1985) between these

1050 Geological Society of America Bulletin, July 1999

faults (area of overlap between northern Vashon Seattle uplift and the Eocene to Quaternary Seat- timation of the amount of relative subsidence of Island and the southern Seattle fault zone) and as- tle basin (Figs. 1, 5, and 6). The northern part of growth strata. However, the amount of material sociated normal faulting may be partly responsi- the Seattle uplift (hanging wall of fault A) locally eroded off the fold crest is probably less than a ble for the significant relief (~500 m) on the base includes steeply north dipping (70°–80°) alluvial- few hundred meters. of the Quaternary within the Seattle uplift from fan and fluvial deposits of the Miocene Blakely 2. The rate estimate only considers slip on fault the center of Puget Sound to its east and west Harbor Formation (Fulmer, 1975; Yount and A within the fault zone, but seismic-reflection flanks (Fig. 6). At its southern end, our data sug- Gower, 1991). Interpretation of industry seismic- data (Fig. 5, ) suggest that faults B and C were gest that offset on fault 1 may transfer to parallel reflection data indicates this unit is 2–3 km thick also active (although in lesser amounts) in Qua- structures to the east (faults 2 and 3). To the north, in the Seattle basin (Johnson et al., 1994; Pratt ternary time. we think it likely that faults 1 and 4 are con- et al., 1997). B. Sherrod and J. A. Vance (1996, 3. The assumption that the base of the Qua- nected, despite the lack of evidence on one cross- oral commun.) obtained a 13.4 Ma fission-track ternary section is 1.9 Ma and has the same age ing seismic profile. date on a tuff from the apparent base of the across Puget Sound is probably not valid. For On the west flank of Puget Sound, the align- Blakely Harbor Formation. Assuming even ex- example, the bases of the Pleistocene and latest ment of discontinuous faults 6, 5, 7, and 8 provides tremely high sediment-accumulation rates for Pleistocene to Holocene sections in central an indication that these structures may have once foreland basins (600 m/m.y.; Johnson, 1985, Puget Sound are at depths of about 1100 and formed a continuous fault. In this scenario, this Table 1), the top of the Blakely Harbor Formation 600 m, respectively. The depth to the base of the continuous fault may have become fragmented should be no older than about 10 Ma. This maxi- latest Pleistocene to Holocene section reflects with increasing transfer of offset to structures in mum inferred age for the top of the Blakely Har- the amount of subglacial erosion (Booth, 1994) eastern Puget Sound. The coincidence of these bor Formation provides a maximum age for initi- associated with the most recent of at least six structures with the prominent bathymetric linea- ation of fault A. Bucknam et al. (1992) showed Quaternary glaciations in Puget Sound (Fig. 3; ment on the west flank of the central Puget Sound that significant uplift occurred during a large Easterbrook, 1994a, 1994b). Assuming compa- trough (Fig. 4) suggests that this discontinuous earthquake along fault A of the Seattle fault ca. rable amounts of erosion in central Puget Sound fault trend has provided an ongoing control on the A.D. 900. Thus, fault A is still active. during the earlier glaciations and uniform subsi- location of glaciofluvial channels and erosion. Seismic-reflection data can be used to estimate dence and sediment-accumulation rates, a Qua- The north-trending faults in central Puget rates of slip on fault A by determining the dimen- ternary section would not begin to be preserved Sound appear to be discrete structures at the sur- sions and geometry of the folded basal Quater- in central Puget Sound until about 1 Ma. This face but are probably connected at depth. This nary reflections at the southern margin of the hypothesis cannot be tested without drilling, but zone of faults is a few kilometers west of and par- Seattle basin. This information can then be used depends on geologically realistic criteria and we allel to the Coast Range Boundary fault, an in- to calculate the horizontal (shortening) and verti- regard it as more likely than a model in which ferred regional crustal block boundary (Fig. 1; cal (relative subsidence) components of struc- the lowest Quaternary strata in the central Puget Johnson et al., 1996), and may be rooted within tural growth (e.g., Schneider et al., 1996, Fig. 10), Sound trough are ca. 1.9 Ma. Assuming an age that contact. In that scenario, this north-trending and the amount of displacement required to pro- of 1 Ma for the base of Quaternary section in zone of strike-slip and normal faulting represents duce this growth. This method is best applied in central Puget Sound yields slip rates of about a portion of a regional distributed shear zone central Puget Sound, where growth elements are 0.8 to 0.9 mm/yr on fault A. The differences in along which the Washington Coast Range is buried by younger strata. Assuming mean veloc- structure and structural relief of Quaternary moving northward relative to the eastern Puget ities in the Pleistocene section of 2000 m/s for strata adjacent to fault A across the Puget Low- Lowland and Cascade Range. line P346 (Fig. 5D), the vertical and horizontal land may thus primarily reflect the age of in- These faults must be considered active or po- components of growth are 640 and 550 m, re- volved strata. tentially active because (1) almost all profiles spectively, and inferred Quaternary fault dis- Fault A of the Seattle fault zone therefore has show the faults extending upward into the Quater- placement is 850 m on a plane dipping 49°. As- a minimum Quaternary slip rate of ~0.6 mm/yr. nary section and (2) these faults displace the suming an age of 1.9 Ma for the base of the Other geologic factors suggest that the actual northernmost active strand of the Seattle fault. Fo- Quaternary section yields a slip rate of about rate for the entire zone is higher, probably about cal mechanisms from the central Puget Lowland 0.4–0.5 mm/yr. 0.7–1.1 mm/yr, significantly higher than that in- indicate a mix of thrust, strike-slip, and normal This method can be applied more loosely in ferred by Pratt et al. (1997; 0.25 mm/yr) for the faulting (Ma et al., 1996), consistent with the eastern and western Puget Sound (Fig. 5, C and 40 m.y. history of the fault. These estimates are complex pattern of faulting shown in Figure 6. E), where the amount of structural relief across not contradictory, but rather indicate variable The north-trending zone extends up the axis of the fault A (900 and 825 m, respectively) provides a slip rates through time. The inferred Quaternary Puget Lowland trough (Fig. 6) and has probably minimum estimate of Quaternary relative subsid- rate suggests that there should be 11.9–18.7 m of provided an element of structural control (zones ence. Assuming a fault dip similar to that for cen- displacement on fault A in the past ~17 k.y., of lithologic weakness more erodible than adja- tral Puget Sound yields Quaternary displace- which, assuming a 49° dip, would yield 9.0–14.1 cent blocks) in the development of Puget Sound. ments of about 1190 and 1095 m, and minimum m of uplift. This amount of uplift is consistent Quaternary slip rates (assumes base of Quater- with Thorson (1996, Fig. 6), who used the con- AGE AND RATES OF FAULT SLIP nary is 1.9 Ma) of about 0.5 to 0.7 mm/yr. tact between the Lawton Clay and Esperance Three problems with this method, all of which Sand (Fig. 3) as a horizontal datum for estimat- Seattle Fault Zone would lead to an underestimation of slip rates, are ing 8–14 m of post-17 ka uplift across the Seat- discussed as follows. tle fault at Alki Point (Fig. 2A). We regard the Stratigraphic and structural relationships indi- 1. On the Puget Sound seismic-reflection pro- resolution on this datum as far more reliable than cate that recent deformation in the Seattle fault files (Fig. 5, C, D, and E), there is an erosional the other potential datums described by Thorson zone is greatest on fault A, the northernmost unconformity within the Quaternary section at (1996) that are based on strandlines of widely splay. This fault forms the boundary between the the crest of the anticline, which leads to underes- spaced proglacial lakes.

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North-Trending Fault Zone (Bucknam et al., 1992) and in the Duwamish mentation could limit the ruptured area in some River Valley along the eastern fault segment Seattle fault earthquakes, a large event ca. A.D. The north-trending strike-slip zone cuts Qua- (J. A. Boughner and B. F. Atwater, 1996, oral com- 900 appears to have involved both segments. Fu- ternary deposits and displaces the youngest splay mun.). Although radiocarbon dating does not pro- ture regional and local earthquake hazard assess- of the Seattle fault (fault A), and must therefore vide the resolution to conclusively determine if ments must incorporate the new information on be considered potentially active. It must also be this uplift occurred as one large event or two fault location, length, geometry, and slip rate re- considered younger than 10 Ma, described here closely spaced events on different fault segments, ported here. as the maximum age for initiation of fault A. the relationships between fault displacement Treated as a whole, cumulative displacement and fault-rupture length defined by Wells and ACKNOWLEDGMENTS across this zone using fault A as a piercing point Coppersmith (1994, Figs. 12 and 13) support a is about 2.4 km. If fault A and the north-trending lengthy fault rupture and the single-event hypoth- This work was funded jointly by Earthquake strike-slip zone were each initiated at 10 Ma, then esis. Rupture across segment boundaries is also Hazards Reduction Program and the Coastal and the cumulative slip on the north-trending zone is consistent with Rubin (1996), who showed that Marine Geology Program of the U.S. Geological about 0.2–0.3 mm/yr. However, the similarity in historic earthquakes generated by intracontinental Survey. We thank Guy Cochrane, Kevin O’Toole, fault A style and geometry from Lake Washing- reverse faults commonly rupture across surface Larry Kooker, Fred Payne, Curtis Lind, and the ton to Dyes Inlet (Fig. 5) suggests that fault A discontinuities such as stepovers and cross faults. captain and crew of the MV Robert Gray for help may have originated and evolved as an unbroken Rubin (1996) concluded that making assumptions in data collection; John Miller and William structure for much of its history. Thus, the cumu- that seismic rupture will initiate and terminate at Stephenson for help in data processing; and lative rate of slip on the strike-slip zone could be major geometric irregularities can lead to an un- Susan Rhea for GIS support. We thank Mobil, 0.5–1.0 mm/yr (for initiation at 5 and 2.5 Ma, re- derestimation of the magnitudes of future large Chevron, and Western Geophysical for providing spectively) or even higher. earthquakes. Conversely, the possibility of smaller, industry seismic-reflection data. We benefited more frequent earthquakes on the Seattle fault from many discussions with John Armentrout, IMPLICATIONS FOR EARTHQUAKE zone with rupture limited to single fault segments Brian Atwater, Rick Blakely, Derek Booth, HAZARDS cannot be ruled out. Joanne Bourgeois, Tom Brocher, David Dethier, Seismic hazard assessments in western Wash- Don Easterbrook, Judith Boughner, Robert The Seattle fault zone and the crosscutting ington must now also incorporate the north- Bucknam, Carol Finn, Arthur Frankel, Ralph north-trending strike-slip zone cut through a ma- trending fault zone in Puget Sound as a potential Haugerud, Mark Holmes, Robert Karlin, William jor urban center, so an understanding of the earth- earthquake source. Using the length for fault 1 Lingley, David Perkins, Christopher Potter, quake hazard is imperative. This investigation defined here and relationships between rupture Thomas Pratt, Brian Sherrod, William Stephen- provides new constraints on the location, lengths, length and moment magnitude (M) defined by son, Timothy Walsh, Craig Weaver, Ray Wells, slip rates, and evolution of local crustal faults that Wells and Coppersmith (1994, Fig. 9), earth- Thomas Yelin, and James Yount. Michael Maler, can be incorporated in new seismic hazard maps quakes of M ~6.5 are possible on this structure. Thomas Pratt, John Stamatakos, and William and assessments (e.g., Frankel et al., 1996). Our Larger events could occur if the strands of this Stephenson provided stimulating reviews. data are most consistent with a slip rate of fault zone connect at depth. 0.7–1.1 mm/yr for the Seattle fault, but provide REFERENCES CITED no information about recurrence interval. If the CONCLUSIONS well-documented ca. A.D. 900 earthquake Aber, J. S., Croot, D. G., and Fenton, M. M., eds., 1989, Glacio- tectonic landforms and structures: Dordrecht, Kluwer (Bucknam et al., 1992), which produced wide- Marine high-resolution, seismic-reflection Academic Publishers, 200 p. spread uplift of ~5–7 m, is characteristic of a reg- data reveal that the Seattle fault forms a 4–6-km- Atwater, B. F., and Moore, A. L., 1992, A tsunami about 1000 years ago in Puget Sound, Washington: Science, v. 258, ular recurrence interval, then the next large earth- wide, west-trending zone of three or more south- p. 1614–1617. quake on the Seattle fault may not occur for dipping reverse faults. The fault zone can be Aydin,A., and Nur, A., 1985, The types and role of stepovers in several thousand years. However, there are no mapped in waterways across the Puget Lowland strike-slip tectonics, in Biddle, K. T., and Christie-Blick, N., eds., Strike-slip deformation, basin formation, and data that argue for or against characteristic earth- for at least 40 km from Dyes Inlet to Lake Wash- sedimentation: Society of Economic Paleontologists and quakes on the Seattle fault, and Thorson (1996) ington. The fault zone was not recognized in Mineralogists Special Publication 37, p. 35–44. argued that the effects of glacial loading and un- Hood Canal. Quaternary sediments have been Blunt, D. J., Easterbrook, D. J., and Rutter, N. W., 1987, Chronology of Pleistocene sediments in the Puget Low- loading may have significantly disturbed the folded and faulted along all faults in the zone. land, Washington: Washington Division of Geology and earthquake cycle in the Puget Lowland. Thus, a Deformation is clearly most pronounced along Earth Resources Bulletin, v. 77, p. 321–353. Booth, D. B., 1991, Geologic map of Vashon and Maury Is- prediction of the timing of the next large event on the northernmost fault (fault A), which forms the lands, King County, Washington: U.S. Geological Survey the Seattle fault is impossible at this time. The es- boundary between the Seattle uplift and Seattle Map MF 2161, scale 1:24 000. timated slip rates suggest that the accumulated basin. Analysis of growth strata deposited across Booth, D. 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W., 1994, Late Mesozoic and possible early Tertiary REVISED MANUSCRIPT RECEIVED SEPTEMBER 1, 1998 ogy, v. 22, p. 71–74. accretion in western Washington state: The Helena- MANUSCRIPT ACCEPTED SEPTEMBER 23, 1998

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