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Timing, Extent, and Continued Evolution of the
Van Zandt Landslide Complex, Whatcom County, Washington
Thesis Proposal for the Master of Science Degree
Department of Geology, Western Washington University
Bellingham, Washington
Geoffrey Malick
September 2, 2015
Approved by Advisory Committee Members:
__________________________________________________________
Dr. Douglas H. Clark, Thesis Committee Chair
__________________________________________________________
Dr. Jackie Caplan- Auerbach, Thesis Committee Advisor
__________________________________________________________
Dr. Robert J. Mitchell, Thesis Committee Advisor
_______________________________________________________
Dr. Scott R. Linneman, Thesis Committee Advisor
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Introduction and Problem Statement
The Pacific Northwest is particularly susceptible to large slope failures because of its high relief
terrain, seismic activity, and an abundance of precipitation (Blais-Stevens et al., 2011). Despite
recent tragedies like the Oso, Washington, slide (3/22/2014; 43 fatalities), our understanding of
the mechanics, timing, and possible triggers of such landslides remains poor. Although several
recent studies (e.g., Iverson et al., 2015; Keaton et al., 2014) have focused on large slope failures
that initiate in unconsolidated glacial deposits, little attention so far has been given to
catastrophic bedrock landslides. To help address this deficiency, I will characterize the Van
Zandt Landslide Complex (VZLC), a high-mobility bedrock landslide in the western foothills of
the North Cascades of Washington (Fig. 1); the goals of my research are to 1) determine ages,
volumes, and potential triggering events for the distinct lobes of the VZLC; 2) test hypotheses
that such large bedrock landslides are linked to other regional landslides, paleoseismic events, or
paleoclimatic trends, and 3) evaluate potential indicators of continued slope deformation in the
source zone.
Dating prehistoric landslides can be difficult because many lack natural exposures and
datable material linked to their formation. Fortuitously, the VZLC deposit contains an abundance
of naturally formed ponds on its debris lobes (Fig. 2); these ponds, as well as several natural
river-cut exposures, provide an exceptional opportunity to collect and date sediments related to
the various slides and to thereby establish a comprehensive chronology of failures at this site.
Such constraints are crucial to test proposed causal links between such slides and paleoseismic or
paleoenvironmental events. Furthermore, previous workers (e.g., Brunengo, 2001) and initial
reconnaissance efforts have identified evidence of ongoing deformation and small slope failures
in the source area. For this study, I will evaluate the relationship between shallow
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microseismicity and tension gaps in the headwall region to quantify progressive strain and test
the potential for future hazards at the VZLC. The results will provide crucial new constraints for
hazard assessments of a poorly understood type of catastrophic landslide that threatens people
and property in the Pacific Northwest, complementing ongoing investigations of sedimentsourced landslides in the region (Iverson et al., 2015; Keaton et al., 2014).
Landslide hazard assessment requires knowledge of the age, frequency, and volume of
previous events, the identification of slopes prone to failure, and data on active deformation in
potential initiation zones (Orwin et al., 2004). The recent devastating debris avalanche-flow in
Oso emphasized the great hazard represented by such high-mobility landslides in the Pacific
Northwest as well as our poor understanding of how and why they occur. The Oso landslide was
extremely destructive because of an exceptionally high travel distance compared to previous
historic events at that location (Keaton et al., 2014). Such long-runout landslides, however, are
not uncommon in the Pacific Northwest. According to lidar evidence, the VZLC appears to have
had at least two large failures with travel distances comparable to that of the Oso slide.
Triggering-event and threshold-condition constraints for long-runout landslides are crucial to
understanding hazard susceptibility throughout the region, especially in the face of growing
population in potentially hazardous areas and increased landslide potential related to greater
rainstorm intensities linked to forecasted climate change (Huggel et al., 2011).
Background
Location and Morphology
The VZLC is located approximately 25 km east of Bellingham, WA, in the western foothills of
the North Cascades (Fig. 1). Preliminary mapping based on 2013 lidar data (Fig. 2) indicates that
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VZLC comprises at least three prehistoric cross-cutting debris lobes that initiated on the steep
western slope of a prominent north-south trending ridge known as the Van Zandt Dike (VZD). In
addition to these larger lobes, there are also several possible smaller deposits and colluvial lobes
formed from shallow slope failures.
The 11 km long VZD is shaped like a north-pointing arrowhead that is ~3 km wide at its
southern end and tapers to ~1.5 km to the north where it forms the VZLC source zone (Fig. 1).
The VZD rises steeply >500 m above the lower reaches of the South Fork Nooksack River
reaching a maximum elevation of 672 m above sea level (asl). The northern half of the VZD (and
the entire VZLC source zone) is made up of the Eocene Chuckanut Formation (sandstone,
mudstone, siltstone, and minor coal intervals) whereas the southern half is Darrington Phyllite
(Tabor et al., 2003).
The crescent-shaped headscarp region for the VZLC is located along the northwestern
edge of VZD and is close to 5 km long. A significant slope break defines the upper reaches of the
steep headwall from the relatively flat top of the VZD (Fig. 2). Along this flat upper region of the
VZD are a series of NNW-SSE trending gentle ridges separated by shallow troughs (Fig. 2). This
alternating ridge and swale topography reflects distinct beds of the Chuckanut Formation where
weaker layers were preferentially scoured by glacial ice (Brunengo, 2001).
The likely source zone for at least one of the large failure events is a wedge-shaped
bedrock hollow (Figs. 2, 3) in steeply dipping beds of the Chuckanut Formation with a top
elevation of 625 m asl and a bottom elevation of 270 m. The hollow is defined by two distinct
intersecting rupture surfaces: a steep (~48°) dip slope (~300 m high) on its southeast side and a
cross-cutting lateral escarpment on its northwest side (Figs. 2, 3) that varies in height from 75-
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110 m. The base of the hollow is a large low-gradient bench formed roughly 240 m above the
valley floor.
The runout zone of the complex comprises at least three landslide debris lobes that
accumulated on the floor of the South Fork valley (Figs. 2, 3). The debris lobes are
distinguishable from the adjacent floodplain by their raised relief and irregular hummocky terrain
and from one another based on cross-cutting morphology of their outer margins and by distinct
morphologic textures. The total area covered by the landslide debris is approximately 4.7 km2.
The morphologically oldest deposit, Debris Lobe 1, is ~1.8km long, approximately 0.60.9 km wide and has an area of at least 0.92 km2 (Fig. 3). This oldest lobe is characterized by a
relatively smooth surface with only a small area of hummocky terrain preserved, mainly in the
northern half of the deposit. The deposit exhibits post-deposition gullying and incision along its
western (distal) edge, characteristics that are largely lacking in the other two lobes.
Debris Lobe 2 overlies the northern part of Debris Lobe 1 and is the largest lobe (Fig. 3).
It is 2.7 km long, approximately 1-1.5 km wide and has an area of ~3.5 km2. The South Fork
appears to have eroded the distal-most edge of this lobe, therefore its ultimate extent is uncertain.
The deposit is quite hummocky with a high density of steep-sided hills, closed depressions and
irregular arcuate ridges. Several of the closed depressions are occupied by small perennial bodies
of water.
Debris Lobe 3 is the youngest lobe and is 1.3 km long, 0.5-0.75 km wide, and has an area
of ~0.45 km2 (Fig. 3) This deposit clearly overprints the northwestern part of Debris Lobe 2 as
well as an alluvial terrace riser associated with the North Fork Nooksack River, both of which
project under the deposit. The individual hummocks on this lobe are smaller both in local relief
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and in dimensions than the hummocks on Debris Lobe 2. This lobe contains at least three small
permanent bodies of water including two perennial lakes and a small bog.
Bedrock Geology
The source rock for the VZLC (Fig. 1) is the Eocene Chuckanut Formation, a sequence of
alternating fine-grained and coarse-grained alluvial strata (Dragovich et al., 1997). Several
studies (e.g., Johnson, 1982; Mustoe, 1997) have divided the Chuckanut Formation into the
Bellingham Bay, Slide, Padden, Maple Falls, and Governors Point members. The entire source
zone of the VZLC is mapped as Bellingham Bay Member (Dragovich et al., 1997); however it is
within 0.5 km of the contact with the Slide Member. Some workers, however, (e.g., Mustoe,
1997; Johnson, 1982) have suggested that these two members are at least partially coeval,
representing the same fluvial system. The only sedimentological difference between the two
members is an overall finer texture of the Slide Member, which rarely contains strata coarser
than medium-grained sandstone (Johnson, 1984). Both units are thick- to thin bedded, well
sorted, rounded to subrounded micaceous fine-grained arkose sandstone alternating with
abundant mudstone, siltstone, and minor coal intervals (Johnson, 1982, 1984; Dragovich et al.,
1997). The sandstone beds throughout the formation consist dominantly of plagioclase,
potassium feldspar, monocrystalline quartz, and biotite with lesser muscovite, chert, and lithic
fragments (Dragovich et al., 1997). The Bellingham Bay Member occasionally contains
polymictic conglomerate consisting of clasts from neighboring units such as the Darrington
Phyllite (Johnson, 1982). The most extensive intact bedrock outcrops in the region occur near the
landslide headscarp where alternating sandstone and finer grained strata strike to the NNE to NE
and dip between 40° and 53° to the NW.
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Schmidt and Montgomery (1994) identified 34 mountain-front-scale landslides that
initiated within the Chuckanut Formation. This unit is particularly susceptible to failure because
extensive folding has caused widespread fracturing and created abundant steep dip-slopes with
highly variable material strengths exposed in a high-relief glacially scoured landscape (Schmidt,
1994; Brunengo, 2001). Despite its local susceptibility to failure, the resistant Chuckanut
Formation forms some of the steepest and most prominent ridges in the region, whereas weaker
low-grade metamorphic rocks (such as the Darrington Phyllite) make up many of the gentler
slopes (Brunengo, 2001). Mountain front-scale mechanical rock strength is highly influenced by
the material properties of its weakest zones (Badger, 2002). In the case of the Chuckanut
Formation, the dominant discontinuities that modulate slope stability are thin intervals of shale,
mudstone and coal, as well as extensive bedrock fractures (Schmidt, 1994).
Structural Geology and Regional Tectonics
Tertiary deformation compressed much of the region into large N-S and NW-SE trending folds,
primarily plunging towards the north and northwest (Brown and Dragovich, 2003).
There are also several Tertiary faults in the vicinity of the VZLC (Fig. 1), some of which have
been reactivated in the Holocene under a new tectonic stress regime (Sherrod et al., 2013). These
faults include the NE-SW trending Boulder Creek Fault, the NW-SE and N-S trending Canyon
Creek Fault, the NE-SW trending Smith Creek Fault, and the low-angle McCauley Creek Thrust
Fault (Sherrod et al., 2013).
Along the Cascadia margin, oblique subduction of the Juan de Fuca plate beneath the
North American plate has created a complex and seismically active volcanic arc and
convergence zone (Wells and McCraffrey, 2013). The foothills of northwestern Washington and
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the Nooksack River Basin lie on the actively migrating fore-arc, an upper-plate block being
rotated and compressed against a rigid backstop of the Canadian Coast Mountains (Wells et al.,
1998). This block migration causes N-S shortening that is accommodated along numerous
shallow-crustal faults throughout the Puget Sound region (Mazzotti et al., 2002). In the foothills
of the North Cascades, this crustal seismicity and deformation is focused along reactivated
Tertiary faults including the Boulder Creek, Canyon Creek, McCauley Creek, and Smith Creek
Faults, all of which are in close proximity to the VZLC (Sherrod et al., 2013).
Glacial Geology
Episodic continental glaciation has also strongly influenced the present landscape. During much
of the Pleistocene, ice from the Cordilleran Ice Sheet extensively scoured and oversteepened the
valley walls of the Nooksack River Basin and surrounding areas (Kovanen and Easterbrook,
2001). This substantial glacial erosion combined with interglacial river incision, particularly at
the base of slopes, left many of the regional mountain fronts at or above their maximum
threshold hillslope height and gradient, making them highly susceptible to failure (Schmidt and
Montgomery, 1994; Montgomery, 2002).
Landslide Triggering
A landslide trigger is an external stimulus such as seismic shaking, intense rainfall, rapid
snowmelt, volcanic eruption, or a freeze-thaw event that causes immediate or progressive slope
failure. Triggering-event constraints are crucial to understanding hazard susceptibility because
they provide critical information about threshold conditions for previous failures and allow
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investigators to estimate probability of failure and factor of safety at a given site (Turner and
Jayaprakash, 1996).
Seismic Triggering
Earthquake scenario reports (e.g., WADNR, 2012; 2013) and my own preliminary limit
equilibrium factor-of-safety analysis demonstrate that local seismically active crustal faults can
potentially produce earthquakes with the peak ground accelerations (>0.2 g) required to trigger
deep-seated bedrock landslides. Trenching data from the Kendall scarp of the Boulder Creek
Fault shows evidence for at least three shallow-focus Holocene earthquakes that were likely Mw
> 6.3, capable of producing damaging ground shaking in northwest Washington (Barnett et al.,
2006; Siedlecki, 2008). Previous studies (e.g., Engebretson et al., 1996; Pringle et al., 1998) have
noted the high concentration of deep-seated bedrock landslides in the Nooksack River Basin
(Fig. 1) and have suggested a causal relationship with nearby shallow-crustal faults.
In addition to shaking from local crustal faults, a partial or complete rupture of the
Cascadia Subduction Zone will produce strong ground shaking throughout the region with a peak
ground acceleration of at minimum 0.2 g (WADNR, 2013; Schmidt, 1994). At least 19 fulllength ruptures of the subduction zone have occurred during the Holocene, along with numerous
partial ruptures (Goldfinger et al., 2012). The Bonneville (Bridge of the Gods) Landslide
Complex along the Columbia River in southern Washington and the Lake Washington Landslide
in Seattle are two examples of dated landslides in the Pacific Northwest that have been linked to
megathrust events (Pringle and Schuster, 1998).
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Rainfall Infiltration Triggering
Although often associated with shallow slope failures in loose or unconsolidated materials,
meteorological events can also induce deep-seated bedrock landslides. Intense and/or longlasting storms with significant infiltration of rainwater can cause highly elevated groundwater
pressures, which increase the driving forces and reduces the resistive force of friction on
potential failure surfaces (Iverson, 2000). Furthermore, rainfall infiltration into bedrock fractures
can increase the mass of an unstable slope, which drives up the shear stress acting upon it,
reducing its factor of safety (FS = resisting forces/ driving forces) and increasing the probability
of failure.
The VZLC headscarp region is located in an area of recent and active timber harvesting.
Forest practices can exacerbate the effects of rainfall events and increase susceptibility to deepseated failure. The removal of vegetation significantly reduces rainfall interception and
evapotranspiration, leading to increased groundwater infiltration and pressures (Keaton et al.,
2014). Logging activity can also increase the risk of shallow failures such as rockfalls, topples,
and debris flows. Tree roots provide support and stability at the ground surface and the removal
of them can reduce “root cohesion” in loose soils and expose fractured rock (Legorreta-Paulin et
al., 2008).
The West Salt Creek Landslide (5/25/2014; 3 fatalities) located in western Colorado is a
recent example of a rainfall infiltration triggered bedrock landslide. This large failure was
induced by two days of heavy rain including a 30-minute period when rainfall intensity was over
20 mm/hr just before the failure (White et al., 2015). The heavy rainfall and overland runoff
infiltrated the shale and marlstone bedrock and rapidly increased pore pressure in and along rock
fractures, joints, and pre-existing shear surfaces (White et al., 2015). Failure occurred along a
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rotational-slip surface causing the rock mass to rapidly disintegrate. The landslide travelled
nearly 3.5 km, making it the longest and one of the most highly mobile (H/L=0.14) landslides in
Colorado’s history (White et al., 2015).
To assess the potential for hydrometerological events as effective landslide triggers in the
region, my study aims to use 14C ages and the robust paleoclimate record (e.g., Steinman, 2014;
Nelson et al., 2011; Mathewes and Heusser, 1981) for the region to determine if the VZLC
failure events are potentially linked to paleoclimatic trends, events, or other dated landslides in
the region.
Although numerous studies (e.g., Geertsema and Schwab, 1997; Huggel et al., 2012;
Zerathe et al., 2014) have examined short-term (decadal) as well as longer-term (centennial to
millennial) paleoclimatic fluctuations and their implications for landslide activity in many of the
world’s mountain belts, little research of this type has been conducted in the North American
Cordillera. Some studies (e.g., Zerathe et al. 2014) have identified clusters of spatially and
temporally concordant landslides and suggest that they are contemporaneous with known periods
of increased precipitation in the paleoclimatic record (e.g., medieval climate anomaly, little ice
age, ENSO fluctuations). The 14C ages presented by this study will contribute to the expanding
inventory of dated landslides in the region and provide new constraints to help assess the
potential for climatic triggering of some paleo-landslides in northwestern Washington.
Other Possible Triggering Mechanisms
Several other natural factors can lead to the genesis of large rock slope failures including rapid
snowmelt, glacial debuttressing, freeze-thaw events, and volcanic eruptions
Rain-on-snow events and/or rapid snowmelt can often have the same effect as heavy
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rainstorms by causing significant surface runoff and groundwater recharge (Hewitt et al., 2007).
However, elevated water output from snowpack typically produces a more continuous supply of
water for recharge compared to infiltration solely from rainfall (Horton, 1938). This is especially
pertinent in the western Washington where a high percentage of liquid precipitation is
intercepted and stored by dense forest vegetation and never reaches the ground surface and
allowed to infiltrate (Storck et al. 1998). Triggering of deep-seated landslides by rapid and
continuous recharge is usually caused by a combination of factors including increased mass of
the unstable block, decreased frictional strength, and the propagation of bedrock fractures. The
Racehorse Creek Landslide (1/7/2009; ~5x105 m3; Crider et al., 2009) is a recent example of a
rain-on-snow triggered mass movement in the Chuckanut Formation at a site that has many
similarities to the VZLC (prehistoric large failures, steeply dipping beds, recent ground cracks).
In January 2009, a particularly strong “pineapple express” rainstorm following snowfall caused
failure along existing bedrock joints, which debuttressed an entire adjacent hillside and initiated
failure along a bedding plane discontinuity (Crider et al., 2009).
Glacial debuttressing as a trigger occurs when a previously glaciated landscape, freshly
exposed and lacking the lateral support once provided by glacial ice, is no longer stable (Panek,
2014). Such paraglacial rock failure typically occurs within centuries to a millennium after
deglaciation and this delay can often make it difficult to conclusively rule out other external
triggering mechanisms (Panek, 2014). The sharply defined morphology of the VZLC debris
lobes expressed in lidar imagery, however, suggests that they are considerably younger than the
most recent (late-Pleistocene) deglaciation of the region (Clague et al., 1997; Kovanen and
Easterbrook, 2001).
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Freeze- thaw cycles can also induce bedrock landslides in mountainous areas. When
stored or flowing water in bedrock fractures, joints, or other discontinuities freezes into ice, it
undergoes volumetric expansion. The increased volume of water causes significantly elevated
pore-water pressures and if complete freezing occurs, the ice itself can exert a pressure on the
surrounding rock (Arsenault and Meigs, 2005). In some cases, these pressures can be enough to
fracture the rock and/or cause slope failure (Matsuoka et al., 1998). Diurnal, seasonal, or longterm freeze-thaw cycles are highly effective at propagating fractures, which can create
preferential flow paths for infiltrating water. Fractures can also greatly reduce bedrock material
strength and a given slope’s angle of friction and factor of safety (Matsuoka et al., 1998).
Although it cannot be ruled out as a contributing factor, freeze-thaw activity is not likely to have
occurred at the likely depths of the failure surface or at the relatively low elevation of the VZLC.
In particular, the initiation points of slides as massive as those at the VZLC must have been tens
to hundreds of meters below ground, well below the depths reached by transient freezing fronts
in the Pacific Northwest.
Volcanic eruptions can trigger large landslides through ground shaking and the
mobilization of large amounts of loose volcanic debris, although this seems unlikely in the case
of the VZLC because the nearest volcano (Mount Baker) is 25 km away and there are no
significant volcanic deposits reported in the landslide debris. Lastly, it is important to keep in
mind that sometimes there is no apparent attributable trigger for a landslide at all or that failure
may have been caused by a combination of processes but not one individual trigger (Hewitt et
al., 2007).
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Long-runout landslides and transport mechanisms
High-mobility landslides have long intrigued and perplexed geologists and hazard planners.
Some of the most disastrous and confounding landslide events in history (e.g., Frank Slide, AB;
Oso, WA) were of this variety (Cruden and Hungr, 1996; Iverson et al., 2015). The long travel
distances are often unexpected and make these events exceedingly destructive (Cruden and
Hungr, 1996; Iverson et al., 2015). Despite considerable advances in understanding landslide
mechanisms, there is a general lack of knowledge as to why these events happen where they do
and what processes trigger and drive the rapid propagation of the debris mass (Hungr et al.,
2005).
A commonly used mobility index for landslides is the H/L ratio where H is equal to the
vertical height of the fall and L is equal to the horizontal displacement of debris measured from
the back of the headscarp (Coraminas, 1996). The majority of documented landslides worldwide
have an H/L > 0.6 and some literature (e.g., Hsu, 1973; Scheidegger, 1978) considers anything
with an H/L ≤ 0.3 as a long-runout landslide (Coraminas, 1996). The devastating Oso, WA
debris-avalanche flow (DAF) had an H/L= 0.105 (Iverson et al., 2015), whereas the two largest
events for the VZLC had only a slightly lower estimated mobility values of H/L=0.160 and H/L=
0.187.
Although my study will not test directly specific runout mechanisms, it will provide
insight into the types of triggering events and conditions that could possibly generate
catastrophic high-mobility landslides. Evaluating temporal links to contemporaneous regional
landslides may also shed light on why certain orogenic belts such as coastal ranges of
northwestern North America have large concentrations of long run-out landslides (Hewitt et al.,
2007).
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Methods
Geomorphic Mapping
During the summer of 2015, I will conduct detailed field mapping of the entire study area
including the head scarp region, potential initiation zones, and runout zones. I will focus much of
my attention on identifying and documenting areas with evidence of recent or ongoing mass
wasting. I will be looking for natural and human-made debris exposures throughout the runout
zone and any other relevant geomorphic features (e.g., evidence of liquefaction, mechanical
striae, intact large boulders), as well as evaluating qualitative indicators of debris movement
(e.g., longitudinal ridges, transverse crevasses) and deposit age (e.g., surface soil development).
Lake Coring
Lidar data acquired in 2013 indicate the VZLC comprises at least three major overlapping debris
lobes and possibly several smaller deposits (Fig. 2). I will obtain minimum limiting ages for the
two younger main lobes of the VZLC by collecting sediment cores from small lakes on the
surface of these deposits. I will focus my coring on the deepest portions of these shallow lakes
because that is where the lacustrine record tends to be the most complete and best preserved. In
order to determine the deepest part of each lake, I will collect detailed bathymetric data using an
inflatable raft, differential GPS, and a handheld bathymetry meter and then create a bathymetric
map using Surfer (v.8.0) data visualization software.
I will collect lake sediments using a modified Livingstone piston corer (Wright, 1967)
from an anchored floating raft to collect multiple cores from each lake with a goal of recovering
the deepest (i.e., oldest) sediments possible. Basal sediments and any terrestrial macrofossils
within them will yield minimum 14C ages for the emplacement of the slide debris (Panek et al.,
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2014; Lang et al., 1999). Although there can be a lag between the slide failure and initial lake
formation, such delays are typically within the margin of error and uncertainty (decades to
centuries) for radiometric dating techniques (Lang et al., 1999). More relevant to my study is the
potential for a lag between lake formation and initial organic sedimentation in the lake (Lang et
al., 1999). To minimize this potential error, I will collect terrestrial macrofossils (wood
fragments, seeds, needles) from the lowest possible strata of the sediment cores, closest to the
underlying landslide debris. Macrofossils are preferable to bulk sediments because they are less
likely to be affected by contamination or other types of errors (e.g., reservoir effects; Bertolini et
al., 2004). In addition to sampling the cores for 14C dating, I will log the visual stratigraphy as
well as the magnetic susceptibility to help establish more complete sedimentary context for the
post-slide deposition in the lakes. These records will help correlate the various cores in each
lake, as well as possibly correlating between lakes. It is also possible that lake sediments on older
slide surfaces may preserve event-strata related to emplacement of younger slide events.
Natural Exposure Dating
At the distal edges of the two largest landslide lobes (Fig. 2), there are natural fluvial incisions of
the landslide debris (Fig. 4) along the South and North forks of the Nooksack River. This
summer (2015), I will investigate these exposures and any others for visible stratigraphy and
collect any datable organic materials included within, or below any exposed slide debris. Basal
contacts, if exposed, will provide valuable constraints for thickness and volume estimates of the
landslide debris lobes (Orwin et al., 2004). Radiocarbon dating of terrestrial macrofossils or
buried logs embedded in the debris can provide direct age constraints on the actual failure event
(Bertolini et al. 2004). The accuracy of the event age depends on the wood fragment or log
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coming from vegetation that was killed by the landside and incorporated into the debris (Lang et
al., 1999). Outermost rings of intact whole logs are preferable to wood fragments because they
would most closely record the actual kill-date of the tree (Bertolini et al., 2004). For fragments, it
is impossible to tell what part of the tree the wood came from. If the sample was sourced from
the inner rings where 14C had already started to decay, it would give an age that is some
unknown number of years older than the actual slide event (Lang et al., 1999). If I am only able
to find and date wood fragments, the ages will be considered a maxima. I will travel to Lawrence
Livermore National Laboratory (LLNL) in the fall or winter of 2015 to prepare and conduct
accelerator mass spectrometry analysis on all 14C samples from both natural exposures and lake
sediment cores.
Slide Volume Estimations
I will estimate the volume of each debris lobe based on thickness estimates from residential well
and gas-pipeline logs, natural exposures, and projections of underlying fluvial terraces imaged in
lidar data (Orwin et al., 2004). Surface topography and areal extents of each lobe are well
defined in the lidar data and high-resolution aerial photography. I will also estimate the volume
from the source area by estimating the pre-event topography and using the cut-fill spatial analyst
tool in ArcGIS on a lidar derived DEM. These calculations will provide an independent test of
my debris-field volume calculations. I expect some discrepancy in the two volume calculations
because erosion has likely removed some debris at distal edge of the largest lobe (Fig. 2).
Counteracting that effect however is the increase in volume that would accompany
disaggregation of the original bedrock source during sliding.
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Active Monitoring
For years, residents of the rural community of Van Zandt, built on top of and adjacent to the
main VZLC debris (Fig. 2), have reported loud noises as well as fresh exposures of rock and
debris in the head scarp area (Oral Communication, Andy Wiser; Wayne Harrison, 2014).
Previous workers (e.g., Brunengo, 2001) have also identified recent tension cracks (Figs. 5, 6) in
various locations near the head scarp, suggesting ongoing deformation and the potential for
progressive slope failure. For this study, I will evaluate the relationship between shallow
microseismicity and tension gaps in the headwall region to test the potential for future hazards,
because large landslides often initiate with slow deformation or creep that culminates in
catastrophic failure (Hewitt et al., 2007). Landslide hazard assessment requires the knowledge of
the age, frequency, and volume of previous events, the identification of slopes prone to failure,
and data on active deformation in potential initiation zones (Orwin et al., 2004).
In the spring of 2015, an array of four high-sensitivity Guralp CMG-6TD seismometers
was installed in and around the VZLC source zone (Figs. 2, 7). Over the following year, these
instruments will record shallow microseismicity related to mass movements including fracturing
of the rock mass, rockfall, or deep-seated gravitational shearing. One of the major advantages of
seismic monitoring over other methods is that it allows for continuous measurement and precise
event timing (Helmstetter and Garambois, 2010).
The seismic data will be downloaded monthly using a field laptop computer and
Seismometer Configuration Real-Time Acquisition and Monitoring (SCREAM) software. I will
use a Matlab script to produce spectrogram graphs and Antelope software to produce waveform
graphs of the seismic data. I will evaluate them to estimate relative location and the timing of any
possible “events of interest” that are recorded on at least two of the four stations. After
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attempting to relate these “events of interest” to any global, regional, and local earthquakes as
well as any known anthropogenic activities, I will eliminate any signals that are clearly not
sourced from the landslide mass. Remaining signals will be categorized based on their
spectrogram and waveform signals. I will then compare the results to preceding and concurrent
hydrometeorological records from the nearby Lawrence and Clearbrook rain gauges to
investigate if the frequency (number of) of seismic events is related to rainfall duration, intensity,
cumulative rainfall, or any combination of these parameters.
To further constrain progressive strain in the source zone, in early October 2015, I will
install Unimeasure JX-PA potentiometric extensometers in several tension gaps in the headwall
region to record displacement and activity of deforming blocks (Fig. 7). Because these
extensometers will be connected to timed dataloggers making hourly measurements, I will be
able to evaluate both whether the fractures are actively forming as well as whether such
movement is episodic or continuous. In addition, I will be able to test if they are linked to
microseismicity recorded by the seismometers. The extensometers have maximum spans of 2-4
m, a standard measurement range of 120 mm, and repeatable precision of 0.15 mm. The main
instrument body can be bolted directly to bedrock and has an extendable draw wire with an
anchor that can likewise be bolted onto rock (Fig. 7). In reconnaissance mapping, I have located
three tension gaps in the head scarp region (Figs. 5, 6) that are well suited for measuring
displacement in the dominant failure direction (WNW-directed slip down the dip of bedding
planes). Sheared soil and tree roots (Fig. 6) indicate that these tension fractures have been
progressively forming and recently active, probably within the last few years. I will download
the data monthly from two Campbell Scientific dataloggers using a field laptop computer and
Campbell Scientific Loggernet software.
19
Anticipated Results
The radiocarbon ages I hope to produce in my study will provide new constraints on the ages of
the two largest debris lobes of the VZLC. The efforts of this project will produce at least limiting
ages and potentially “event” ages for the triggering of large rock slope failures of the VZLC.
These ages may correlate with known paleoseismic events, paleoclimatic intervals and/or other
regional dated landslides (e.g., Pringle et al., 1998; Orwin et al., 2004; Pringle and Schuster,
1998; Engebretson et al., 1996). I expect to find similar minimum ages from the two coring
targets on the largest debris lobe and for these dates to be distinct from the minimum age found
for the youngest debris lobe (Fig. 2). I anticipate that the two main debris lobes will be younger
than ca. 2,000 years B.P. based on their sharply defined topography, thin soils, and poorly
developed drainage network as apparent both on lidar imagery (Fig. 2) and in the field.
Although it is possible that the source zone may remain quiescent over the relatively
short length of my study, I am hopeful that the active monitoring component of this study will
provide evidence for ongoing deformation in the source area of the VZLC. I expect that the
seismometers will show intermittent high-frequency seismic events at all four stations,
potentially indicating local rockfalls, and bedrock fracturing or creep. I expect that the installed
wire extensometers will show that tension gaps in the head scarp region are expanding as active
blocks are gravitationally slumping along discontinuity slip surfaces. If active, it is reasonable to
expect that any movement will be seasonally variable, with increased activity in the wetter and
colder winter months, especially during strong storms with heavy precipitation and rapid
groundwater recharge, or during periods of diurnal freeze-thaw cycles. It will be particularly
crucial to determine if displacement along tension cracks is synchronous with shallow ground
movements recorded by the four seismometers, particularly if it occurs during or shortly after a
20
large rain event. Although it may be difficult to precisely constrain rainfall threshold conditions,
this study will shed light on which parameters (intensity, duration, cumulative rainfall) have the
greatest influence on mass movements and are most likely to trigger larger failures in the VZLC
and other regional deep-seated bedrock landslides.
Potential Problems
One of the greatest hurdles to completing all of the goals of this project is landowner permission
and access. The Washington State Department of Natural Resources (DNR) owns the majority of
the source area and has granted me permission to access, install instruments, and conduct lake
coring on their land (Appendix A). The runout zone, however, is divided into numerous small
parcels owned by dozens of different private landowners. Permission to core at least one lake on
each of the two main debris lobes is the most critical, along with access to any natural exposures
of the landslide debris. General access to land in the runout zone is also important for completing
geomorphic mapping. To date, I have had good success gaining access to many of the crucial
regions on the slide.
Lake sediment coring can present a range of challenges involving the sediment and lake
characteristics, the equipment, and sediment recovery. For example, one of the lakes that is
located at the base of a steep slope (Fig. 2) may have been repeatedly affected by post-slide
rockfall or tree-falls. The Livingston coring system is not well suited to core through sediments
that contain coarse debris or large tree trunks; in such a situation, it may be difficult or
impossible to reach the basal most sediments most closely tied to the slide.
Reservoir effects are often a concern with 14C dating in terrestrial lakes or ponds. Even
small amounts of carbonate minerals or other sources of “old” carbon (radiocarbon dead)
21
percolating into the lake can cause 14C ages on aquatic plants to be inaccurate (Wohlfarth et al.,
1998). Although the Chuckanut Formation does not contain anything more than trace amounts of
carbonate minerals, it does have an abundance of ancient organic-rich strata (Mustoe, 1997).
The sensitive nature of the Guralp CMG-6TD seismometers makes them highly effective
for recording small-scale proximal ground movements. However, this sensitivity can also cause
the instruments to record unwanted anthropogenic “noise” which can dilute the seismic signal.
All four instruments are within 1.5km of human habitations and are in an area that includes
active logging, hunting, agriculture, a railroad, and a state highway (Figs. 2, 3).
I also acknowledge that external factors (e.g., weather) over the course of my study may
not reach threshold conditions needed to cause responses measurable with my instrumentation.
For example, the strong El Nino forecast for the coming winter (2015-2016) will likely be
warmer and dryer than average, significantly reducing the effects related to both heavy
precipitation as well as freeze-thaw events.
Project Timeline
The array of four seismometers was installed during the spring of 2015. I will complete monthly
data downloads from these instruments through at least the spring of 2016. Lake coring, field
mapping, and sample collection from debris exposures will commence in the summer of 2015
and continue into the fall. I will install the wire extensometers in early October and will collect
those data at the same time I download data from the seismometers. 14C analysis will likely take
place during the fall or winter of 2015, depending on the schedule of Lawrence Livermore
National Lab. Interpretation of both the 14C results and active monitoring data will take place
during the spring of 2016.
22
References Cited
Arsenault, Ann M., and Andrew J. Meigs. 2005."Contribution of deep‐seated bedrock
landslides
to erosion of a glaciated basin in southern Alaska." Earth Surface Processes and
Landforms 30, no. 9: 1111-1125.
Atwater, Brian F., Bobb Carson, Gary B. Griggs, H. Paul Johnson, and Marie S. Salmi. 2014.
“Rethinking Turbidite Paleoseismology along the Cascadia Subduction Zone.” Geology
42 (9): 827–30.
Badger, Thomas C. 2002. “Fracturing within Anticlines and Its Kinematic Control on Slope
Stability.” Environmental & Engineering Geoscience 8 (1): 19–33.
Barnett, E. A., H. M. Kelsey, B. L. Sherrod, R. J. Blakely, J. F. Hughes, E. R. Schermer, R. A.
Haugerud, C. S. Weaver, and E. Siedlecki, 2006, Active faulting at the northeast margin of
the greater Puget lowland: A paleoseismic and magnetic-anomaly study of the Kendall
scarp, Whatcom County, Northwest Washington, Eos Trans. AGU, 87(52), Fall Meet.
Suppl., Abstract S31A-0183.
Bertolini, Giovanni, Nicola Casagli, Leonardo Ermini, and Claudio Malaguti. 2004.
“Radiocarbon Data on Late glacial and Holocene Landslides in the Northern Apennines.”
Natural Hazards 31 (3): 645–62.
Blais-Stevens, Andrée, Reginald L. Hermanns, and Courtney Jermyn. 2011. “A 36Cl Age
Determination for Mystery Creek Rock Avalanche and Its Implications in the Context of
Hazard Assessment, British Columbia, Canada.” Landslides 8 (4): 407–16.
Brown, Edwin H., and Joe D. Dragovich, 2003. Tectonic elements and evolution of northwest
Washington. Washington Division of Geology and Earth Resources.
Brunengo, M.J. Guidebook for the Northwest Geological Society Field Trip: June 2-3, 2001: Part
2. The Van Zandt Dike Landslide. Northwest Geological Society (2001).
Clague, John J., R. W. Mathewes, J.-P. Guilbault, I. Hutchinson, and B. D. Ricketts. 1997. “PreYounger Dryas Resurgence of the Southwestern Margin of the Cordilleran Ice Sheet,
British Columbia, Canada.” Boreas 26 (3): 261–78.
Corominas, J, J Moya, A Lloret, J. A Gili, M. G Angeli, A Pasuto, and S Silvano. 2000.
“Measurement of Landslide Displacements Using a Wire Extensometer.” Engineering
Geology 55 (3): 149–66. doi:10.1016/S0013-7952(99)00086-1.
Crider, Juliet G., Tucker, David, Linneman, Scott L. Clark, Doug, 2009. "The 2009 Racehorse
Creek Landslide: Forensic Dynamics of a Large, Complex Catastrophic Mass
Movement." 2009 Portland GSA Annual Meeting.
23
Cruden, D. M., and O. Hungr. 1986. “The Debris of the Frank Slide and Theories of Rockslide–
avalanche Mobility.” Canadian Journal of Earth Sciences 23 (3): 425–
Dragovich, J.S. 1997, Geologic map and interpreted geologic history of the Kendall and Deming
7.5-minut quadrangles, western Whatcom County, Washington. Washington, Division of
Geology and Earth Reseurces.
Engebretson, D.C., Easterbrook, D.J., and Kovanen, D.J. 1996, Triggering of very large, deepseated, bedrock landslides by concentrated, shallow earthquakes in the North Cascades,
Washington: Geological Society of America Abstracts with Programs, v. 28, no. 5, p. 64.
Geertsema, Marten, John J. Clague, James W. Schwab, and Stephen G. Evans. 2006. “An
Overview of Recent Large Catastrophic Landslides in Northern British Columbia,
Canada.” Engineering Geology, Large Landslides: dating, triggering, modelling, and
hazard assessment, 83 (1–3): 120–43.
Geertsema, M., and Schwab, J.W., 1997 "Retrogressive flowslides in the Terrace-Kitimat,
British Columbia area: from early post-deglaciation to present and implications for future
slides." Proceedings of 11th Vancouver geotechnical society symposium, Vancouver.
Goldfinger, Chris, C. Hans Nelson, and Joel E. Johnson. 2012. “Cascadia Great Earthquake
Chronology Based on the Turbidite Event Record.” Boulder, CO, United States:
Geological Society of America GSA.
Hewitt, Kenneth, John J. Clague, and John F. Orwin. 2008. “Legacies of Catastrophic Rock
Slope Failures in Mountain Landscapes.” Earth-Science Reviews 87 (1–2): 1–38.
Horton, R.E., 1938. “Phenomena of the contact zone between the ground surface and a layer of
melting snow” Assoc. Internationale d’Hydrol Scientifique, Paris 244: 545-561
Hsü, K. J. 1975. “Catastrophic Debris Streams (Sturzstroms) Generated by Rockfalls.”
Geological Society of America Bulletin 86 (1): 129–40.
Hsu, K.J. , 1978. Albert Heim: observations on landslides and relevance to modern
interpretations. In: Voight, B. (Ed.), Rockslides and Avalanches. In: Natural Phenomena,
vol. 1. Elsevier, Amsterdam, pp. 71–93.
Huggel, Christian, John J. Clague, and Oliver Korup. 2012. “Is Climate Change Responsible for
Changing Landslide Activity in High Mountains?” Earth Surface Processes and
Landforms 37 (1): 77–91.
Hungr, O., Evans, S.G., 2004. Entrainment of debris in rock avalanches: an analysis of the longrunout mechanism. Geol. Soc. Am. Bull. 116, 1240–1252.
Hungr, Ol., Fell, R.,, Couture, R.,and Eberhardt, E. 2005. Landslide Risk Management. CRC
Press.
24
Johnson, S.Y., 1984, thesis, University of Washington, Seattle (1982); Can. J. Earth Sci. 21, 92
Iverson, R. M. 2000. “Landslide Triggering by Rain Infiltration.” Water Resources Research 36
(7): 1897–1910.
Iverson, R. M., D. L. George, K. Allstadt, M. E. Reid, B. D. Collins, J. W. Vallance, S. P.
Schilling, et al. 2015. “Landslide Mobility and Hazards: Implications of the 2014 Oso
Disaster.” Earth and Planetary Science Letters 412 (February): 197–208.
Johnson, Samuel Y. 1984. “Stratigraphy, Age, and Paleogeography of the Eocene Chuckanut
Formation, Northwest Washington.” Canadian Journal of Earth Sciences 21 (1): 92–106.
Keaton, J. R. et al. The 22 March Oso Landslide, Snohomish County, WA; GEER
Summary. NSF Geotechnical Extreme Events Reconnaissance, 172 pp.
Report
(2014).
Kovanen, D. J., and D. J. Easterbrook. 2001. “Late Pleistocene, Post-Vashon, Alpine Glaciation
of the Nooksack Drainage, North Cascades, Washington.” Geological Society of America
Bulletin 113 (2): 274–88. doi:10.1130/0016Lang, Andreas, José Moya, Jordi Corominas, Lothar Schrott, and Richard Dikau. 1999. “Classic
and New Dating Methods for Assessing the Temporal Occurrence of Mass Movements.”
Geomorphology 30 (1–2): 33–52.
Legorreta-Paulin, Gabriel, Carol Serdar, and Venice Goetz. 2008. “An Overview of the
Washington State Department of Natural Resources, Forest Practices Division’s
Landslide Hazard Zonation Project Methodology and Products for Identification of
Unstable Slopes.” Boulder, CO, United States: Geological Society of America GSA.
Mathewes, Rolf W., and Linda E. Heusser. 1981. “A 12 000 Year Palynological Record of
Temperature and Precipitation Trends in Southwestern British Columbia.” Canadian
Journal of Botany 59 (5): 707–10.
Matsuoka, Norikazu, Kazuomi Hirakawa, Teiji Watanabe, Wilfried Haeberli, and Felix Keller.
"The role of diurnal, annual and millennial freeze-thaw cycles in controlling alpine
slope instability." In Proceedings of the
Seventh International Conference on
Permafrost, Yellowknife, Canada, Collection Nordicana, vol. 57, pp.
711-717.
1998.
Montgomery, David R. 2002. “Valley Formation by Fluvial and Glacial Erosion.” Geology 30
(11): 1047–50.
Mustoe, G.E, 1997 Paleogeography and Paleontology of the Early Tertiary Chuckanut
Formation, Northwestern Washington, Washington Geology, v.25, no. 3, pg.3
Nelson, Daniel B., Mark B. Abbott, Byron Steinman, Pratigya J. Polissar, Nathan D. Stansell,
25
Joseph D. Ortiz, Michael F. Rosenmeier, Bruce P. Finney, and Jon Riedel. 2011.
“Drought Variability in the Pacific Northwest from a 6,000-Yr Lake Sediment Record.”
Proceedings of the National Academy of Sciences 108 (10): 3
Orwin, John F., John J. Clague, and Robert F. Gerath. 2004. “The Cheam Rock Avalanche,
Fraser Valley, British Columbia, Canada.” Landslides 1 (4): 289–98.
Pánek, Tomáš. 2014. “Recent Progress in Landslide Dating A Global Overview.” Progress in
Physical Geography, October, 0309133314550671.
Pringle, P. T.; Schuster, R. L., 1998, A new radiocarbon age for the Bonneville landslide,
Columbia River Gorge, Washington [abstract]: Association of Engineering Geologists,
41st Annual Meeting, Program with Abstracts, p. 116.
Pringle, P.T., Schuster, R.L., and Logan, R.L.1998, New radiocarbon ages of major landslides in
the Cascade Range, Washington, Washington Geology, v.26, p. 31-39.
Scheidegger, A., 1973. On the prediction of the reach and velocity of catastrophic landslides.
Rock Mech. 5, 231–236
Schmidt, Kevin M., 1994, Mountain Scale Strength Properties, Deep-Seated Landsliding, and
Relief Limits, Thesis (M.S), University of Washington, Seattle, WA--Digital, Available
at: http://www.dnr.wa.gov/publications/fp_tfw_sh10_95_001.pdf
Schmidt, Kevin M., and David R. Montgomery. 1995. “Limits to Relief.” Science 270 (5236):
617–20.
Sherrod, Brian L., Elizabeth Barnett, Elizabeth Schermer, Harvey M. Kelsey, Jonathan Hughes,
Franklin F. Foit, Craig S. Weaver, Ralph Haugerud, and Tim Hyatt. 2013. “Holocene
Tectonics and Fault Reactivation in the Foothills of the North Cascade Mountains,
Washington.” Geosphere 9 (4): 827–52.
Siedlecki, E. M. 2008, The geometry and earthquake history of the Boulder Creek fault: A
paleoseismic, seismic refraction and ground penetrating radar study, Whatcom County,
Washington, Thesis (M.S)--Western Washington University, Bellingham, WA-Print
Steinman, Byron A., Mark B. Abbott, Michael E. Mann, Joseph D. Ortiz, Song Feng, David P.
Pompeani, Nathan D. Stansell, Lesleigh Anderson, Bruce P. Finney, and Broxton W.
Bird. 2014. “Ocean-Atmosphere Forcing of Centennial Hydroclimate Variability in the
Pacific Northwest.” Geophysical Research Letters 41 (7): 2014GL059499.
Steinman, Byron A., Mark B. Abbott, Michael E. Mann, Nathan D. Stansell, and Bruce P.
Finney. 2012. “1,500 Year Quantitative Reconstruction of Winter Precipitation in the
Pacific Northwest.” Proceedings of the National Academy of Sciences 109 (29): 116. 19–
23.
Storck, P., Bowling, L., Wetherbee, P., and Lettenmaier, D., 1998, Application of a GIS-based
26
distributed hydrology model for prediction of forest harvest effects on peak stream flow
in the Pacific Northwest: Hydrological Processes, v. 12, p. 889–904.
Tabor, R., Haugerud, R., Hildreth, W., and Brown, E., 2003, Geologic map of the Mount Baker
30- by 60-minute quadrangle, Washington: Geologic Investigations Series I-2660. US
Geological Survey.
Turner, A. Keith, and G. P. Jayaprakash. 1996. “LANDSLIDES: INVESTIGATION AND
MITIGATION. CHAPTER 2 - Assessment.” Transportation Research Board Special
Report, no. 247. http://trid.trb.org/view.aspx?id=462499.
Varnes D.J.,1978. Slope movement types and processes. In: Schuster RL, Krizek RJ (eds).
Landslides, analysis and control, special report 176: Transportation research board,
National Academy of Sciences, Washington, DC., pp. 11–33
WADNR seismic scenario report for a M=6.8 Earthquake on the Boulder Creek Fault,
Northwest Washington, 2012, Division of Geology and Earth Resources.
WADNR seismic scenario report for a M=9.0 Cascadia Subduction Zone Earthquake, 2013,
Division of Geology and Earth Resources.
WADNR Significant Deep-Seated Landslides in Washington State – 1984 to 2014, 2014, Division
of Geology and Earth Resources.
http://wa-dnr.s3.amazonaws.com/publications/ger_list_large_landslides.pdf
Wells, Ray E., Craig S. Weaver, and Richard J. Blakely. 1998. “Fore-Arc Migration in
Cascadia and Its Neotectonic Significance.” Geology 26 (8): 759–62.
Wells, Ray E., and Robert McCaffrey. 2013. “Steady Rotation of the Cascade Arc.” Geology 41
(9): 1027–30.
White, J.L., Morgan, M.L, and Berry, K.A., “The West Salt Creek Landslide” A Catostrophic
Rockslide and Rock/Debris Avalanche in Mesa County, CO” Colorado Geological
Survey Bulletin 55:1-55
Wohlfarth, B., Skog, G., Possnert, G., Holmquist, B., 1998. Pitfalls in the AMS radiocarbondating of terrestrial macrofossils. J. Quat. Sci. 13, 137–145. doi:10.1002/(SICI)10991417(199803/04)13:23.0.CO;2-6
Wright, H.E., 1967. A square-rod piston sampler for lake sediments. Sediment. Res. 37, 975–
976.
Zerathe, S., Lebourg, T, Braucher T., , and Bourlès, D. 2014. “Mid-Holocene Cluster of LargeScale Landslides Revealed in the Southwestern Alps by 36Cl Dating. Insight on an
Alpine-Scale Landslide Activity.” Quaternary Science Reviews 90 (April): 106–27.
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Figures
Figure 1. Simplified regional geologic map of the northern Nooksack River Basin. The VZLC is one of several large landslide
complexes in the Nooksack River Basin. Multiple reactivated Tertiary faults are within close proximity to the VZLC and the
other large landslides in the area (Sherrod et al., 2013). Map data (geologic units and faults) from Dragovich, 1997.
28
a.
b.
700
A
A'
Elevation (m)
600
L
500
400
H
300
200
100
0
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Horizontal Distance (m)
Figure 2. Preliminary mapping (a) and longitudinal topographic profile (b) of the Van Zandt Landslide Complex. a. Airborne lidar
hillshade map indicates three discrete overlapping landslide debris lobes, a general landslide source zone, several lacustrine coring
targets (at least three on each of the younger two lobes), and a colluvial lobe that was likely formed by small-scale failures. Natural
exposures of landslide debris along the South Fork Nooksack River may also provide datable material and will aide in debris
thickness and volume estimations. Road system provides ready access and indicates residential areas potentially at risk. Trench logs
of the pipeline excavation should provide further subsurface constraints. b: Longitudinal topographic profile (A-A’) indicates runout
length of debris-lobe 2 and factors H and L along transect A-A’. Vertical exaggeration is zero. Source Information: Lidar data (PSLC,
2006; 2013). Mapping by G. Malick.
29
Figure 3. Headscarp and main debris lobes of the VZLC. Imagery from Google Earth, capture date 5/4/2014, facing
west at oblique angle.
Fig 4. Contact between the VZLC debris and underlying alluvial deposits in a river-incised natural exposure along the
South Fork Nooksack River.
30
Fig 5. Tension fracture in the headscarp region of the VZLC. Note soil probe for scale.
Fig. 6. Tension Fracture in the headscarp region of VZLC showing torn roots and disturbed trees.
31
Fig 7. Schematic diagram of the VZLC headscarp region with tension fracture perpendicular to the dominant failure
direction (down the dip of bedding planes). Thin intervals of shale, mudstone, and coal represent potential rupture
surfaces and together with extensive bedrock fractures modulate slope stability in the VZLC. A: Example of
seismometer configuration. The instrument is buried ~1 m below the surface and is preferably resting on bedrock.
Power is sourced from a 12 V battery and solar panel. B: Example of extensometer configuration. The main instrument
body is bolted directly into bedrock and a draw wire extends across the gap and is anchored on the opposite wall of the
tension fracture. A 12 V battery supplies power to a data logger and the potentiometric sensor.
32