Seismic Stability of the Duwamish River Delta, 

Seattle, Washington

Professional Paper 1661-E

U.S. Department of the Interior

U.S. Geological Survey
Other than this note, this page left intentionally blank.
Earthquake Hazards of the Pacific Northwest Coastal and Marine Regions

Robert Kayen, Editor

Seismic Stability of the Duwamish River Delta, 

Seattle, Washington

By Robert E. Kayen and Walter A. Barnhardt

The delta front of the Duwamish River valley near Elliott Bay and Harbor Island is 

founded on young Holocene deposits shaped by sea-level rise, episodic volcanism, and 

seismicity. These river-mouth deposits are highly susceptible to seismic soil liquefaction and are potentially prone to submarine landsliding and disintegrative flow failure.

Professional Paper 1661-E

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

This report and any updates to it are available at:

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Manuscript approved for publication, May 29, 2007

Text edited by Peter Stauffer

Layout by David R. Jones

Suggested citation:

Kayen, R.E., and Barnhardt, W.A., 2007, Seismic stability of the Duwamish River delta, Seattle, Washington: 

U.S. Geological Survey Professional Paper 1661-E, 11 p.

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FRONT COVER

Color shaded-relief bathymetry of the Duwamish delta in Elliott Bay, offshore Seattle, Washington. Contour interval 20 m. 

Onshore part of image is from a U.S. Geological Survey digital orthophotoquad (DOQ).
iii

Contents

Abstract...........................................................................................................................................................1

Introduction and geologic setting...............................................................................................................1

Sediment supply.............................................................................................................................................1

Earthquakes....................................................................................................................................................3

Triggering seismic soil liquefaction............................................................................................................3

Disintegrative flow failure potential ...........................................................................................................5

Discussion and conclusions ........................................................................................................................9

Acknowledgments.........................................................................................................................................9

References......................................................................................................................................................9

Figures

1. Map of the Puget Sound region, showing locations of major fault zones 

and three major river deltas ....................................................................................................................2

2. Model of Holocene evolution of the Duwamish delta and the influence 

of episodic volcanism...............................................................................................................................3

3. Diagram of idealized structure of the Duwamish River delta at Elliott Bay 

before industrial development ................................................................................................................4

4. Diagrams showing assessment of initial liquefaction using earthquake-induced 

cyclic stress or Arias intensity................................................................................................................5

5. Geographic Information Systems (GIS) plots showing the distribution of factor 

of safety interpolated from seven boreholes arrayed on the Duwamish River 

delta at Elliott Bay......................................................................................................................................6

6. Diagrams illustrating the behavior of soils, after liquefaction during an earthquake, 

under different conditions of stress, strain, and porosity..................................................................... 7

7. Cross-sectional diagram showing state analysis of the Duwamish delta front 

along the B1-B7 profile parallel to shore. .............................................................................................8
Other than this note, this page left intentionally blank.
By Robert E. Kayen1 and Walter A. Barnhardt2

Abstract

A highly developed commercial-industrial corridor, 

extending from the City of Kent to the Elliott Bay/Harbor 

Island marine terminal facilities, is founded on the young 

Holocene deposits of the Duwamish River valley. The deposits 

of this Holocene delta have been shaped not only by relative 

sea-level rise but also by episodic volcanism and seismicity. 

Ground-penetrating radar (GPR), cores, in situ testing, and 

outcrops are being used to examine the delta stratigraphy and 

to infer how these deposits will respond to future volcanic 

eruptions and earthquakes in the region. A geotechnical investigation of these river-mouth deposits indicates high initial 

liquefaction susceptibility during earthquakes, and possibly 

the potential for unlimited-strain disintegrative flow failure of 

the delta front.

Introduction and Geologic Setting

This paper examines the geomorphology and seismic 

stability of the Duwamish River delta at the Port of Seattle in 

western Washington State. This river, and those forming the 

two principal Holocene deltas to the south, the Puyallup and 

Nisqually Rivers, originate on the slopes of Mount Rainier, 

a large and active stratovolcano of the Cascade Range, and 

flow into southern Puget Sound (fig. 1). Fluvial sediment 

derived from extensive drainages in this volcanic terrain has 

formed thick young deltaic deposits, which record a complex 

sequence of valley fill. Our main objective is to determine 

how volcanoes and earthquakes have affected the evolution of 

Holocene deltas in Puget Sound and to evaluate the potential 

for the natural deposits that comprise the Duwamish River 

delta to undergo initial liquefaction and associated large-strain 

flow failure.

The development of river systems in the Puget Lowland 

has been strongly influenced by the preexisting topography 

and tectonic setting of the region. Multiple glaciations in Quaternary time greatly modified the landscape, covering it with 

thick glacigenic deposits and sculpting deep, linear troughs 

1

 Menlo Park, California

2

 Woods Hole, Massachusetts

comprising many present-day lakes and the numerous arms of 

Puget Sound (Booth, 1994). Subaerial segments of the glacial 

valleys are now occupied by rivers, which deliver abundant 

sediment to the heads of laterally constrained marine embayments. Holocene deltas in these embayments became choice 

sites for industry and commerce in Seattle and Tacoma.

The Duwamish River, before engineering modifications 

at the start of the 20th century, was a natural distributary 

channel of the Cedar and Green Rivers, as well as at times 

the White River. These rivers originate on the flanks Mount 

Rainier, a large active stratovolcano of the Cascade Range. 

After deglaciation, a fjord canyon occupied the present location of the Duwamish River valley. This was rapidly infilled 

since the mid-Holocene, largely by sediment derived from 

Mount Rainier. Episodic volcaniclastic debris flows (lahars) 

from Mount Rainier caused the delta front to prograde rapidly 

towards its present position at the southeast end of Elliott Bay 

(fig. 1).

The nearby city of Vancouver, British Columbia, lies in 

a similar deltaic setting. Although the deltas suffered little 

damage during deep earthquakes that struck in the middle 

1900s, they may be subject to greater damage from recently 

discovered sources of earthquakes in the Puget Sound area. 

Chief among these additional sources are the Seattle Fault, 

which directly underlies the Duwamish delta (fig. 1), and the 

boundary between the subducting Juan de Fuca Plate and the 

overriding North America Plate, which is at a depth of about 

50 km beneath the City of Seattle.

Sediment Supply

Abundant sediment supply from Mount Rainier, including lahars (volcanic debris flows), has fueled rapid progradation of Holocene deltas in southern Puget Sound (Dragovich 

and others, 1994; Barnhardt, and others, 1998). Landslides and 

debris flows, not necessarily attended by eruptions, produce 

lahars that occasionally choke river channels and blanket valley bottoms with sediment (Scott and others, 1995). The largest known of these events was the Osceola Mudflow (Crandell, 

1971; Vallance & Scott, 1997), which occurred approximately 

5,700 years ago and covered 505 km2 of the Puget Lowland 

with cohesive lahar deposits up to 31 m thick. On the basis of 

numerous soil borings, Dragovich and others (1994) traced 

Seismic Stability of the Duwamish River Delta, 

Seattle, Washington
2 Seismic Stability of the Duwamish River Delta, Seattle, Washington

Figure 1. Map of the Puget Sound region, 

showing locations of major faults and 

three major river deltas. The Duwamish 

River delta is located at Elliot Bay in the 

City of Seattle and is underlain by the 

Seattle Fault Zone.

the Osceola Mudflow within Holocene valley fill. Using the 

distinctive mudflow deposits as a stratigraphic marker, they 

estimated that the Duwamish and Puyallup deltas have grown 

seaward 51 and 14 km, respectively, since that time (fig. 2). 

These high progradation rates (9.0 and 2.5 m/yr, respectively) 

are similar to that of the Fraser River delta (2.4–6.5 m/yr), 

which for much of its history was also confined to a valley of 

limited width (Williams and Roberts, 1989).

Other middle to late Holocene lahars also contributed 

abundant sediment to the Duwamish, Puyallup, and Nisqually 

Rivers (Scott and others, 1995). These sandy lahar deposits 

have a strong influence on groundwater and engineering properties of the valley soils, yet their full extent is poorly known. 

For example, deposits of massive, coarse sand that occur in 

the Puyallup Valley clearly originated as lahars (Pringle and 

Palmer, 1992). The sands consist of angular grains having a 

volcanic provenance, based on mineralogy and the presence 

of scoria, pumice, and lapilli. Small twigs from the lahar sand 

deposit yielded an uncorrected age of 2,320 ±120 14C yr BP 

(Palmer and others, 1991), which is time correlative with 

a block-and-ash-flow deposit in the upper Puyallup valley 

(Crandell, 1971). Cores and trench exposures 65 km downstream of Mount Rainier indicate this lahar flood deposit is 

as much as 7.5m thick and represents a significant volume of 

rapidly deposited sediment (Palmer and others, 1991; Pringle 

and Palmer, 1992).

MAP

AREA

British

Columbia

WA

OR

Seaward edge

of subduction

zone

Elliott

Bay

Harbor 

Island

Duwamish delta 

(Port of Seattle)

Puyallup

Seattle

Fault Zone

Nisqually delta

Puyallup delta

(Port of Tacoma)

Bainbridge

Island

Devils Mountain

Fault Zone

SEATTLE

Tacoma

Duwamish River

Mount Rainier

Olympia

Auburn

Puyallup River

Nisqually River

Lower Green River

Green River

White River

Cedar River

Tacoma

Fault Zone

Puget S

o

u

n

d

Pacific

Ocea

n

0 20 KILOMETERS

47°

48°

123° 122°
Earthquakes 3

Earthquakes

Although the central Puget Sound region has been historically quiescent, abundant evidence exists for a large earthquake there about 1,000 years ago. A raised marine platform 

on Bainbridge Island, 5 km west of Seattle (fig. 1), indicates 

7m of sudden, coseismic uplift during this event (Bucknam 

and others, 1992). In addition, subaqueous landslides (Jacoby 

and others 1992; Karlin and Abella, 1992) and tsunamis 

(Atwater and Moore, 1992) are apparently related to prehistoric movement along the Seattle Fault Zone, a major reverse 

fault that passes directly beneath the Duwamish delta and 

downtown Seattle (Johnson and others, 1994; fig. 3). Tectonic 

displacement along this fault is also suggested by uplifted, 

shell-rich deposits that are observed several meters above 

the high-tide level (Campbell, 1981). These raised estuarine 

sediments are capped with a thick layer (locally greater than 

3 m) of black, well-sorted sand (Campbell, 1981). Pringle and 

others (1997) inferred a volcanic origin (reworked lahars) for 

these surficial deposits of clean sand, which probably correlate 

with upstream deposits of the Deadman Flat lahar assemblage 

(~1,000 14C yr BP). These deposits are displayed at the surface 

by swarms of liquefaction dikes from prehistoric ground 

failure in the Duwamish along the Duwamish waterway at and 

near Kellogg Island. The dikes intrude tidal-marsh deposits 

that postdate the uplift along the Seattle Fault 1,000 years ago. 

Some of the dikes can be traced upward to lenses of silty sand 

that are probably the flanks of vented-sand volcanoes (sand 

boils) erupted from the dikes. The largest of the dikes is as 

much as 18 cm wide and at least 6m long. They contain dark, 

fine to medium sand that becomes finer grained upwards and 

resembles sand derived from Mount Rainier.

Other known sources of strong motion for the Puget 

Lowland capable of triggering ground failures in the sediment 

of the Duwamish River include the Tacoma Fault to the south, 

and the Cascadia megathrust (Brocher and others, 2000, 2001). 

Tensional faulting within the downgoing slab of the Casadia 

megathrust was the source of theM 6.8 Nisqually earthquake 

event of February 28, 2001 (Bray and others, 2001; Filiatrault 

and others, 2001; Nisqually Earthquake Clearinghouse Group, 

2001). Considerably further away from, and northward of, the 

Duwamish Valley is the Devils Mountain Fault Zone (Johnson 

and others, 2001).

Triggering Seismic Soil Liquefaction

Liquefaction of deltaic deposits of the Duwamish River 

valley during earthquakes of moderate to large magnitude 

can have severely adverse affects on structures and lifelines. 

Liquefaction damage often leads to separation of pipeline 

conduits, settlement of foundations, bearing-capacity failure, 

lateral movement of ground and bridge piers, and uplift of 

storage tanks and other positively buoyant structures. Initial 

liquefaction, the transient increase in pore-water pressures 

and corresponding loss of effective confining stress and soil 

strength, results in soil behaving in a fluidlike manner, with 

the potential for large strain deformation (disintegrative flow 

failure). In some cases, disintegrative flow failure of ground 

following initial liquefaction has led to catastrophic landslides 

on sloping land, embankments, and earth dams.

To characterize the geotechnical properties of the subsurface delta, standard penetration test (SPT) and cone penetration test (CPT) data are used. Following the guidelines of Seed 

and his colleagues (1983, 1984), Cetin and others (2004), 

and Moss and others (2006). We use the effective-stressnormalized, 60-percent energy efficiency standard penetration 

resistance (N1)60 for liquefaction assessment. In our study, 

we currently use SPT and CPT data made available by others working on local site investigations of Duwamish River 

5

Present

Slow

growth

4

~2,300 and 1,000 yr B.P.

(two largest volcanic events)

Rapid

growth

3

Mid to Late Holocene

(post-Osceola Mudflow)

Former

position

Slow

growth

2

~5,700 yr B.P.

(Osceola Mudflow)

Rapid

growth

1

Early Holocene

(pre-Osceola Mudflow)

Marine

embayment

Puget

Sound

Ancient

delta

Mount

Rainier

Figure 2. Model of Holocene evolution of the Duwamish delta and the influence of episodic volcanism.
4 Seismic Stability of the Duwamish River Delta, Seattle, Washington

soils (unpublished Report 3528 by Hart Crowser, Inc., 1910 

Fairview Avenue East, Seattle, WA 98102-3699, 1993).

Liquefaction typically occurs in sandy, water-saturated 

soils such as flood-plain deposits, delta deposits, alluvial 

sediments, and landfill. Much of the worst damage from the 

1989 Loma Prieta, California, earthquake and the 1995 Kobe, 

Japan, earthquake resulted from settlement and collapse of 

port facilities where the soil beneath them liquefied. During 

earthquake shaking, water pressure in tiny spaces, or pores, 

within the soil is elevated temporarily, commonly for only a 

matter of seconds or minutes. These high pore pressures can 

fully negate interparticle contact stresses, resulting in a loss 

of strength that can potentially cause the soil to flow like a 

liquid. Soil that had previously supported large structures 

cannot support them when it is in a liquefied state. The cause 

of these excess pore pressures during shearing is the tendency 

towards fabric collapse and volumetric reduction of pore 

space in loose soils. Fabric collapse of the soil grain skeleton 

loads the pore water of the soil interstices. If the collapse 

occurs more rapidly than the soil can dissipate excess water 

pressures, as in the case of earthquake shaking, water pressure can rise while the corresponding particle contact stresses 

proportionally drop. In contrast, dense soils tend either to not 

generate high excess pore water pressures, or to rapidly regain 

strength with minor levels of deformation. Initial liquefaction 

(National Research Council, 1985; Youd and others, 2001) 

occurs when elevated pore pressures, in excess of hydrostatic, 

reduce the effective stresses acting on an undeformed cohesionless soil mass to zero.

The liquefaction potential of a soil deposit tends to 

decrease with increasing age of the deposit, degree of particle 

cementation, and depth to the water table. Each of the above 

factors tends to densify the soil structure, increase rigidity, 

and reduce the potential for generation of high pore pressure 

(Youd and Perkins, 1978). At the Duwamish River delta, a 

preponderance of factors, including extremely young and 

thick deposits of sand, rapid deposition, and high water table, 

have resulted in the formation of a loose deposit that is highly 

susceptible to generation of excess pore water pressure during 

earthquakes.

For the assessment of initial liquefaction potential at the 

delta front we used SPT data collected at each of the soil boring sites offshore Harbor Island. These measurements, a proxy 

for soil residual strength, were then related to measures of the 

intensity of earthquake shaking likely to affect the delta front. 

Two approaches were used to assess the intensity of seismic 

shaking at the study sites: (1) the conventional cyclic stress 

method (CSR) of Seed and others (1984) and Cetin and others 

(2004) and (2) an approach using the Arias intensity of ground 

motion of Kayen and Mitchell (1997). The first method 

Figure 3. Idealized structure of the Duwamish River delta at Elliott Bay before industrial development 

(vertical exaggeration ~20x). The Seattle Fault here is blind, not exposed.

Upland

(till, glacial outwash)

Oxbow Buried

channel

River Uplifted estuarine deposits

Seattle Fault Zone

Pleistocene

Delta

front Holocene

Tertiary

D2

D2

D2

D1

D1

Point-bar and channel-fill deposits

Salt marsh, tidal flat

Deltaic deposits of different ages, 

formed at different sea levels

Osceola Mudflow

N 0 2 KILOMETERS
Disintegrative Flow Failure Potential 5

employed a simplified uniform cyclic stress ratio to represent 

the complex and irregular earthquake-induced stress-time 

history. The second approach employed a quantitative measure of earthquake shaking intensity, termed Arias intensity. 

The Arias intensity measure is the sum of the energy-per-unit 

weight absorbed by an evenly spaced population of idealized 

undamped simple oscillators in response to the earthquake 

motion (Kayen and Mitchell, 1997).

In assessment of seismic soil liquefaction triggering, we 

associate CSR and Arias intensity (Ihb) profiles with the SPT 

liquefaction field performance data, (N1)60, from the soil boring sites using two SPT-based boundary curve correlations for 

the assessment of soil liquefaction potential (fig. 4). The CSR 

and Ihb at the liquefaction boundary in figure 4 represent the 

minimum threshold seismic intensity required to induce liquefaction in soil. The CSR curves presented in figure 4A show 

the probability (likelihood) of liquefaction for a magnitude 

7.5 earthquake for sandy soil with 5 percent or less silt and 

clay particles. We compared the minimum level of shaking 

intensity needed to liquefy soil of a given (N1)60 with the likely 

intensity of shaking induced by an earthquake affecting the 

Duwamish River delta to determine the factor of safety against 

initial liquefaction. Computed factors of safety in excess of 

1.0 indicate liquefaction is unlikely given the design-basis 

earthquake, whereas values equal to or below 1.0 indicate 

that liquefaction is likely. Results from the stress-based and 

Arias-intensity-based analysis are presented here as crosssections along the line of cores (fig. 5). For aM 7+ earthquake, 

with accelerations of 0.2 g and greater, there is a broad zone 

extending to a subbottom depth of approximately 20m that is 

predicted to liquefy. TheM 6.8 Nisqually earthquake of February 28, 2001, produced accelerations of approximately 0.2 g

on Harbor Island, resulting in numerous liquefaction related 

features on the surface, ground settlement and lateral displacements (Bray and others, 2001). Given the proximity of the 

delta front to the Seattle Fault, which passes directly beneath 

it (Johnson and others, 1994), and other sources, the potential 

for surface ground motions to reach above the 0.2-g range of 

the Nisqually earthquake event is considered to be guaranteed. 

The 1996 Pacific Northwest map of 2-percent exceedance 

probability for peak acceleration in the next 50 years, developed by the U.S. Geological Survey National Seismic Hazards 

Mapping Project, predicts accelerations at approximately a 

0.6-g level for the Duwamish Delta. 

Disintegrative Flow Failure Potential

Liquefaction susceptibility analyses, like those of Seed 

and others (1983), Cetin and others (2004), and Kayen and 

Mitchell (1997), predict the conditions of transient porepressure rise and initial liquefaction, and the breakdown of 

soil structure to a residual strength state. However, other 

means must be used to shed light on how a sediment mass will 

behave after initial liquefaction occurs. Specifically, what is 

the potential for transformation of earthquake-induced liquefied material into a disintegrative flow?

The type of mass movement that occurs following the initial liquefaction involves a potential continuum of mechanical 

behavior ranging from limited strain-hardening deformation to 

unlimited viscous flow. Whitman (1985) described two ideal 

behavioral states, disintegrative flow failure and nondisintegrative failure. Disintegrative flow failure is a condition in which 

a sediment mass deforms continuously under a static shear 

stress (downslope gravitational stress in this case) that equals 

or exceeds the resistive residual shear strength of the sediment. 

This sediment shear resistance at high strains, Sr, has been 

CYCLIC STRESS RATIO, FOR = 1 TON PER SQUARE FOOT 

ARIAS INTENSITY, IN METERS PER SECOND

0.0 0.1

0.1

0.2

0.3

0.4

0.5

0.6

PL

0 10 20 30

MODIFIED PENETRATION RESISTANCE, N1, IN BLOWS PER FOOT

0 10 20 30 40

1

5 A B

LIQUEFACTION

LiquifiedMarginal Non-liquified

 Pre-1985 data

“New” data

LIQUEFACTION

NO LIQUEFACTION

NO LIQUEFACTION

80% 20%

95% 50% 5%

80% 20%

95% 50% 5%

Figure 4. Initial liquefaction, the complete loss 

of cyclic strength and effective confining stress 

caused by pore-pressure rise at zero strain, can 

be assessed using field data and an estimated 

measure of either (A) the earthquake-induced 

cyclic stress (Cetin and others, 2004) or (B) Arias 

intensity (Kayen and Mitchell, 1997). Both plots 

are presented for sand with fines content of 5 

percent or less. Figure 4A presents curves that 

show the probability of liquefaction occurrence 

(PL) between 5 and 95 percent.
6 Seismic Stability of the Duwamish River Delta, Seattle, Washington

Figure 5. Geographic Information Systems (GIS) plots showing the distribution of factor of safety, under varied conditions of 

maximum acceleration, interpolated from seven boreholes arrayed parallel to the shoreline at Elliott Bay on the Duwamish 

River delta. Borehole B7 is to the east and borehole B1 is to the west. In assessing the potential for initial liquefaction, a 

factor of safety less than 1.0 indicates a likelihood for liquefaction. I

hb is the Arias intensity.

referred to as steady-state shear strength (Poulos, 1981; Poulos 

and others, 1985) and residual strength (Seed, 1987; Seed 

and Harder, 1990; Baziar and Dobry, 1995). Slopes that have 

undergone disintegrative flow failure experience large strains 

such that the soil mass flows (Whitman, 1985). Nondisintegrative failure is a condition of momentary instability of a moreor-less coherent block. Soil movement (inertial deformation 

during earthquakes) can occur during nondisintegrative failure, 

but only for relatively short distances.

In order for disintegrative flow failure to occur, the 

residual shear strength, Sr, must be less than or equal to the 

downslope gravitational shear stress,τs. The sediment state 

in which τs ≥ Sr and in which no drainage (inflow or dissipation of pore water) occurs is referred to as State I (Whitman, 

1985). According to Whitman (1985), a disintegrative flow is 

possible under locally undrained conditions (constant volume) 

only if the sediment is in State I.

Within State II sediment, the residual shear strength is 

greater than the downslope gravitational shear stress, that is 

Sr >τs. Downslope movement can occur only while external 

downslope stresses, earthquake loading in this case, are acting. A condition of momentary instability of a coherent block 

results as sediment undergoes nondisintegrative strain during 

transient earthquake loading (Whitman, 1985). Sediment states 

Factor of Safety

<1

1-2

2-3

3-4

4-5

5+

DEPTH, IN METERS

Ihb ≈ 0.6 m/s, amax = 0.2g

B7 B6 B5 B4 B3 B2 B1

40

30

20

10

0

500 400 300 200 100 0

DISTANCE, IN METERS

Factor of Safety

<1

1-2

2-3

3-4

4-5

5+

DEPTH, IN METERS

B7 B6 B5 B4 B3 B2 B1

40

30

20

10

0

500 400 300 200 100 0

DISTANCE, IN METERS

Factor of Safety

<1

1-2

2-3

3-4

4-5

5+

DEPTH, IN METERS

B7 B6 B5 B4 B3 B2 B1

40

30

20

10

0

500 400 300 200 100

V.E. 10x

V.E. 10x

V.E. 10x

V.E. 10x

0

DISTANCE, IN METERS

Factor of Safety

<1

1-2

2-3

3-4

4-5

5+

DEPTH, IN METERS

B7 B6 B5 B4 B3 B2 B1

40

30

20

10

0

500 400 300 200 100 0

DISTANCE, IN METERS

Ihb ≈ 0.3 m/s, amax = 0.1g

Ihb ≈ >5 m/s, amax Ihb ≈ 2.0 m/s, amax = 0.3g = 0.4g
Disintegrative Flow Failure Potential 7

can be distinguished by the undrained effective stress response 

to applied shear (Whitman, 1985).

Disintegrative flow (State I sediment behavior) can occur 

in contractive sediment if the resistive shear stress at high 

strain, Sr, falls below the downslope gravitational shear stress, 

τs (Whitman, 1985). For example, in figure 6A, τs is initially 

less than the sediment peak undrained strength, su, and hence 

the slope is stable. During earthquake loading, additional 

stresses are superimposed upon any preexisting stresses. The 

peak shear strength, su, is reached and then shear resistance 

falls to Sr because of the increased pore pressure and strain 

softening resulting from liquefaction. If Sr <τs, straining of 

sediment can continue after the earthquake inertial loads are 

removed, because of the downslope stress disequilibrium. The 

result is an accelerating and disintegrating soil gravity flow.

A useful diagram for presenting those responses is the 

effective stress path, a plot of the shear stress, q, versus the 

average 2-dimensional effective confining stress, p' (fig. 6). 

Applied shear stress and normal effective confining stress 

are calculated for a plane of maximum shear stress during a 

triaxial compression test. During undrained shear, loose and 

volumetrically contractive sediment develops positive pore 

pressure that lowers the effective confining stress and causes 

the stress path to bend to the left (fig. 6B). At the onset of 

shearing of such sediment, a peak shear stress, su, is quickly 

reached, typically within 1 to 2 percent strain (Castro, 1969). 

At further strain, sediment fabric collapses on the interstitial 

pore water so rapidly that excess pore pressure cannot be 

expelled. A common result of undrained, contractive behavior 

is a marked drop in shear resistance (figs. 6A,B).

In contrast, State II sediment typically exhibits only 

slightly contractive to dilative behavior. Dilation, or the 

tendency toward volumetric increase during shear, causes a 

reduction of pore-water pressure. The result is a monotonic 

increase in shear stress that is supported even at high strains 

(fig. 6A). The stress path for dilative soil in State II climbs 

continuously toward and along the Mohr-Coulomb failure 

envelope on figure 4B. State II sediment develops a Sr at large 

strains greater than τs. If the earthquake loads that initiated 

failure are removed, the remaining shear resistance is sufficient to offset the downslope gravitational stress, and straining ceases. That is, the sediment mass becomes stable after 

external loading conditions are removed.

To assess the potential for flow failure of the Duwamish delta slope, a balance must be made of the downslope 

gravitational stresses induced by self-weight and the resistive residual strength of the soil mass. At a given subbottom 

depth, the downslope gravitational driving shear stress is a 

function of the sediment buoyant density and sea-floor slope 

angle. The resistive residual strength of the delta sediment 

was estimated by the relation proposed by Baziar and Dobry 

(1995) for silty sand. Using the soil boring logs of the delta 

front, we computed residual strengths from the SPT dataset 

for units of clean sand and silty sand deposits using a mean 

profile through the zone of likely residual strengths reported 

by Baziar and Dobry (1995).

Figure 6. Diagrams illustrating the behavior of soils after 

liquefaction during an earthquake, under different conditions of 

stress, strain, and porosity. After initial liquefaction occurs and 

inertial loading stops, the soil will have undergone some level of 

strain. If the high-strain residual strength falls below the local 

slope-induced stress (gray zones in A and B), flow-sliding can 

occur. Loose soils, like those of the Duwamish delta, contract 

under applied stress and are susceptible to flow-sliding if the 

slope-induced stress is large enough (C). Denser soils dilate under 

stress and do not flow-slide unless they are transformed into a 

looser state.

Shear strain

State II Zone: No slide or coherent slide,

limited postseismic deformation

STATE I ZONE

Flow slide, large to unlimited postseismic deformation

Residual strength, Sr

Down slope gravitational shear stress, 

Gravitational shear stress, 

τs

Shear stress

Average effective confining stress

Steady-state line

Consolidation line

Void ratio (porosity)

Loose

 sediment

Loose sediment

Loose (contractive) sediment Dense (dilative) sediment Dense sediment

Dense sediment

A B

C

Initial effective

confining stress

Undrained stress path to shear failure

and high strain deformation

State path (undrained) to high strain deformation

Mohr envelope (Kf

) line

DILATIVE ZONE CONTRACTIVE ZONE

Peak strength, Su

τs

τs
8 Seismic Stability of the Duwamish River Delta, Seattle, Washington

The ratio of Sr to τs leads to a binary ‘State’ solution, 

in which soil elements with values greater than 1.0 were 

assigned a State II condition (coherent movement without 

flow failure potential) and those with values equal to or below 

1.0 are assigned a State I condition (flow-failure potential). 

Zones between boreholes were interpolated with a linear 

stretch-fit using the program ArcInfo. In the upper 10m of 

the sediment column at the delta front, the resisting residual 

strength exceeds the downslope gravitational stress for almost 

the entire cross section (fig. 7). Disintegrative flow failure in 

this upper portion of the delta sediment is not predicted on 

the basis of the state property analysis. Rather, the sediment 

could move small distances in an inertially driven landslide 

event as a relatively coherent mass; that is, it is largely in 

State II.

At depths below 10 m, State I and State II conditions 

are interspersed throughout the soil mass. Although a State I 

condition implies that disintegrative flow failure can occur in 

a specific soil element within this zone, isolated State I zones 

within a matrix of State II soil will likely respond in a State II 

deformational mode. However, at the Duwamish River delta 

front, the analysis suggests that State I soil elements extend 

broadly, though not continuously, throughout the cross-section. If inertial earthquake loads are sufficiently strong (that 

is, close-in Mw > 7.0 and amax > 0.2 g to 0.3 g) and can strain 

deeper sediment into a residual-state condition, then disintegrative flow failure may occur. If this is the case, we expect a 

slide morphology of relatively coherent material rafted upon 

a deep-seated disintegrative-flow layer. Movement would 

cease when the slope angle diminished or if the slide mass 

thinned during shear, either of which reduces τs. This analysis 

suggests that the overlying material, perhaps including portions of the waterfront facilities, would be transported down 

the flank of the Duwamish delta slope and might not restore 

gravitational stability before becoming submerged beneath 

the waters of Puget Sound.

Figure 7. State analysis of the Duwamish delta front along the B1-B7 profile parallel to shore. 

State I indicates high disintegrative flow failure potential, whereas State II indicates potential 

for failure in a nonflow mode.

 40

 30

 20

 10

0

DEPTH, IN METERS

 500 400 300 200 100 0

DISTANCE, IN METERS

B6 B5 B4 B3 B2 B1

Vertical exaggeration 10x

EAST WEST

Harbor Island West Seattle

STATE I

STATE II

Water

Sediment

B7
Discussion and Conclusions 9

Discussion and Conclusions

Stratigraphic investigation of the Duwamish River valley, 

as well as those of the Puyallup and Nisqually Valleys currently 

under investigation by the authors, form the basis of an evolutionary model for Holocene deltas in Puget Sound. Evolution 

of the deltas was strongly influenced by the relic topography of 

the Puget Lowland, especially the geometry of glacial valleys 

that were deeply cut into Pleistocene outwash and till. Marine 

embayments replaced Late Pleistocene ice in the deeper parts 

of the valleys, which then became the loci of intense deltaic 

deposition. Large pulses of sediment, mainly the result of 

lahars from Mount Rainier, were funneled into the narrow 

valleys. The meandering river channels, like those that exist 

today, were probably overwhelmed by the episodically high 

volumes of volcaniclastic debris (Barnhardt and others, 1998; 

Barnhardt and Sherrod, 2006). Unable to transport the massive 

load, the single channel was replaced by a braided stream, with 

numerous shallow channels spread across a wide flood plain. 

After some undetermined time, sediment supplies decreased 

and a meandering system was reestablished, albeit in a different 

location than before. Although their full extent is unclear, the 

abandoned preeruption channels apparently meander across the 

muddy deltas as narrow ribbons of sand (fig. 3), which remain 

susceptible to liquefaction during large earthquakes.

The deltas grew rapidly seaward and, as sea level rose, 

aggraded and progressively buried older deltas (figs. 2,3). On 

the basis of the present location of the Duwamish delta relative to its positions at 5,700 years ago, Dragovich and others 

(1994) estimated an averaged progradation rate of 6.0 km/k.y. 

Progradation rates may be significantly higher for short time 

intervals during the main impact of lahar events.

Earthquake-induced failure of the Duwamish delta front is 

a significant threat, especially where port facilities are built on 

thick, unconsolidated, sandy sediment adjacent to submarine 

slopes of 4–6 degrees. Analysis of the stability of the Holocene 

deltaic deposits using field penetration test data indicates that 

extensive soil liquefaction and ground failure of native deltaic 

deposits are likely during moderate to large earthquake events. 

Under expected levels of seismic loading, the analysis of deltaic 

material of the Duwamish river indicates a high potential for liquefaction for the delta under likely earthquake scenarios for the 

City of Seattle. An investigation of the state properties of native 

soil using field data indicates that large-strain flow failure may 

occur at the delta front along the northern end of Harbor Island.

Acknowledgments

This study was supported by the Western Coastal and 

Marine Geology project on CASCADIA Earthquake Hazards. 

John Tinsley and Brad Carkin are thanked for their reviews of 

the manuscript. Carol Reiss, Dianne Minasian, and Florence 

Wong contributed their expertise in producing GIS crosssections of the Duwamish river delta. Homa J. Lee, Phoebe 

Deyhem, and Byron Richards assisted in the acquisition of 

radar and seismic field data.

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— Kayen and Barnhardt—Seismic Stability of the Duwamish River Delta, Seattle, Washington —Professional Paper 1661-E