# Shoreline Stratigraphy of Lake Washington: Implications for Holocene Crustal Strain and Earthquake Recurrence in the Cascadia Forearc

Robert M. Thorson. Department of Geology and Geophysics, University of
Connecticut, Storrs CT, 06269. <thorson@geol.uconn.edu> Phone
860-486-1396.

*Text of Poster Session for Cascadia Penrose Conference, June 2000.*

**Abstract**

The frequency of great earthquakes on the outer coast of the Cascadia
Subduction Zone has been established using turbidites and marsh
stratigraphy (Atwater and Hemphill-Haley, 1997). Theoretically, the same
techniques can be applied to the benthic sediments and shorelines of
Lake Washington, which extends 26 km across all strands of the Seattle
Fault and the Seattle Basin within the continental forearc crust of the
Puget Lowland. Karlin and Abella (1996; and more recent abstracts) have
carefully documented and dated as many as 42 distinct "silt bands"
within the benthic stratigraphy; they interpret them as seismic
turbidites suggesting an earthquake recurrence interval of 300-500
years. There is no question that one of the prominent late Holocene
"seismites" in Lake Washington was associated with the 1100 yr BP
crustal earthquake associated with abrupt vertical motions along the
Seattle Fault, tsunami deposits, rockfall avalanches, and subaqueous
landslides. This association, however, appears to be an indirect one, at
least in the vicinity of Juanita Bay, where the benthic silt band
represents the basin-ward advection of silt during a brief, but intense,
phase of shoreline erosion. This erosion, which produced a new spit at
Nelson Point, was initiated by an abrupt, 1-2 meter rise in lake level
caused by some combination of coseismic submergence within Seattle Basin
and coseismic uplift of its southern outlet. Mechanistically, this
basin-side silt band suggests the primary linkage...displacement 
submergenceerosionadvectionbenthic sedimentation ...rather than the
linkage...displacementstrong ground motionlandslidesturbidites
benthic sedimentation.

Prior episodes of submergence are indicated by repeated reversals from
woody to gyttja-rich peat horizons in freshwater marshes removed from
landslide sites. For example, in Mercer Slough, there are 21 abrupt
transitions within the interval 6100+/- 160 to 1030 +/-100 14C yr BP,
with an average submergence depth of 0.52 meters, yielding a recurrence
interval of 270 years, comparable to the recurrence interval for benthic
silt bands of Core TT147-1. At Yarrow Bay, there are nine such
submergences prior to a prominent concentration of silicate mud dated to
the 1100 year submergence event. The submergence history, although
driven primarily by the sea level rise and progradation of the Duwamish
delta between 9 and 3 ka, was punctuated by avulsion events (Smith et
al., 1989) at the lake outlet, which complicate paleoseismic
interpretations. Some silt bands are coseismic; some may even be
turbidites. However, a non-seismic origin for silt bands prior to 3 ka
is consistent with the lack of evidence for movement on the Toe-Jam
strand of the Seattle Fault (Nelson et al., 2000) during the early to
mid-Holocene time, as well as with the evidence for limited tectonic
warping of the Lake Washington shoreline since deposition of the Mazama
tephra ca. 6.9 ka.

Atwater, B.F., and Hemphill-Haley, E., 1997, Recurrence intervals for
great earthquakes of the past 3500 years at northeastern Willapa Bay,
Washington: U.S. Geological Survey Professional Paper 1576, 108 p.

Karlin, R.E., and Abella, S.E. B., 1996, A history of Pacific Northwest
earthquakes recorded in Holocene sediments from Lake Washington: Journal
of Geophysical Research 101; B3: 6137-6150.

Nelson, A.R., Johnson, S.Y., Pezzopane, S.K., Wells, R.E., Kelsey, H.M.,
Sherrod, B.L., Koehler, R.D., Bradley, L-A., Bucknam, R.C., Laprade,
W.T., Cox, J.W., and Narwold, C.F., Postglacial and late Holocene
earthquakes on the Toe Jam strand of the Seattle Fault, Bainbridge,
Island, Washington: Geological Society of America Abstracts with
Programs 32; 5.

Smith, N.D., Cross, T.A., Dufficy, J.P., and Clough, S.R., 1989, Anatomy
of an avulsion: Sedimentology 36: 1-23.

# 

# 

# Background

## 

## *Cascadia Thrusts*

Strong earthquakes repeatedly strike the outer coast between Vancouver
Island and northern California. Each involves a large (Mw\>7) Cascadia
thrust event associated with coseismic subsidence, tsunamis, and strong
ground shaking (Atwater and Hemphill-Haley, 1997). Although variable in
frequency, they have an average recurrence interval of 500-540 years.
Earthquakes also occur at depth below the Puget Lowland within the
subducted slab.

# 

# *Upper Plate Earthquakes*

A major earthquake threat is also associated with faulting of the
shallow crust within the upper plate, especially along known structures
such as the Seattle Fault, where a paleoseismic event 1100 yr BP was
associated with ground rupture, strong ground shaking, landsliding,
local tsunamis, and liquefaction. Stratigraphic offsets within trenches
on Bainbridge Island (Nelson et al., 2000), subaqueous landslides and
turbidites (Prunier et al., 1997), the range of dates associated with
landslides in the Cascades, and fault slip calculations (Johnson et al.,
1999) indicate that additional late Holocene events.

# 

# Holocene Stratigraphy - Lake Washington)

Lake Washington occupies the eastern part of the Seattle Basin and
extends across all strands of the Seattle Fault. The benthic sediments
of this deep lake consist primarily of highly organic, diatomaceous
oozes punctuated by discrete intervals of increased terrigenous
sediment, primarily clay and fine silt. Karlin and Abella (1996)
interpret these strata as seismic turbidites associated with
destabilization (primarily subaqueous landsliding) of unconsolidated
sediments. Based on an excellent C14 chronology, as many as 42 "silt
bands" in core TT147-1 have an average recurrence interval of 300-400
years.

**Co-seismic Turbidites ?**

# Implications

If the silt bands are co-seismic turbidites, their quasi-regular
frequency suggests one of three possibilities, each with a different
implication for seismic risk management:

-   That the heterogeneous upper plate is capable producing a regular
    earthquake sequence, in spite of the evidence for earthquake
    displacements prior to late Holocene time.

-   That remote shaking associated with subduction thrust events (either
    from the outer coast or at depth within the subducted slab) is
    strong enough to regularly destabilize sediments in Lake Washington.

-   That there is a mechanical linkage associated with either stress
    transfer or earthquake triggering in which Cascadia thrusts control
    forearc earthquakes.

# Immediate Problems

-   The regular recurrence interval for Lake Washington (200-400 yr)
    does not match that of the outer coast (500-540 yr)

-   Silt bands are persistence throughout the lake, throughout the
    mid-late Holocene. A lake-wide source of sediment is suggested,
    rather than multiple local sources such as landslides, debris flows,
    and slumps.

-   Texturally, the "turbidites" are not graded, sand is generally
    absent, clay is more anomalous than silt, and discrete pulses are
    sustained for as long as several hundred years. The bands are
    visually indistinct compared with known seismic turbidites and
    sesimically generated silt bands (Doig (1991).

-   The silt band associated with the 1100 year event does not suggest
    any relationship between the location of known slumps and the
    magnitude of the siltation peak.

-   The W-shaped cross-section of deep parts of the lake would have
    directed sediment density flows away from the center of the lake.

-   Smaller lakes do not have a comparable stratigraphic record.

# 

# Origin of the "Silt" Bands 

#### Benthic Stratigraphy

[Core C-1]{.underline} (Karlin and Abella, 1996; Leopold et al., 1982)
is centrally located in the lake basin, is furthest removed from
shoreline sources, and is the most well characterized
sedimentologically. It exhibits two prominent peaks; the highest is
associated with the 1100 year earthquake; the lower dates about 8 ka
based on the age model for the lake, an age consistent with its
stratigraphic position just below the Mazama ash (6.9 ^14^C yr BP; 7645
cal yr BP = 7.6 ka). [Core TT147-1]{.underline}, the best dated shows
less prominent but more regular peaks in terrigenous input.

Our studies indicate that the younger peak was due primarily to sediment
erosion associated with submergence. The older peak coincides with the
reversal from emergence to submergence in marine sea level and
correlated with a sustained doubling of Al~2~O~3~ content.

### **The 1100 year old silt band**

Late Holocene deposits and landforms in the central and northern part of
Lake Washington prove that the 1100 yr BP silt band was associated with
an abrupt coseismic submergence of 1-2 meters, in addition to the well
known association between landslides and slumps on the lake bottom.

-   A prominent spit at Nelson Point, south of Juanita Bay
    (Chrzastowski, 1983), was built "seaward" of the older, lower spit.
    Clearly, submergence must be responsible for the increased height of
    the spit. The "basin-ward" step in the spit indicates an
    extraordinary pulse of gravelly sediment from the high bluffs to the
    south. This pulse of erosion was initiated by submergence.

-   At the head of Juanita Bay, organic tributary alluvium graded to a
    lake level 1-2 m below present was truncated by wave erosion and
    overlain by sands associated with a newer, higher beach landward of
    the former site. A conventional C14 date on a log at the top of the
    alluvium yielded an age of 1460+/-90 yr BP. This relationship links
    shoreline erosion to a very late Holocene earthquake, probably the
    1100 year event.

-   Marshes behind the Nelson Point spit and south of the beach at
    Juanita Bay were inundated with mineral "silt" (fine sand to clay),
    representing the fine-grained component of the eroded material
    deposited distal to the exposed spit. Marshes elsewhere (e.g. Yarrow
    Bay and Calkin Point in northeastern Mercer Island), were also
    flooded with mineral silt in very late Holocene time (\>1390 +/- 90
    (not including a carbon residence time correction up to several
    hundred years) at Yarrow Bay and at Calkin Point (\<1520 +/- 60).
    Sustained deposition (20-30 cm of silty sediments) at the level of
    the shoreline, and the singularity of the silt band at multiple
    remote sites indicate that the silt was derived by shoreline
    erosion, rather than by subaqueous slumping or coseismic tsunamis.

-   Deposition of the silt at remote sites requires transport of
    suspended fine silt and clay into the center of the lake followed by
    settling under slackwater conditions.

##### **The \~8 ka silt band**

The older sediment pulse is more poorly dated because it is not
associated with a dramatic event like the 1100 year earthquake. This
pulse, however, marks the boundary between two prominent patterns of
lake sedimentation in the benthic and shoreline stratigraphy (Newman,
1983; Leopold, 1986).

##  Deepwater Shallow

**Pre 8-9 ka:** Rapid sedimentation Slow sedimentation

Low proportion terrigenous High proportion terrigenous

Limited "siltation" events Monotonous marsh flora

Short carbon reservoir age Fern-alder dominated shore

**Post 8-9 ka:** Slow sedimentation Rapid sedimentation

High proportion aluminosilicate Low proportion

Abundant "siltation" events Highly variable marsh flora

########  Longer carbon reservoir age Sedge-Salix near shore

The boundary between these two sediment domains in the nearshore
stratigraphy coincides with a prominent concentration of silt at a depth
of 1610 cm in Mercer Slough. We correlate this silt bed to the prominent
peak in the benthic record.

We interpret this silt bed and the abrupt transition in sediment regime
as the time of maximum emergence above relative sea level, dated at 8-9
ka throughout the Puget --Fraser Lowland (Dragovich et al., 1994). Prior
to this time, the land had been rising slowly relative to the sea. After
the nadir of relative sea level, however, sediment stored on abandoned
littoral wave-cut platforms and deltas was increasingly available for
reworking into the lake basin. We cannot rule out a seismic event being
partially responsible for this silt band, which is prominent only in
core C-1, although we know of no evidence for it.

***Interval between the silt bands***

Between these two prominent peaks in Core C-1 are many of smaller
magnitude.

-   Nearshore sediments from Mercer Slough (Core B) post-dating the
    Mazama Ash, exhibit multiple, abrupt reversals from woody to
    non-woody peat. Although they cannot be correlated one-for-one,
    these reversals match those from other marshes (Yarrow Bay, Juanita
    Bay, Sammamish delta, Luther Burbank Park) in character and
    frequency. This suggests that net transgression represents the sum
    of discrete events, averaging 40-70 cm in magnitude.

-   The pollen record from Mercer Slough Core B (Newman, 1983) shows
    dramatic, sample-by-sample changes in the abundance of taxa
    indicative of wet conditions, and shoreline woody taxa, suggesting
    alternations between wetter and drier conditions on the marsh.

-   The frequency of submergence events in Mercer Core B (21 events
    between 6100 and 1100 14C yr BP; recurrence of 238 yr) matches that
    of the most well dated benthic core (TT147-1 18 events 6000-1100 yr
    BP; recurrence of 272 yr). This suggests that the silt bands and the
    minor submergences are responding to the same forcing.

**Avulsion/Aggradation Model**

**A Freshwater "marine" embayment**

Conventional wisdom holds that Lake Washington is an isolated body of
freshwater impounded by an alluvial fan at the mouth of the Cedar River,
on which the city of Renton is now built (Mullineaux, 1970). A recently
constructed submergence curve for the lake (Thorson, 1998), however, one
taken from the tectonically stable north end of the, shows that its
transgressing freshwater marshes have tracked sea level for most of the
postglacial interval, suggesting that Lake Washington is essentially a
freshwater embayment of the sea separated from it by a sand shoal. The
mechanism by which Lake Washington can simultaneously remain freshwater
yet track sea level requires a negative feedback system in which the
rate of rate of sea-level rise be lower than the rate of fan
aggradation. Presently, the sediment transport for the lower Cedar River
is estimated at nearly 10^6^ tons/yr (Booth, 1993). Given the size of
the fan, this sediment flux is high enough to raise the fan at the
required rate.

#### Fan Avulsion

The alluvial fan at the mouth of the Cedar River occurs at the junction
between the basin of lake Washington and the basin draining towards the
Duwamish, and now occupied by the Black River. This junction is bisected
by the upland northwest of Renton, which forces the flow either to the
north (Lake Washington) and west (Duwamish Valley). Sustained growth of
the delta in either direction would inevitably result in avulsion of the
active channel system towards the other basin. A recent avulsion of the
Duwamish System (Palmer et al., 1994), must have had an impact on the
Cedar River, perhaps forcing it to avulse towards Lake Washington.

Abrupt changes in the fan stratigraphy near Renton are indicated in all
near-surface boreholes. One of these abrupt changes, too place at both
ends of the Black River sometime after 810 +/- 70 years.

Alternatively, symmetrical progradation into both basins would create a
zone of slackwater during major floods in which channel aggradation
would occur. (One such flood was captured by the Renton metropolitan
district in 1911.)

Both the avulsion and symmetrical progradation models four force
aggradation to take place in discrete steps. Abrupt changes in alluvial
sedimentation are characteristic of boreholes everywhere on the
floodplain.

Aggradation/avulsion events lead to abrupt increases in the height of
the lake shoreline, each at the scale of decimeters. Detailed work in
other lakeshore settings indicates that events of this magnitude lead to
pulses of shoreline erosion which peak early, then diminish through
time.

Paleoseismic Record

####### Origin of Silt Bands

The silt bands in the benthic strata of Lake Washington have multiple
origins, even within single cores. Although the most prominent late
Holocene silt band was associated with a seismic event (1100 yr old), we
interpret that this pulse of terrigenous sediment resulted primarily
from co-seismic submergence, rather than from co-seismic shaking,
although the effects of the latter may have been locally important.

The other silt bands as also most likely due to discrete pulses of
submergence, which may or may not be associated with crustal movements.
The avulsion/aggradation model provides a more parsimonious explanation
for the silt bands because we know that pulsed transgression of the
shoreline was occurring at that time. In contrast, the silt bands appear
too abundant (42 events) and too regular (200-400 years) to be due to
crustal earthquakes.

###### **Singularity of the late Holocene event**

# 

# *Co-seismic Water level changes*

There is only [one]{.underline} dramatic Holocene spit in the entire
lake at Nelson point. This suggests only one episode of sediment flux
sufficiently high to prograde the shoreline. There is only
[one]{.underline} dramatic influx of mineral silt to the marshes. There
is only [one]{.underline} compelling regression in the nearshore
stratigraphy; it takes place near central Mercer Island.

Radiocarbon dates from littoral sediments in Lake Washington are
consistent with an age assignment to the 1100 year event. However, it is
equally likely that the event took place sometime near 1400 years BP.

Other silt bands and gravel bars are present and may reflect less
dramatic events than those of the 1100 year event.

# *Tsunami at Kenmore (Sammamish River Outlet*

The high marsh of the Sammamish Delta at Kenmore contains a massive,
locally graded sand "sheet "locally up to 40 cm thick that was present
between 1.5 and 1.8 m depth in all four cores taken from this vicinity.
This pebble-bearing, cleanly washed coarse gray sand is similar to that
described by Atwater and Moore (1995) in Cultus Bay (Whidbey Island),
and is thickest in the most landward core, becoming thinner and with
multiple beds in deeper cores. Pending further work, this sand is
interpreted as evidence for a single tsunami of Late Holocene age
generated by uplifts of the Seattle Fault crossing Mercer Island.

# Long-term shoreline deformation

The elevation of the Mazama tephra within marsh peats at the Sammamish
Delta ranges from --5.45 to --9.35 asl, with a mean value of --6.96 m.
The tephra lies at -5.61 m depth in Union Bay, -6.61 m in Mercer Slough,
and --8.61 m at the Renton Delta. This suggests net strain along
shoreline has been minimal (\<3 m +/-) during the last 7 ka.

Limited crustal motion is also suggested by the shoreline morphology of
submerged spits, wave-cut benches, ravines, river deltas (Cedar and
Sammamish), and tributary fan-deltas associated with minimum stand of
relative sea level. Collectively, these features support the early
interpretations of Gould et al (unpub.) indicating a paleoshoreline at
--10 to-14 m elevation in the lake at the time of the reversal in
relative sea level.

There is a prominent exception to this trend, at Newcastle Creek, east
of the center of Mercer Island. There, the platform of a tributary fan
delta is less than 4 m depth. If this tributary delta platform
correlates with the others, it suggests a local uplift of about 6 m. In
this locality, there is stratigraphic evidence for an abrupt regression
of the shoreline sometime after 1410+/-50 years BP. The absence of thick
marsh peats and the absence of the Mazama tephra flanking Mercer Island
suggests net uplift of the shoreline between strands of the Seattle
Fault.

######### **Speculations**

There is no doubt that multiple earthquakes have occurred in the late
Holocene that have affected the sedimentation of Lake Washington, and
that the 1100 year earthquake caused subaqueous landslides and slumps
along the lake margin. The shoreline record, however, contains evidence
for only one dramatic event during late Holocene time. This shoreline
event is probably coeval with the 1100 year event, but it might be up to
several hundred years older, owing to uncertainties in 14C reservoir
ages (726 +/- 252 14C yr) and atmospheric corrections (ca. --100 yr). An
older age of ca. 1400 yr BP is better supported by the shoreline dates,
and is within the range of uncertainty for dating the 1100 year old silt
band. If true, the major late Holocene displacement of the Seattle fault
near Mercer Island may have preceded its counterpart at Restoration
Point by several centuries.

# Acknowledgments

# 

# This work was supported by grants from the U.S. Geological Survey National Earthquake Hazards Reduction Program, and was carried out in collaboration with Estella Leopold (U.W. Botany) and her students. 

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