U.S. Department of the Interior

U.S. Geological Survey

Bedrock Mapping and Seismic Hazard Assessment at 

Gold Basin Landslide, Washington

By Lydia M. Staisch 

Open-File Report 2018–1132
U.S. Department of the Interior

RYAN K. ZINKE, Secretary 

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018 

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Suggested citation:

Staisch, L.M., 2018, Bedrock mapping and seismic hazard assessment at Gold Basin landslide, 

Washington: U.S. Geological Survey Open-File Report 2018–1132, 19 p.

https://doi.org/10.3133/ofr20181132. 

ISSN 2331-1258 (online)
iii

Contents

Background ................................................................................................................................................... 1

Scope of Investigation ................................................................................................................................... 5

Bedrock Mapping ....................................................................................................................................... 3

Seismic Hazard Assessment...................................................................................................................... 3

Bedrock Mapping........................................................................................................................................... 3

Methods ..................................................................................................................................................... 3

Field Observations ..................................................................................................................................... 6

Rock Mass Strength Results.................................................................................................................. 8

Seismic Hazard Assessment ....................................................................................................................... 12

Probability of Ground Shaking Exceedance ............................................................................................. 14

Bedrock Characteristics and Coseismic Hillslope Failure......................................................................... 16

References Cited......................................................................................................................................... 18

Figures

1. Topographic map of Gold Basin, Washington, overlain by lidar hillshade map from the 2005 

Snohomish and 2013 Tulalip lidar surveys of the Gold Basin landslide area.................................... 2

2. Preliminary geologic map of the South Fork Stillaguamish river centered over the Gold Basin 

landslide, Washington....................................................................................................................... 5

3. Statistical plots of Schmidt hammer measurements from bedrock sites within the Gold Basin 

landslide study area, Washington................................................................................................... 10

4. Plots of landslides and slope values by geologic unit in the Gold Basin landslide study area ........ 12

5. Probabilistic Seismic Hazard map of Washington State ................................................................. 13

6. Probabilistic seismic hazard curve extracted from the U.S. Probabilistic Seismic Hazard 

Assessment map............................................................................................................................ 15

7. Slope map of the Gold Basin landslide study area, South Fork Stillaguamish River, Washington.. 16

Tables

1. Rock Mass Strength parameters and relative rating......................................................................... 6

2. Field sites, discontinuity orientations, Schmidt hammer measurements, and RMS values............... 8

3. Discontinuity orientations with respect to hillslope and Rock Mass Strength parameters for bedrock 

field sites......................................................................................................................................... 11

Bedrock Mapping and Seismic Hazard Assessment at 

Gold Basin Landslide, Washington

By Lydia M. Staisch 

Background

The Gold Basin landslide is located along the South Fork Stillaguamish River, within the Mount 

Baker-Snoqualmie National Forest in western Washington State (fig. 1). Recent concerns related to 

slope stability after the 2014 State Route 530 Landslide near Oso, Washington, forced the closure of the 

U.S. Forest Service Gold Basin Campground in May of 2014. In addition to safety concerns for 

National Forest visitors, the landslide-derived sediment pulses shed into the South Fork Stillaguamish 

River may harm migrant salmon spawning grounds (Shannon & Wilson Engineers, 1954; Benda and 

Collins, 1992), an important resource for the Stillaguamish Tribe of Indians and for public anglers. 

The Gold Basin landslide is composed of three active lobes and has an approximate footprint of 

566,560 m2 (Anchor QEA LLC, 2012). Each lobe consists of steep topographic escarpments contained 

largely within Pleistocene glacial outwash sediments (Booth, 1989; Drury, 2001; Benda and Collins, 

1992) and debris flow and earth flow deposits at the base. Glacial outwash sediments range broadly in 

character, from laterally continuous homogenous clay-rich sediments to lenticular, discontinuous 

packages of well-rounded and lithologically heterogeneous gravel. Together, the steep local relief and 

fine-grained strata of the Gold Basin landslide contribute to its large sediment delivery to the South 

Fork Stillaguamish River (Benda and Collins, 1992; Drury, 2001). 

Several previous investigations focused on slope stability of the Gold Basin landslide and nearby 

landslide complexes and have estimated the mechanisms and volumes of sediment delivered from the 

Gold Basin landslide into the South Fork Stillaguamish River. Miller (1999) found that the lateral and 

vertical heterogeneity of Pleistocene glacial outwash strata leads to high groundwater flow in localized 

areas. Specifically, this is thought to occur when permeable sand and gravel is deposited over relatively 

impermeable clay and silt (Miller, 1999). McCabe (2016) measured stratigraphic columns within the 

middle lobe of the Gold Basin landslide and observed that sand and gravel deposited over fine 

impermeable layers did indeed correlate to perched ground water flow. The stratigraphy and 

groundwater observations suggest that erosion within the Gold Basin landslide may be concentrated 

along the upper extent of the exposed headwalls (McCabe, 2016). In addition to the concentration of 

groundwater flow, the shrinking and swelling of clay and silt-rich layers during wet and dry seasons 

leads to vertical cracking and block failure within the Gold Basin landslide (Shannon & Wilson 

Engineers, 1954). 

The sediment and slope characteristics of the Gold Basin landslide lead to an overall large 

contribution of sediment delivery to the South Fork Stillaguamish River, estimated to be 25 percent of 

the total annual sediment load (40,000 tons per year from the Gold Basin landslide) (Benda and Collins, 

1992). In a more quantitative effort, McCabe (2016) used terrestrial lidar scanning (TLS) and measured 

a net volumetric loss of 4,800 m3 of the middle lobe of the Gold Basin landslide over a 6-month 

investigation (July to January). With the given density of dry glacial outwash (2,160 kg/m3, Savage and 

others, 2000), this amounts to an annual contribution of about 11,500 tons from the middle lobe alone.
2

Figure 1. Topographic map of Gold Basin, Washington (2017 Whitehorse Mountain, 2017 Helena Ridge, 2017 

Silverton, and 2017 Mallardy Ridge quadrangles), overlain by lidar hillshade map from the 2005 Snohomish and 

2013 Tulalip lidar surveys (http://lidarportal.dnr.wa.gov/) of the Gold Basin landslide area. Inset map shows location 

of Gold Basin in Washington State. Red outline shows the three main lobes of the Gold Basin landslide. Blue 

outline shows area of mass wasting that is less well defined than the three lobes.

Assuming the three lobes equally supply sediment to the South Fork Stillaguamish River and 

that the Gold Basin landslide sediment contribution is similar over the other six months of the year, the 

TLS results may suggest an overall annual sediment delivery over 34,000 tons, only slightly less than 

suggested by Benda and Collins (1992). The main mechanisms of sediment delivery to the South Fork 

Stillaguamish River are from debris flows and erosion of exposed headwalls and prior landslide deposits 

(Benda and Collins, 1992; Miller, 1999). 
3

In addition to landslides confined within the Pleistocene glacial strata, bedrock landslides are 

also apparent on lidar imagery of the study area (fig. 2). Bedrock landslides may pose additional hazard 

to the area, either during stochastic hillslope failure or during strong ground motion events (Meunier and 

others, 2007, 2008). Potential seismic sources include the proximal Darrington-Devils Mountain and 

southern Whidbey Island fault zones, as well as the offshore Cascadia Subduction Zone (Atwater and

Hemphill-Haley, 1997; Witter and others, 2003; Sherrod and others, 2008; Personius and others, 2014). 

Previous analyses of hillslope stability in the Cascade Range suggests that rock mass strength (RMS) is 

a useful way of characterizing bedrock and fracture patterns in order to understand potential landslideprone landscape (Schmidt and Montgomery, 1996). 

Scope of Investigation

The goals of this investigation are to assess the glacial strata and bedrock geology of the Gold 

Basin landslide and adjacent areas and to assess how the geology and geomorphology within the study 

area affect the likelihood of coseismic landsliding. 

Bedrock Mapping

Bedrock mapping initiated with the review of existing geologic maps and the current state of 

knowledge on rock units within the project area, followed by field work and the assessment of potential 

geologic influence on slope stability. Methods required field verification of rock units underlying the 

landslide to determine how glacial stratigraphy and bedrock geology can affect the likelihood of 

coseismic landslides, and to use quasi-quantitative RMS measurements to assess slope stability in 

various geologic units. 

Seismic Hazard Assessment

Seismic hazard assessment initiated with a review the known seismicity in the area and to assess 

nearby structural features that could affect landslide behavior in the Gold Basin landslide vicinity during 

seismic events. This was followed by an estimation of maximum credible ground acceleration and 

discussion of expected duration of inter-event intervals for seismic events.

Bedrock Mapping

Methods

Fieldwork was conducted during the week of August 7, 2017. The focus of fieldwork was to 

assess Pleistocene glacial outwash sediment stratigraphy and slope stability and to assess bedrock 

strength near and within the Gold Basin landslide complex.

Pleistocene glacial outwash field sites were located within the Gold Basin landslide and along 

the eroded cut banks of the South Fork Stillaguamish River. Several key observations were noted at 

each site: (1) local stratigraphy, including bedding thickness and composition, (2) ground water flow out 

of the glacial outwash sediments, (3) fracture density and orientation with respect to hillslope, (4) 

maximum cliff height, and (5) evidence of recent hillslope failure.
4
5

Figure 2. Preliminary geologic map of the South Fork Stillaguamish river centered over the Gold Basin landslide, 

Washington. Basemap is a lidar hillshade image derived from the Snohomish 2005 and Tulalip 2013 lidar datasets 

(http://lidarportal.dnr.wa.gov/) and the U.S. Geological Survey National Elevation Dataset (NED). Original mapping 

is from Pessl and others (1989) and Tabor and others (1988, 2002). New field sites are denoted by colored dots 

and site locations and descriptions are available in table 2).

Bedrock field sites were located within the South Fork Stillaguamish watershed within a 15 km 

radius of the Gold Basin landslide. To assess the potential for deep-seated landslides in bedrock, 

hillslope strength was assessed using the RMS scheme from Selby (1980) and adapted by Schmidt and 

Montgomery (1996). Measurements include: (1) the strength of intact bedrock, assessed using Schmidt 

hammer measurement; (2) the degree of bedrock weathering; (3) the orientation of discontinuities with 

respect to hillslope, including bedding, foliation, and cleavage; (4) the width of observed discontinuities; 

(5) the lateral and vertical pervasiveness and degree of clay infilling of discontinuities; and (6) the 
6

movement of groundwater out of the rock mass (table 1). Observations were made during the dry season 

of 2017, which was a particularly dry year in western Washington, and therefore groundwater flow was 

observed at near-minimum flow levels.

Following fieldwork, bedrock mapping was entered and digitized in Esri ArcGIS, guided by 

existing 1:100,000 scale mapping (Pessl and others, 1989; Tabor and others, 1988, 2002). Landslides 

were mapped remotely using available lidar imagery and the landslide mapping protocol from Burns 

and Madin (2009). Aside from landslide mapping, geologic contacts were only changed from the 

original mapping where field observations or lidar imagery clearly showed need for amendment. The 

resulting geologic map is to be considered preliminary (fig. 2). 

Table 1. Rock Mass Strength (RMS) parameters and relative rating (R)

Parameter Very strong Strong Moderate Weak Very weak

Intact rock strength Very coherent Coherent Moderate 

coherent

Weak Very weak

R=20 R=18 R=14 R=10 R=5

Weathering Unweathered Slightly 

weathered

Moderately 

weathered

Highly 

weathered

Completely 

weathered

R=10 R=9 R=7 R=5 R=3

Discontinuity 

Spacing

>3 m 3–1 m 1–0.3 m 0.3–0.05 m <0.05 m

R=30 R=28 R=21 R=15 R=8

Discontinuity 

orientation with 

respect to 

hillslope

Moderate dip into 

slope

Steep dip into 

slope

Horizontal or 

vertical dip

Steep dip out of 

slope

Moderate dip out 

of slope

R=20 R=18 R=14 R=9 R=5

Discontinuity 

width

<0.1 mm 0.1–1 mm 1–5 mm 5–20 mm >20 mm

R=7 R=6 R=5 R=4 R=2

Discontinuity 

pervasiveness 

and infilling

None pervasive Few pervasive Pervasive, 

no infill

Pervasive, 

thin infill

Pervasive, 

thick infill

R=7 R=6 R=5 R=4 R=1

Outflow of ground 

water

None Trace <25 liters per 

m/102

25–125 liters per 

m/102

>125 liters per 

m/102

R=6 R=5 R=4 R=3 R=1

Total 100–91 90–71 70–51 50–26 <26

Field Observations

Thirty-one field sites were visited within the Gold Basin landslide area. Table 2 includes a list of 

field site locations and measurements taken. Field site locations are also shown on figure 2 and colorcoded based on geologic unit. An expanded list of pertinent field observations is included below: 

Site 2–1. Steep exposure of glacial outwash. Strata are mostly sandy with some pebble-rich layers. 

Horizontal bedding. Cliffs are ~13 m high. 

Site 2–2. Steep exposure of glacial outwash. More conglomeratic layers than site 2–1.

Site 2–3. Steep 21.4-m-high cliff of glacial outwash. Base is conglomerate with pebble- to cobblesized clasts. Strata fine upward into sand. 

Site 2–4. Outcrop at this site is higher in glacial outwash stratigraphy. Units are sandier and seem 

more indurated. Vertical joints are more closely spaced. Horizontal bedding is more closely 

spaced.
7

Site 2–5. Cliff exposure consists of 130-m of glacial outwash. Hummocky wet clay landslide and 

debris flow deposits at base of cliffs. Debris flows at base of escarpment are very water

laden. Stratigraphy at base of cliffs is thick and sandy with interbedded cobble-rich lenses. 

Generally fines upward. Beds are finely laminated near top. Cliffs fail as flakes along 

vertically oriented joint planes. Joint spacing appears to decreases in finer units towards top 

of cliffs.

Site 2–6. Small landslide in glacial outwash. Deposit is mainly composed of sand and pebble sized 

gravel. 

Site 3–2. Glacial outwash exposure along South Fork Stillaguamish River. 16.8-m-high exposure. 

Base is pebble-cobble conglomerate fining upwards into sand. Upper layers are finely 

laminated 205-cm-thick beds. Crest of cliff is boulder conglomerate with scour at base. 

Energetic channel fill layer mimics topography.

Site 3-7. Glacial outwash exposure along Mountain Loop Highway. Base of outcrop is pebble 

conglomerate that fines to medium-coarse sand. Top is cobble conglomerate with both very 

well rounded and angular clasts. 

Site 4–1. Glacial outwash exposure along South Fork Stillaguamish River. Fine-medium sand holds 

overhanging cliff angle. Swaley cross-bedding, up to 1-m-thick beds. Block failure calves off 

along vertically oriented fractures. Blocks are 1–10 cm thick. 

Site 4–2. Glacial outwash exposure along South Fork Stillaguamish River. Very clay-rich unit at river 

level, extended upwards several meters. Top of clay-rich exposure is not clear at outcrop. 

Several interbedded gravel layers are present, non-cliff-forming exposure.

Site 4–3. Underlying competent bedrock exposure [KJph(w)] at crest of ridge. Glacial outwash likely 

a thin veneer at this site.

Site 4–4. Glacial outwash exposure along South Fork Stillaguamish River. Cliff is 41.8 m high. 

Cobble-rich layers do not hold cliff angle as well as sandy layers. Sandy units may have fine 

cementation, perhaps glacial rock flour or clay. Water trickling out of base of cobble-rich 

layers with finer strata beneath. Cobble-rich layers act as local aquifer. Observed water flow 

out of cliff up to 30.2 m above river level. 

Site 4–5. Small outcrop of glacial outwash along South Fork Stillaguamish River. Gray clay-rich unit 

with interbedded cobble-rich layers. Unclear at outcrop whether cobble layers are 

interbedded or if they are more recent channel fill.

Site 4–6. Glacial outwash exposure capped by Quaternary alluvium. Glacial outwash exposure extends 

2.4 m above river level. From 2.4–8.0 m, alluvium. Glacial outwash is fine and flakey clayrich strata with few to no interbedded pebble clasts. 

Site 4–7. Glacial outwash exposure along Marten Creek. Cliff exposure is 16.8 m high. Strata is very 

clay rich and dark gray in color. Glacial strata are capped with 4.4 m of Quaternary alluvium 

(cobble size fraction). Glacial clay strata are water laden.

Site 4–11. Near top of local glacial outwash elevation exposure limit. Top 7.8 m exposed is light gray 

in color and sandy. Locally more well indurated than most previous outcrops. Fractures are 

vertically oriented. Below sandy unit, 2 m of strata that are similarly gray but more clay rich 

than overlying sands. Induration is less well developed. Cobble conglomerate with sandy 

matrix 5.8 m below last exposure. Hillslope here and below is much more easily eroded. 

Finer strata hold steeper hillslope angle.
8

Table 2. Field site locations, discontinuity orientations, Schmidt hammer measurements, and RMS values

[Latitude and longitude in decimal degrees north and west. RMS, rock mass strength; no., number]

Site Latitude Longitude Unit Foliation Joint set 1 Joint set 2 Joint set 3 Schmidt Hammer RMS Strike Dip Strike Dip Strike Dip Strike Dip Average No.

1–1 48.07003 121.71677 KJph(w) 333 62 245 66 - - - - 35.5 11 62

2–1 48.08147 121.73220 Qgo - - - - - - - - - - 49

2–2 48.08044 121.73182 Qgo - - - - - - - - - - 50

2–3 48.08033 121.72911 Qgo - - - - - - - - - - 58

2–4 48.08120 121.72797 Qgo - - - - - - - - - - 48

2–5 48.08205 121.72724 Qgo - - - - - - - - - - -

2–6 48.07364 121.66096 Qgo - - - - - - - - - - -

2–7 48.07259 121.65417 J^mt(e) 310 48 - - - - - - 24.7 12 65

2–8 48.06582 121.70238 KJph(w) 338 9 195 83 292 89 - - 42.8 10 77

2–9 48.06465 121.70314 KJph(w) 18 22 269 83 110 77 - - 59.2 13 70

3–1 48.09281 121.74638 Qgo - - - - - - - - - - -

3–2 48.08536 121.76214 Qgo - - - - - - - - - - 55

3–3 48.07760 121.74988 KJph(w) 348 17 205 37 296 44 - - 32.2 11 64

3–4 48.03886 121.68658 KJmm(w) 236 16 64 71 17 78 117 79 46.4 15 77

3–5 48.03689 121.69669 KJmm(w) - - - - - - - - 43.4 15 -

3–6 48.02554 121.68753 KJmm(w) 356 88 70 76 28 25 - - 59.1 12 85

3–7 48.04906 121.62963 Qgo - - - - - - - - - - -

3–8 48.04907 121.63261 KJph(w) 124 57 - - - - - - 23.7 17 62

3–9 48.04667 121.64910 KJph(w) 189 19 305 87 106 49 - - 56.5 11 77

3–10 48.05255 121.66433 KJph(w) 84 58 278 85 - - - - 44.6 7 62

4–1 48.07055 121.72056 Qgo - - - - - - - - - - 52

4–2 48.07003 121.71804 Qgo - - - - - - - - - - 53

4–3 48.07005 121.71787 KJph(w) - - - - - - - - - - -

4–4 48.07295 121.71619 Qgo - - - - - - - - - - -

4–5 48.07242 121.66409 Qgo - - - - - - - - - - -

4–6 48.07101 121.65158 Qgo - - - - - - - - - - -

4–7 48.07199 121.60693 Qgo - - - - - - - - - - -

4–8 48.07391 121.59732 J^mt(e) 166 85 251 65 - - - - 55 14 67

4–9 48.10513 121.56806 Oit(s) - - - - - - - - 55.2 13 -

4–10 48.08428 121.77922 KJph(w) 224 57 319 73 29 34 - - 54.5 10 69

4–11 48.08346 121.77143 Qgo - - - - - - - - - - -
9

Rock Mass Strength Results

Hillslope strength is set by several characteristics of bedrock, including intact rock strength and

discontinuity spacing, width of separation along the joint, pervasiveness, and orientation with respect to 

hillslope (Schmidt and Montgomery, 1996). The RMS scheme outlined in table 1 (Selby, 1980; Schmidt 

and Montgomery, 1996) was used to assess RMS values. Each parameter listed bears on hillslope 

stability, however, Schmidt and Montgomery (1996) found that intact rock strength and discontinuity 

orientation with respect to hillslope are particularly key parameters. For instance, hillslopes with 

anaclinal discontinuity orientations, where discontinuities dip into the hillslope, are more stable than 

cataclinal orientations, where discontinuities dip out of the hillslope (Schmidt and Montgomery, 1996). 

Intact rock strength was measured using a Schmidt hammer, which measures the recoil distance 

of a small piston hammer. Multiple Schmidt hammer measurements were taken at each bedrock site. 

Measurements were discarded if they were erroneously measured on flakes or otherwise non-intact rock 

at each outcrop (thus the exclusion of measurements in glacial outwash sediments). Bedrock field sites 

were located within one of three units: KJph(w), Jurassic to Cretaceous low-grade phyllite of the 

western mélange belt; KJmm(w), Jurassic to Cretaceous marine metasedimentary rocks of the western 

mélange belt; and J^mt(e), Triassic to Jurassic metasedimentary and metavolcanic rocks of the eastern 

mélange belt. A summary of Schmidt hammer measurements is shown in figure 3. 

Schmidt hammer measurements taken from KJph(w) ranged from low, weak values (10–20) to 

competent values (60–70), indicating extremely variable rock strength within this particular phyllite unit 

(fig. 3A, B). Measurements taken from KJmm(w) were less variable, and ranged typically between 30 and 

65 (fig. 3A, C), showing overall greater rock competence than KJph(w). Discontinuity spacing was 

generally smaller and more pervasive in KJph(w), decreasing its overall RMS score. Finally, Schmidt 

hammer measurements taken on J^mt(e) varied widely but were also strongly bimodal, with one 

population between about 20 and 31 and another population between 48 and 65 (fig. 3A, D). This suggests 

that rock competence may be generally quite strong for J^mt(e), but local weathering or bedrock 

characteristics can weaken the local strength of the unit and, therefore, decrease hillslope stability.

To assess discontinuity orientation with respect to hillslope, foliation and joint-set orientations 

were measured using a Brunton compass (table 2). In general, the strongest joint-set pattern trends 

northwest-southeast for measured sites and for previously mapped foliations (fig. 2; table 2). Jurassic to 

Cretaceous units (KJmm(w) and KJph(w)) often had 2–3 prominent joint-set orientations, whereas the 

Triassic to Jurassic units (J^mt(e)) had fewer prominent joint-set patterns. However, few sites were 

visited and these observations may not be characteristic of the entire study area. 

Hillslope aspect and slope was measured at each site from existing digital elevation models and 

compared to discontinuity orientations (table 3). Table 3 reports the absolute difference of hillslope 

versus discontinuity strike orientation (Δ Strike) and the angular divergence (Δ Dip) between hillslope 

gradient and discontinuity dip. Low values of absolute orientation difference (Δ Strike <90°) are 

suggestive of cataclinal slopes. Low values of angular divergence (Δ Dip = 10°–40°) are particularly 

susceptible to deep-seated landsliding (Schmidt and Montgomery, 1996). 

Overall, most bedrock sites were assigned “Moderate” RMS values (tables 1, 2). Sites that were 

assigned “Strong” RMS values were contained within either KJph(w) or KJmm(w) units (tables 1, 2). 

For a nearby study on existing slope failure and its correlation to RMS values, Schmidt and 

Montgomery (1996) found that landslide sites consistently had RMS values below 69. Fifty percent of 

RMS values at bedrock sites are below 69 (table 3). Several RMS values were assessed for glacial 

outwash sites where “intact rock strength” is essentially at a minimum. These RMS values are 

consistently less than or equal to 50 and considered weak (tables 1, 2). 
10

Figure 3. Statistical plots of Schmidt hammer measurements from bedrock sites within the Gold Basin landslide 

study area, Washington. A, Box and whisker plots showing the mean value (red line) and range (black bracketed 

lines) of Schmidt hammer measurements for three different geologic units. B–D, Probability density functions (blue 

lines) and histograms (gray boxes) of individual Schmidt hammer measurements of KJph(w) (B), KJmm(w) (C), and

J^mt(e) (D). 
11

Table 3. Discontinuity orientations with respect to hillslope and Rock Mass Strength (RMS) parameters for bedrock field sites

[Bold values indicate low values of absolute orientation difference, angular divergence, or RMS values that may suggest slopes susceptibility to deep-seated 

landsliding. WRT, with respect to; RMS, rock mass strength; no., number]

Site Unit

Hillslope 

orientation

Foliation 

WRT slope

Joint set 1 

WRT slope

Joint set 2 

WRT slope

Joint set 3 

WRT slope Schmidt Hammer RMS

Strike Dip Δ Strike Δ Dip Δ Strike Δ Dip Δ Strike Δ Dip Δ Strike Δ Dip Average No.

2–7 J^mt(e) 136.3 6.9 173.7 41.1 - - - - - - 24.7 12 65 Moderate

4–8 J^mt(e) 80.0 15.5 86.0 69.5 171.0 49.5 - - - - 55.0 14 67 Moderate

3–4 KJmm(w) 289.1 3.6 53.1 12.4 225.1 67.4 272.1 74.4 172.1 75.4 46.4 15 77 Strong

3–6 KJmm(w) 182.9 0.2 173.1 87.8 112.9 75.8 154.9 24.8 - - 59.1 12 85 Strong

1–1 KJph(w) 78.9 2.9 254.1 59.1 166.1 63.1 - - - - 35.5 11 62 Moderate

2–8 KJph(w) 236.0 6.2 102.0 2.8 41.0 76.8 56.0 82.8 - - 42.8 10 77 Strong

2–9 KJph(w) 296.8 6.0 278.8 16.0 27.8 77.0 186.8 71.0 - - 59.2 13 70 Moderate

3–3 KJph(w) 290.6 5.2 57.4 11.8 85.6 31.8 5.4 38.8 - - 32.2 11 64 Moderate

3–8 KJph(w) 322.9 4.0 198.9 53.0 - - - - - - 23.7 17 62 Moderate

3–9 KJph(w) 235.0 4.0 46.0 15.0 70.0 83.0 129.0 45.0 - - 56.5 11 77 Strong

3–10 KJph(w) 304.9 7.5 220.9 50.5 26.9 77.5 - - - - 44.6 7 62 Moderate

4–10 KJph(w) 296.9 3.1 72.9 53.9 22.1 69.9 267.9 30.9 - - 54.5 10 69 Moderate
12

To obtain a larger-scale view of bedrock controls on slope stability, I analyzed the number of 

mapped landslides that originate in various lithologic units (fig. 4) and how slope values vary with 

lithologic unit. The results show that Pleistocene glacial strata have a greater number of landslides 

contained within the units relative to other bedrock units, followed by Jurassic to Cretaceous

metamorphosed mélange units (KJmm(w) and KJph(w)). The relatively large number of landslides 

mapped within Pleistocene glacial strata may be due to the unconsolidated nature of these units. Jurassic 

to Cretaceous mélange may also be more susceptible to landsliding than other units owing to the greater 

number of prominent joint sets observed, which may contribute to degraded slope stability if cataclinally

oriented with hillslope. Slope values analyzed for various geologic units are generally inversely correlated 

with landslide abundance, which may suggest that the relatively unconsolidated strata and highly fractured 

bedrock units are less likely to maintain steep hillslope angles. While Jurassic to Cretaceous and 

Pleistocene units also compose a large proportion of the area, the number of landslides does not entirely 

correlate to the percentage of analyzed area composed of each lithology (fig. 4). 

Figure 4. Plots of landslides and slope values by geologic unit in the Gold Basin landslide study area (see fig. 2 

for locations of units). A, The number of landslides with significant area and (or) headscarps originating in each

geologic unit and the percentage of land composed of each lithology (hatched bars). B, The mean and standard 

deviation of slope values analyzed for each geologic unit. The number of landslides and slope angles are broadly 

inversely correlated.
13

Seismic Hazard Assessment

Seismic hazards in western Washington State come from two main sources: megathrust 

earthquakes along the Cascadia Subduction Zone (Peterson and others, 2008), located offshore to the 

west, and smaller but shallower earthquakes along upper crustal faults in the Puget Sound lowland 

(Sherrod, 2001; Sherrod and others, 2000, 2004; Nelson and others, 2003a,b; Sherrod and Gomberg; 

2014; Personius and others, 2014). Consequently, a recent Probabilistic Seismic Hazard Assessment 

(PSHA) of the United States shows seismic hazard potential in western Washington is relatively high 

(fig. 5) (Peterson and others, 2008). The magnitude of ground shaking correlates well with the volume 

and aerial extent of coseismic landsliding (Keefer, 1994, 2002; Meunier 2007, 2008) owing to the 

addition of short-lived stresses that can exceed the cohesion and frictional strength of hillslopes 

(Newmark, 1965). Thus, estimates of the probability of peak ground acceleration (PGA) magnitude and 

frequency are useful for identifying and characterizing landslide hazards. 

Figure 5. Probabilistic Seismic Hazard map of Washington State (from Peterson and others, 2008). Cool to 

warm color gradient shows the peak ground acceleration (PGA) values with a 10 percent probability of expected 

exceedance within the next 50 years. Quaternary active faults are from the U.S. Geological Survey Quaternary fault 

and fold database (USGS, 2006).
14

For the Gold Basin landslide area, nearby active upper crustal faults include the DarringtonDevils Mountain fault zone to the north, which ruptured a M6.7–M7.0 earthquake about 2,000 years ago 

(Personius and others, 2014), and the southern Whidbey Island fault zone to the south, which generated 

four post-glacial surface rupturing earthquakes (Sherrod and others, 2008). Offshore, the Cascadia 

Subduction Zone is capable of producing great earthquakes (M >8) (Atwater and Hemphill-Haley, 1997; 

Witter and others, 2003). Coseismic landsliding during the 1994 M6.7 Northridge and 1999 M7.6 ChiChi earthquakes has occured when PGA values exceed 1 meter per second squared (m/s2

), equivalent to 

0.102 g (where g is the acceleration due to gravity), and on slopes that exceed 20° (Meunier and others, 

2007, 2008). The Northridge and Chi-Chi earthquake locations are ideal for analysis because they 

represent case studies of coseismic landsliding in dry and wet conditions, respectively, at magnitudes 

expected from upper crustal faults in Washington. Therefore, critical PGA and slope values from 

Meunier and others (2007) were used in the following hazard analysis. 

Probability of Ground Shaking Exceedance

In order to assess coseismic landslide hazard posed by onshore and offshore earthquake sources 

for the area proximal to the Gold Basin landslide, I extracted the 50-year seismic intensity hazard curve 

from the U.S. Geological Survey 2008 PSHA model (fig. 6) (Peterson and others, 2008). From the 

hazard curve, I extracted two horizontal dissections at 2 percent and 10 percent probability (fig. 6). The 

resulting probability density functions (PDFs) represent the 2 percent and 10 percent probability of 

ground shaking intensity to be exceeded over the next 50 years, respectively. These curves indicate that 

there is an estimated 2 percent probability of PGA greater than approximately 0.42 g in the study area in 

the next 50 years. There is an estimated 10 percent probability of PGA greater than approximately 0.215 

g in the study area in the next 50 years. Both of these PGA values are greater than the critical PGA 

estimated from the Northridge and Chi-Chi earthquakes, above which coseismic landsliding occurred 

(Meunier and others, 2007). 

Since the values at 2 percent and 10 percent probability of exceedance are above the critical 

PGA for coseismic landsliding (~0.102 g), I extracted a vertical dissection of the hazard curve at 0.1 g, 

which shows the distribution of exceedance probabilities for the specific value of shaking intensity (fig. 

6). The resulting PDF shows that there is approximately a 37.5 percent probability of exceeding the 0.1 

g ground shaking intensity in the next 50 years. From this, I infer that there is approximately a 37.5 

percent probability coseismic landsliding will occur in the study area over the next 50 years on slopes 

greater than 20° (fig. 6). 

Slope values in the study area are fairly steep on average (fig. 7). Of the lidar footprint analyzed 

(~1,047 km2

), over 530 km2 of the area is steeper than 20°. Therefore, over half the total area shown in 

figure 7 is at a slope angle susceptible to coseismic landsliding if ground acceleration exceeds 1 m/s2

. 

Bedrock Characteristics and Coseismic Hillslope Failure

The above seismic hazard assessment does not necessarily suggest that most or all hillslopes 

above 20° are likely to fail during a seismic event in the study area, but rather suggests that a 

considerable portion of the landscape in the area is available for failure. Local characteristics of the 

substrate, including rock type and orientation of discontinuities with respect to hillslope angle, are 

important as well. Based on fieldwork and analysis of bedrock characteristics, some bedrock sites 

appear to be more prone to slope failure, whether during a seismic event or not. 
15

Figure 6. Probabilistic seismic hazard curve extracted from the U.S. Probabilistic Seismic Hazard Assessment

map (Peterson and others, 2008) for the Gold Basin landslide study area, Washington. Values were extracted at

lat. 48.1° N., long 121.8° W. Blue lines are the probabilistic curved generate for site-specific probability and peak 

ground acceleration (PGA) values. Red vertical line on the main plot is the distribution of exceedance probabilities 

for a specific value of ground shaking intensity (0.1 g ≈ 1 m/s2 [where g is acceleration due to gravity]). Red curve

below main plot shows the PDF of expected probabilities of exceedance for 0.1 g. Horizontal green and orange 

lines represent the PGA values extracted for specific probabilities (10 percent and 2 percent, respectively). These 

horizontal dissections are plotted as green and orange probability density functions (PDFs) below.
16

Figure 7. Slope map of the Gold Basin landslide study area, South Fork Stillaguamish River, Washington. 

Hillslopes between 10° and 20° are highlighted in darker purple. Hillslopes above 20° are highlighted in lavender. 

Bedrock, glacial outwash, and landslide field sites locations are shown with dots colored by potential landslide 

hazard. Red dots show sites in active landslides or unconsolidated glacial outwash strata, which indicates they are 
17

active or have characteristics for potential shallow to deep-seated landslide; orange dots show bedrock sites with 

cataclinal discontinuity orientations, low dip divergence, and low rock mass strength (RMS) values, which indicates 

they have conditions for potential deep-seated hillslope failure; yellow dots show bedrock sites with low RMS 

values and anaclinal discontinuity orientations, which indicates they have some conditions characteristic of 

potential deep-seated hillslope failure; green dots show bedrock sites with high RMS values and anaclinal 

discontinuity orientations, which indicates they have few to no characteristics for potential hillslope failure.

The slope map in figure 7 also shows bedrock and landslide field sites color-coded by 

characteristics that may lead to slope failure. Sites that are either active landslides or are underlain by 

weak, unconsolidated Quaternary glacial strata are shown in red. Owing to the weak material properties, 

low RMS values, and already active landslides at these sites, the adjacent hillslopes appear to be 

relatively unstable. In addition, slope analyses suggest these strata do not typically maintain steep 

hillslope angles (fig. 4). Field sites in bedrock with low RMS values (RMS <69), low discontinuity dip 

divergence (10°–40°), and cataclinal discontinuity orientations are shown in orange. At these sites, 

vertically oriented topographic stresses will decrease the relative rock strength because joints and 

bedding layers (discontinuities) are optimally oriented to produce shear dislocation surfaces (Schmidt 

and Montgomery, 1996). Bedrock field sites with low RMS values and anaclinal discontinuity 

orientations are shown in yellow. While the intact rock strength and discontinuity characteristics suggest 

these sites are underlain by a relatively weak substrate, the anaclinal discontinuity orientations are such 

that vertically oriented topographic stresses increase relative rock strength. Finally, bedrock sites with 

relatively high RMS values and anaclinal discontinuity orientations are shown in green and are 

considered the most stable under passive conditions. 

In the event of strong ground shaking, vertical and horizontal PGAs have the potential to push 

hillslopes closer to failure. This suggests that hillslopes with anaclinal discontinuity orientations are 

susceptible to coseismic landsliding if the dynamic seismically induced stresses sufficiently decrease 

cohesion or the internal angle of friction to allow for brittle failure (Newmark, 1965). Therefore, the 

characterization of field sites based on bedrock and hillslope measurements should be considered 

general and not indicative of absolute stability or instability.
18

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Manuscript approved on August 13, 2018

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