Ground Failure Triggered by Shaking during the November 30, 

2018, Magnitude 7.1 Anchorage, Alaska, Earthquake

Open-File Report 2020–1043

U.S. Department of the Interior

U.S. Geological Survey
Cover. Photograph of a complex of sand boils along the mouth of the Little Susitna River, north of Anchorage, Alaska. 
Ground Failure Triggered by Shaking during the November 30, 

2018, Magnitude 7.1 Anchorage, Alaska, Earthquake

By Alex R.R. Grant, Randall W. Jibson, Robert C. Witter, Kate E. Allstadt, Eric M. Thompson, 

and Adrian M. Bender

Open-File Report 2020–1043

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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Suggested citation:

Grant, A.R.R., Jibson, R.W., Witter, R.C., Allstadt, K.E., Thompson, E.M., and Bender, A.M., 2020, Ground failure 

triggered by shaking during the November 30, 2018, magnitude 7.1 Anchorage, Alaska, earthquake: U.S. Geological 

Survey Open-File Report 2020–1043, 21 p., https://doi.org/10.3133/ofr20201043.

Associated data for this publication:

Grant, A.R.R., Jibson, R.W., Allstadt, K.E., Thompson, E.M., Witter, R.C., Bender, and A.M., Schmitt, R.G., 2020, 

Ground failure triggered by shaking during the M7 Anchorage earthquake: U.S. Geological Survey data release, 

https://doi.org/10.5066/P99ONUNM.

ISSN 2331-1258 (online)
iii

Contents

Introduction............................................................................................................................................................1

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

Observations ..........................................................................................................................................................6

Landslides .....................................................................................................................................................6

Disrupted Landslides.........................................................................................................................6

Coherent Landslides..........................................................................................................................8

Flow Landslides ..................................................................................................................................9

Landslides from the 1964 Alaska Earthquake.............................................................................12

Liquefaction ................................................................................................................................................12

Sand Boils..........................................................................................................................................12

Extensional Cracks with Associated Sand Ejecta.....................................................................14

Extensional Cracks without Associated Sand Ejecta...............................................................14

Other Observations....................................................................................................................................16

Discussion ............................................................................................................................................................18

Summary...............................................................................................................................................................18

References Cited.................................................................................................................................................19

Figures

1. ShakeMap produced by the U.S. Geological Survey Alaska Earthquake Center of the 

Cook Inlet region, Alaska, for the magnitude 7.1 earthquake on November 30, 2018 .......2

2. Maps of estimated ground failure probabilities in the Cook Inlet region, Alaska, 

following the magnitude 7.1 Anchorage earthquake..............................................................3

3. Map of GPS tracks of field work in the Cook Inlet region, Alaska .......................................4

4. Overview map showing locations of named places and images discussed in the text ...5

5. Photograph of ground failure of an engineered fill-slope embankment .............................5

6. Photographs of rockslides, rock avalanches, and rockfalls .................................................6

7. Photographs of disrupted landslides and coastal bluff collapses .......................................7

8. Digital terrain models derived from lidar data acquired before and after the 2018 

Anchorage earthquake used to estimate elevation changes induced by slope failure 

along the Eagle River ...................................................................................................................8

9. Photographs of rockfalls and coastal bluff landslides...........................................................9

10. Photographs of coherent landslides .........................................................................................9

11. Photographs of roadway damage............................................................................................10

12. Photographs of the Potter Hill flow slides..............................................................................10

13. Digital terrain models derived from lidar data acquired before and after the 2018 

Anchorage earthquake used to estimate elevation changes caused by the Potter Hill 

landslides .....................................................................................................................................11

14. Photographs of minor cracking along 1964 landslide grabens in Anchorage .................13

15. Photograph of a complex of sand boils along the mouth of the Little Susitna River north 

of Anchorage ...............................................................................................................................13
iv

 16. Photographs of sand boils and possible relict sand boils observed following the 
Ground Failure Triggered by Shaking during the November 30, 

2018, Magnitude 7.1 Anchorage, Alaska, Earthquake

By Alex R.R. Grant, Randall W. Jibson, Robert C. Witter, Kate E. Allstadt, Eric M. Thompson, and Adrian M. Bender

Introduction

The magnitude (M) 7.1 Anchorage, Alaska, earthquake 

occurred at 8:29 a.m. local time on November 30, 2018, about 

12 kilometers (km) north of Anchorage. The focal depth of 

about 47 km and the normal focal mechanism indicate that the 

earthquake occurred in the subducting slab of the Pacific Plate. 

The U.S. Geological Survey (USGS) ShakeMap (Worden 

and Wald, 2016) of this event1

 (fig. 1) shows severe (intensity 

VIII) shaking near the epicenter; this caused significant 

damage in the urban and infrastructure center of Alaska and 

was felt throughout the Anchorage and Matanuska-Susitna 

Valley areas. The USGS Prompt Assessment of Global 

Earthquakes for Response2 (PAGER; Wald and others, 2010) 

system gave an “orange” alert within an hour of the origin 

time, indicating estimated economic losses in the range of 

$100 million to $1 billion with few fatalities. No fatalities 

were reported following the 2018 Anchorage earthquake, 

but considerable damages to roads, railway alignments, and 

homes were observed throughout the epicentral region. The 

2018 Anchorage earthquake was one of the most damaging 

earthquakes in the United States in the past decade. 

USGS near-real-time ground-failure products provide 

immediate estimates of the severity and extent of potential 

landslides and liquefaction following a major earthquake 

(Allstadt and others, 2016, 2017). Near-real-time maps depict 

landslide and liquefaction potential based on statistical models 

(Zhu and others, 2017; Nowicki Jessee and others, 2018) and 

are based on USGS ShakeMap ground-motion estimates and 

susceptibility proxies; the maps do not incorporate post-event 

observations of actual earthquake-related ground failures. 

Within half an hour of the M7.1 Anchorage earthquake, 

USGS ground-failure maps3

 predicted a limited area (10–100 

square kilometers [km2]) of potential landslides with little 

or no population exposure (<100 people) and a significant 

area (100–1,000 km2

) of potential liquefaction with limited 

population exposure (1,000–10,000 people). These estimates 

increased after the magnitude was refined and strong groundmotion data was incorporated into ShakeMap: the landslide 

model increased to significant area (>10 km2

) and the 

liquefaction alert remained the same (although the estimated 

area affected increased). Overall, the population exposure 

levels did not change substantially. Spatial extents of ground 

failures predicted by these rapidly generated hazard products 

(fig. 2) included landslides along the Chugach Mountains 

front east and northeast of Anchorage, the southwest Talkeetna 

Mountains, and along the Turnagain Arm of Cook Inlet. The 

models predicted widespread liquefaction along river valleys, 

tidal flats, and estuaries, mostly in remote areas, but with some 

elevated probabilities within Anchorage and the Matanuska 

River valley. To improve the accuracy of future USGS nearreal-time earthquake products, such as the landslide and 

liquefaction alerts and maps shown in figure 2, correlation of 

observed ground failure with predictions by real-time products 

is necessary. 

This report documents our observations of ground failure 

caused by the earthquake. We conducted field work from 

December 1 to 10, 2018, during the 10 days immediately after 

the earthquake. A team of six USGS scientists and engineers 

verified the presence or absence of ground failures throughout 

the area by conducting three helicopter overflight missions 

and several days of ground inspections. We focused our 

efforts on specific areas identified by citizen reports as well 

as where near-real-time products (fig. 2) predicted ground 

failure. Because of the history of large damaging earthquakes 

affecting Anchorage, we were interested in determining if 

landslides from past earthquakes may have been reactivated. 

We assessed the response of large translational landslides that 

moved during the 1964 M9.2 Alaska earthquake (Hansen, 

1965; Jibson and Michael, 2009) to shaking that occurred 

during the 2018 Anchorage earthquake. 

Preliminary comparisons of digital terrain models 

developed from airborne light detection and ranging (lidar) 

data4

 acquired before and after the earthquake characterize the 

largest landslides observed in Anchorage and Eagle River. 

1

Available at https://earthquake.usgs.gov/earthquakes/eventpage/

us1000hyfh/shakemap/intensity.

2

Available at https://earthquake.usgs.gov/earthquakes/eventpage/

us1000hyfh/pager.

3

Available at https://earthquake.usgs.gov/earthquakes/eventpage/

us1000hyfh/ground-failure/summary. 4

Available at https://elevation.alaska.gov.
2 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 1. ShakeMap produced by the U.S. Geological Survey Alaska Earthquake Center of the Cook Inlet region, Alaska, for the 

magnitude 7.1 earthquake on November 30, 2018. Star indicates the epicenter, box shows the outline of the rupture extent, colors 

depict the level of shaking. km, kilometer; peak acc., peak acceleration (in terms of the percentage of gravitational acceleration 

at the Earth’s surface [%g]); peak vel., peak velocity (in centimeters per second [cm/s]).
Methods 3

Figure 2. Maps of estimated ground failure probabilities in the Cook Inlet region, Alaska, following the magnitude 7.1 Anchorage earthquake. 

A, Liquefaction probabilities based on the Zhu and others (2017) model. B, Landslide probabilities based on the Nowicki Jessee and others 

(2018) model. km, kilometers.

Methods

Our goal was to document the presence or absence of ground 

failure (liquefaction and landslides) throughout the strongly shaken 

upper Cook Inlet region. Our investigation of ground failures 

extended south out of Anchorage to Portage and Ingram Creek, east 

along the Matanuska River, and north to the town of Willow along 

roadways and throughout the upper Cook Inlet area via helicopter 

(table 1; fig. 3). We were able to conduct one flight within 

36 hours of the earthquake on December 1, 2018, to ensure rapid

documentation of potential ground failure. We prioritized ground 

and overflight reconnaissance locations based on multiple criteria: 

• Locations where USGS ground-failure models predicted 

significant hazard (fig. 2).

• Landslide locations from the 1964 Alaska earthquake and 

other areas known to be susceptible to ground failure in 

Anchorage (Hansen, 1965; Jibson and Michael, 2009).

• Areas susceptible to ground failure that lacked significant 

predicted hazard (including looking for the limits of 

ground failure). 

• Areas where citizens reported ground-failure observations.

Ground-failure reconnaissance was conducted in teams of 

two to three scientists. During ground-based reconnaissance, 

we collected georeferenced images of liquefaction and (or) 

landslide features and mapped individual ground-failure 

features (using ArcCollector, a mobile geographic information 

system [GIS] application) as polygons of failure extent, mainscarp or tension-crack lines, or points with notes about the 

ground failure. We also noted observations of ground failure 

non-occurrence, as these are also especially important for 

calibrating ground-failure prediction models. Sites discussed 

in this report, and locations of field photographs in subsequent 

figures, are shown in figure 4. 

Ground-failure features in this event were short lived for 

several reasons:

• The Alaska Department of Transportation and Public 

Facilities rapidly repaired road damage caused by 

landslides (fig. 5). 

• Snow falling in the days immediately following the 

earthquake obscured slope failures, ground cracks, and 

sand boils. Snow in mountainous areas was significant 

enough to prevent observation of large features, such as 

the Rainbow Peak landslide (fig. 6A), captured shortly 

after the earthquake by local residents. 

• Most sand boils were initially observed within tidal flats 

around the Cook Inlet where an ~10 meter (m) tidal range 

quickly removed sand ejecta or evidence of liquefaction. 

In light of these issues, the timeline of making our 

observations was critically important. Confidence in our 

observations of the absence of ground failures diminishes as 

time from the earthquake increases, which can be highlighted 
4 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 3. Map of Global Positioning System (GPS) tracks of field work in the Cook Inlet region, Alaska. Colors vary by date. Solid lines 

indicate ground-based fieldwork; dashed lines indicate aerial reconnaissance using a helicopter. GPS tracks were not collected on the 

December 4th reconnaissance within Anchorage. 

Table 1. Field and aerial reconnaissance in response to the November 30, 2018, Anchorage earthquake.

[km, kilometers]

Date Method1 Objectives

12/1/18 Aerial Overflight reconnaissance of Municipality of Anchorage and Matanuska-Susitna Valley areas the day after the earthquake (346 km)

12/4/18 Ground Assess 1964 Turnagain Heights and Government Hill landslides

12/5/18 Aerial Identify landslides in the Chugach Mountains (563 km) 

12/5/18 Ground Map liquefaction features along Knik Arm

12/6/18 Aerial Identify liquefaction in the greater Cook Inlet region (332 km) 

12/6/18 Ground Assess Turnagain Heights landslide, map ground failures in northern Anchorage

12/7/18 Ground Map liquefaction and landslide features in southern Anchorage

12/7/18 Ground Map liquefaction features along the Knik and Matanuska Rivers, assess levee damage near Palmer

12/8/18 Ground Map landslides near Peters Creek and Eagle River

12/8/18 Ground Map landslides and liquefaction in southern Anchorage

12/9/18 Ground Identify the limits of ground failure east along the Glenn Highway

12/9/18 Ground Map liquefaction features along Turnagain Arm 

12/10/18 Ground Identify the limits of ground failure north along Parks Highway

12/10/18 Ground Map additional liquefaction features within Anchorage 

1

Aerial reconnaissance was conducted in a Robinson R-44 helicopter, distance flown given in parentheses; ground reconnaissance included sites accessible by road and 

investigated on foot.
Methods 5

Figure 4. Overview map showing locations of named places and images (listed by figure number) discussed in the text.

Figure 5. Photograph of ground failure 

of an engineered fill-slope embankment 

that damaged about 70 meters of the 

northbound offramp from Minnesota 

Drive to International Airport Road, in 

Anchorage, Alaska. On December 1, 

2018, the day after the earthquake and 

time this photograph was taken, Alaska 

Department of Transportation crews 

had already begun repairs to the road. 

Location shown in figure 4.
6 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 6. Photographs of 

rockslides, rock avalanches, 

and rockfalls. A, Rockslide on 

Rainbow Peak (photograph by 

Jeremiah Drage, resident). B, 

Rock avalanche below Eagle 

Peak (photograph by Colin Tyler, 

resident). C, Rockfall above Eagle 

River. D, Rockfall from Peak 

4,710’ (photograph by Heather 

Thamm, Chugach National Forest 

Avalanche Information Center). 

Locations shown in figure 4.

by two striking examples. First, the initial helicopter flight 

(December 1, 2018) documented extensive sand boils at the 

mouth of the Little Susitna River, but several tide cycles erased 

these features prior to a ground survey of this location on 

December 6, 2018. Second, citizen observations documented 

a large earthquake-triggered rockslide at Rainbow Peak (see 

fig. 6) above the Seward Highway, but snowfall obscured this 

feature prior to the second flight (December 5, 2018), when the 

rockslide was no longer visible. 

Observations

We developed an initial inventory of 153 ground-failure 

features from the November 30, 2018, M7.1 Anchorage 

earthquake based on ground-based observations shortly after the 

earthquake (December 5–10) that includes the presence or absence 

of liquefaction, landslides, and individual crack traces of lateral 

spreads and incipient landslides. Field-mapped ground-failure 

observations and a set of 565 georeferenced and annotated images 

from both ground and overflight reconnaissance are available on 

ScienceBase (Grant and others, 2020). 

Landslides

The landslide model (fig. 2) predicted localized zones of 

moderate to high landslide hazard within the western Chugach 

and southwest Talkeetna Mountains above Anchorage and the 

Matanuska Valley, respectively. The highest predicted landslide 

hazard was along the fairly densely populated lower reaches 

of Eagle River, Peters Creek, and steep slopes surrounding 

Anchorage (figs. 2 and 3). We discuss our landslide observations 

based in part on the landslide groupings of Keefer (1984), 

who used specific landslide nomenclature from Varnes (1978): 

disrupted landslides (falls and slides in rock and debris), coherent 

slides (block slides and slumps in rock and earth), and flow-type 

slides (debris and earth flows).

Disrupted Landslides

Rockfalls and disrupted soil- and rockslides typically are 

the most common types of landslides triggered by earthquakes 

(Keefer, 1984). Although these types of landslides were triggered 

by the 2018 Anchorage earthquake, they were neither numerous 

nor widespread. That observation must be qualified because fresh 

snow cover and limited light impeded our observations, but such 

a paucity of landslides for this type of earthquake has precedent. 

The 2001 Nisqually, Washington, earthquake, similar in many 

respects to the 2018 Anchorage earthquake (M6.8, normal focal 

mechanism, 50 km depth), also failed to trigger widespread 

landsliding (Highland, 2003).

The disrupted landslides we observed were shallow, 

involving only the uppermost few meters of rock and soil, 

and appeared to have moved rapidly because of their general 

occurrence on steep slopes. We observed disrupted landslides 
Observations 7

primarily along coastal bluffs, roadcuts along the Seward 

Highway, and the steep slopes within the Chugach Mountains. 

Nothing we saw suggested concentrated landslide activity in 

the Chugach Mountains; however, fresh snowfall completely 

obscured most high-elevation areas, especially in the southwestern 

Chugach Mountains, by the time of our overflight on December 

5, 2018. We noted no signs of large valley-blocking landslides or 

freshly exposed source areas on steep slopes during our surveys. 

Such features likely would have been visible despite snow cover.

One of the largest known disrupted slides was photographed 

the day of the earthquake by a local resident: a rockslide off 

Rainbow Peak, visible from the Seward Highway (fig. 6A). This 

landslide was completely obscured by snow by the time of our 

overflight on December 5, 2018. The slide is visible under a thin 

coating of fresh snow in DigitalGlobe satellite imagery taken on 

December 3, 2018; examination of this image allows us to estimate 

a total length from the top of the source area to the distal end of 

the deposit of nearly 700 m and a total area of about 50,000 square 

meters (m2

). Landslide deposits reached the drainage, but the 

catchment is small, and deposits appear thin and coarse. We do not 

expect the deposits to impound a significant amount of water, but 

we cannot completely rule out this possibility without more data.

Several larger rockfalls and rockslides, photographed by a 

local resident on November 30, occurred along the steep valley 

walls of the Eagle and Eklutna Rivers. The largest was a rockslide 

with a debris trail several hundred meters long on the flank of a 

glacial valley leading up to Eagle Peak (fig. 6B); deposits were 

thin and trailed off before reaching the valley floor, which suggests 

a small source area. Though this rockslide was obscured by 

snow by the time of our overflight, we noted numerous signs of 

additional rockfalls and rockslides farther up Eagle River valley 

that likely were triggered by the earthquake (fig. 6C). Most slope 

failures consisted of small detachments of rock onto existing 

talus slopes or long, thin, trailing deposits formed because of the 

steepness of these valleys. We saw no signs of major drainage 

blockages. Many of these slope failures started as falls and 

transitioned to flows as they reached shallower slopes, and most 

source areas were in bedrock. We noted a similar density and 

style of slope failures in the Eklutna River valley upstream from 

Eklutna Lake. Mountainous areas south of these two valleys 

showed no signs of similarly intense rockfall activity at the time 

of our December 5 overflight, but differences in snow cover 

could have biased our assessment. The Chugach National Forest 

Avalanche Information Center reported earthquake-triggered 

snow avalanches and rockfalls in mountainous terrain surrounding 

Girdwood (fig. 6D) 

Other notable disrupted slides included raveling of new 

material from steep canyon walls cut into glacial drift by the 

Eklutna River downstream from the dam (fig. 7A) and failures in 

steep slopes along Eagle River between the Glenn Highway bridge 

and Eagle River Loop Road (fig. 7B). Preliminary comparisons of 

digital terrain models derived from lidar acquired before and after 

the earthquake show several shallow debris slides and avalanches 

in 40- to 50-m-high valley walls along Eagle River (fig. 8). These 

slope failures occurred in unstable colluvium, consisting of silt, 

sand, and gravel eroded from adjacent glacial drift that mantles 

Figure 7. Photographs of disrupted

landslides and coastal bluff 

collapses. A, Disrupted landslides 

in glacial drift along Eklutna River 

bluffs. B, Landslides in bluffs along 

the Eagle River west of Glenn 

Highway. C, Coastal bluff collapses 

along south side of Turnagain Arm. 

D, Coastal bluff collapses along 

west side of the Knik Arm. Locations 

shown in figure 4.
8 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 8. Digital terrain models (DTMs) derived from light detection and ranging (lidar) data acquired before and after the 2018 Anchorage 

earthquake used to estimate elevation changes induced by slope failure along the Eagle River (lidar available at https://elevation.alaska.gov). 

Maps use the North American Datum of 1983 (NAD83) Universal Transverse Mercator Zone 6 North coordinate system, in meters (m). a, Lidar 

DTM developed from data acquired for the Municipality of Anchorage in May 2015. b, Lidar DTM of the same area developed from data acquired 

in December 2018, after the earthquake. c, DTM showing elevation change between the 2015 and 2018 lidar data. Landslides occurred on steep 

slopes as much as 35 m high in the Eagle River canyon. Warm colors show negative elevation changes, greatest near landslide headscarps, and 

cool colors show positive elevation changes, where landslide deposits occur. The total range of elevation change varied from 4.6 to –12.7 m. d, 

Profile A–A’ shows the elevation across a landslide before and after the earthquake. Location shown in c. 

steep to very steep slopes (Schmoll and others, 1972; 1980). 

Schmoll and others (1980) specifically point out the susceptibility 

of the colluvium along Eagle River to slope failure, “Landslides 

and earthflows have been common in the past in these areas and 

may be reactivated, especially if the slope is disturbed.”

During helicopter overflights we also observed collapsed 

coastal bluffs along the south side of the Turnagain Arm (fig. 7C) 

and the west side of the Knik Arm (fig. 7D). Few rapid, disrupted 

slides directly impacted infrastructure, except for several rockfalls 

along the Seward Highway (fig. 9A) that caused a temporary road 

closure (removed prior to our field reconnaissance) and failures 

along steep coastal bluffs (fig. 9B).

Coherent Landslides

Few deep (>3 m) coherent landslides were observed after 

the earthquake, although at least one led to significant impact 

to the built environment. Eight homes in the River Heights 

neighborhood of Eagle River were red tagged (unsafe for 

occupation) because of landslide movement in the steep slope 

behind the houses (fig. 10A). Our observations indicate that the 

slide plane may have been several meters deep. The basal slip 

surface appeared parallel to the slope in the upper part of the 

slide, but it extended below the houses at the base of the slope 

and appears to have curved upward at the toe, indicating possible 
Observations 9

Figure 9. Photographs of rockfalls and coastal bluff landslides. 

A, Rockfall along Seward Highway. B, Coastal bluff landslides 

along Knik Arm west of Anchorage. Locations shown in figure 4.

Figure 10. Photographs of coherent landslides. A, Coherent 

landslide that propagated into structures in the town of Eagle 

River; several structures below this slope were red tagged 

because of damage from deformation of this slope. Red arrows 

show the headscarp of the structure-impacting landslide, cyan 

arrows show additional extensional cracks above the slope. B, 

Landslide complex along the south-facing bluffs of Kincaid Park 

with many internal scarps. Primary scarp shown with red arrows. 

Locations shown in figure 4.

rotational movement. Some of the houses were moved several 

decimeters off their foundations and were pushed toward the 

street by the landslide. Backyard patios, fences, and outbuildings 

were heavily damaged by tilting in different directions and 

differential uplift. Utility connections were severed or distressed 

in some homes. Additional homes not directly impacted by the 

landslide across the street were yellow tagged (restricted use) 

until further investigation could determine the extent of the 

landslide area and the potential for additional movement.

In the southwest corner of Anchorage, we documented 

additional evidence of coherent landslides along the southern 

coastal bluffs of Kincaid Park. We observed evidence of 

extensional cracks along the edge of the nearly 80-m-high bluffs 

in the southeastern corner of Kincaid Park, and a complex of 

cracks have as much as 1.5 m vertical and 1 m horizontal offsets 

along the southwestern corner of Kincaid Park at the top of a 

~70-m-high bluff (fig. 10B). Incipient cracks were also noted in 

several slopes near the Glenn Highway crossing of the Eagle River 

and along the tops of talus slopes along the lower Eklutna River. 

Flow Landslides

The most damaging landslides triggered by this earthquake 

involved both natural and fill materials that developed complex, 

flow-like, landslide deposits. The most widely publicized were 

failures in engineered road fills at the north-bound Minnesota 

Drive offramp to International Airport Road in Anchorage (fig.5)

and along Vine Road south of Wasilla (fig. 11A). At Minnesota 

Drive, collapse of about 70 m of the offramp embankment 

appeared to occur from loss of bearing strength in underlying, 
10 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

poorly drained silt and fine sand of glacioestuarine origin 

(Combellick, 1999). The Alaska Department of Transportation 

had already begun repairs by the time of our first overflight on 

December 1. A similar engineered embankment constructed 

across a saturated bog also failed during earthquake shaking 

caused by loss of bearing strength in underlying deposits, which 

caused damage to about 90 m of Vine Road (fig. 11A). At Vine 

Road, saturated silt, fine sand, and (or) organic deposits fill a 

bog, which occupies a poorly drained area in glacial terrain 

confined by drumlins. Similar types of failures occurred in the 

railroad grade along the coastal bluffs 3 km north of the town of 

Chugiak. These failures appeared to initiate as slumps and then 

transitioned into a more flow-like behavior (fig. 11B). Several 

such slides occurred along a few kilometers of the railroad in this 

area and were being repaired at the time of our aerial overflights.

In the Potter Hill neighborhood of southern Anchorage, we 

observed several large flow slides (fig. 12). Near Potter Hill, a 

railroad grade has been cut approximately halfway up ~35-m-high 

coastal bluffs, creating a 15–20 m wide bench. The 1964 Alaska 

earthquake triggered destructive landsliding and ground cracking 

that extended about 3 km along this bluff (fig. 12B; Hansen, 

1965; Hansen and others, 1966). The 1964 Potter Hill landslides 

Figure 11. Photographs of roadway damage. A, Earthquake 

shaking caused loss of bearing strength in bog sediment that 

disrupted Vine Road in Wasilla, Alaska. B, Flow-slides along a 

railway alignment near Peters Creek. Locations shown in figure 4.

Figure 12. Photographs of the 

Potter Hill flow slides. A, Aerial 

photograph of two flow landslides 

near Potter Hill. B, Landslides 

triggered by the 1964 Alaska 

earthquake disrupting nearly a 

mile of the Alaska Railroad at 

Potter Hill (photograph by Reuben 

Kachadoorian of the U.S. Geological 

Survey, from Hansen and others, 

1966). C, Main scarp of a slump/

flow slide at Potter Hill. Railroad 

grade is to the left, tidal flat to the 

right. Disrupted slide material and 

ponded drainage are ubiquitous. D, 

Tilted trees on landslide deposits at 

Potter Hill indicate multiple modes 

of movement: slumping, siding, and 

toppling. Location shown in figure 4.
Observations 11

were described as having 5–10 m deep, steep slope-parallel slip 

surfaces and that they transitioned into flows after failure. Hansen 

(1965) documented several previous episodes of landsliding along 

this stretch of bluff from both previous earthquakes and heavy 

rainstorms. In 1964 and in some previous episodes of slope failure, 

segments of railroad track were damaged or destroyed. Hansen’s 

(1965) description of the Potter Hill landslides indicates that they

“...consisted of elongate fragmented slump blocks 

rotated backward and broken into many pieces toward 

the base of the slope. In places at the toe, they turned into 

earth and mudflows derived partly from the intertidal silt 

of Turnagain Arm and partly, perhaps, from the clay-siltsand sequence in the lower part of the bluff.”

This description perfectly matches our 2018 observations. 

The landslides we observed here extended along the same 

reach of bluff and had the same morphology as the Potter Hill 

landslides described in 1964 (figs. 12A, 12B, 13); they appear 

to have initiated as coherent slumps that then disaggregated and 

transitioned into flows (fig. 12C). Landslides occurred only on the 

lower part of the bluff below the railroad grade. Trees were askew 

and tilted in chaotic orientations (fig. 12D) indicating multiple 

movement geometries—slumping, sliding, and toppling. The 

2018 landslides involved both failure of new bluff material and 

reactivation of some parts of the 1964 landslide deposits. The 2018 

landslides were smaller, less extensive, and had shorter runout 

distances than those triggered in 1964. The railroad track was 

Figure 13. Digital terrain 

models (DTMs) derived from 

light detection and ranging 

(lidar) data acquired before 

and after the 2018 Anchorage 

earthquake used to estimate 

elevation changes caused by 

the Potter Hill landslides (lidar 

available at https://elevation.

alaska.gov). Maps use the 

North American Datum of 1983 

(NAD83) Universal Transverse 

Mercator Zone 6 North 

coordinate system, in meters 

(m). a, Lidar DTM developed 

from data acquired for the 

Municipality of Anchorage in 

May 2015. The Alaska Railroad 

runs along the road bed cut 

into the coastal bluff and 

showed no slumping in 2015 

before the 2018 earthquake. b, 

Lidar DTM of the same area 

developed from data acquired 

in December 2018 after the 

earthquake. c, DTM showing 

the elevation change between 

the 2015 and 2018 lidar data. 

The Potter Hill landslides cut 

into the bench seaward of the 

railroad but did not damage 

the roadbed or tracks. Warm 

colors show negative elevation 

changes, greatest near 

landslide headscarps, and cool 

colors show positive elevation 

changes, where landslide 

deposits occur. The total range 

of elevation change varied 

from 3.7 to 6.0 m. Profile A–A’ 

shows the elevation across a 

landslide before and after the 

earthquake. 
12 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

not damaged in the 2018 earthquake largely because the railroad 

grade has been widened since 1964. However, future episodes of 

landsliding could pose a risk to the tracks with additional recession 

of the bluffs. 

Landslides from the 1964 Alaska Earthquake

We visited several of the large, deep landslides triggered by 

the 1964 Alaska earthquake (M9.2) to determine if any of them 

showed evidence of reactivation during the 2018 earthquake. In 

summarizing observations from many studies of landslides from 

the 1964 earthquake, Jibson and Michael (2009) concluded that 

these large, deep landslides are triggered only by subduction-zone 

earthquakes that generate very long durations (several minutes) 

of strong shaking. Documenting the response of these landslides 

to the 2018 shaking provides an opportunity to test this argument 

and assess the hazard from reactivation of these landslides in other 

types of earthquakes that have shorter shaking durations.

The 1964 Government Hill landslide is located about 

1 km north of downtown Anchorage on the bluff that forms 

the northern edge of Ship Creek channel (figs. 4 and 14A). 

The original landslide formed as large blocks along the bluff 

translated outward along a subhorizontal basal shear surface and 

a broad, deep graben formed between the main scarp and the 

largest of the landslide blocks (Hansen, 1965). During our field 

investigation, we discovered a new continuous crack that extends 

about 30 m along the crown of the main scarp of the 1964 

landside at Sunset Park (fig. 14A). Additional discontinuous 

cracking no more than a few meters long was visible elsewhere 

along the old main scarp. Separation across the cracks was no 

more than 1–2 centimeters (cm), and we saw no downslope 

deformation, indicating no significant downslope movement.

We saw no evidence of remobilization of the Turnagain 

Heights landslide (fig. 4 and 14B). The Turnagain Heights 

landslide of 1964 devastated more than 0.5 km2

 of a residential 

neighborhood and destroyed about 75 homes; the slide is located 

about 5 km southwest of downtown Anchorage along the bluffs 

on the south coast of Knik Arm in Cook Inlet. The original slide 

was a fairly deep (~60 m) translational block slide that created 

a highly disrupted landscape of chaotic landslide blocks and 

intervening grabens of differing sizes (Hansen, 1965; Updike 

and others, 1988). We walked an axial transect through an 

unmodified part of the original landslide and observed fresh, 

laterally continuous, cracks bounding several grabens (fig. 14B). 

However, graben-bounding cracks generally had no more than 

about 1 cm of extension. We saw neither extensional cracking 

along the main scarp nor compressional deformation near the toe 

of the slide complex. 

We also did not observe significant remobilization or 

displacement at other large, deep landslides from the 1964 

earthquake in downtown Anchorage (Hansen, 1965) during 

our aerial reconnaissance. We were unable to field-verify this 

observation for every major 1964 landslide, and many of these 

areas have been significantly modified and urbanized since 

1964, which might mask subtle deformation; thus, we cannot 

determine with certainty whether deformation occurred on these 

landslides. Absence of reporting or observations in our work 

and others (Franke and others, 2019) suggests remobilization of 

large 1964 landslides was not a significant phenomenon in the 

2018 earthquake. 

Liquefaction

Following large earthquakes, significant economic losses 

can result from liquefaction-induced settlement and lateral 

displacements because of the overlap of infrastructure with areas 

most susceptible to liquefaction, namely fluvially deposited 

sands and fill. Widespread evidence of liquefaction was observed 

after the 2018 Anchorage earthquake in natural fluvial-estuarine 

environments along the lower reaches of several rivers and 

streams draining into the Cook Inlet as well as in natural and fill 

deposits within Anchorage. Liquefaction likely played a role in 

damages to dozens of residential dwellings in southern Anchorage. 

Reconnaissance by others found decimeter-scale settlement and 

surficial cracks caused by liquefaction throughout the Jewel and 

Sand Lake regions of southern Anchorage resulting in settlement 

and tilting of nearby dwellings (see Franke and others, 2019, for 

details and locations of these features).

We documented three categories of liquefaction observations: 

(1) sand boils, (2) extensional cracks with sand ejecta, and(3)

extensional cracks without sand ejecta, where liquefaction 

cannot be conclusively identified without additional subsurface 

investigations. Extensional cracks without sand ejecta were most 

commonly observed. Similarities in appearance between the 

extensional cracks in the presence of sand boils and extensional 

cracks without sand ejecta provides some evidence that these 

cracks likely resulted from liquefaction. We cannot rule out the 

possibility that some of these extensional cracks were caused by 

non-liquefaction-related slumping or sliding.

Sand Boils

The most notable evidence of liquefaction was seen at the 

mouths of the Little Susitna River (fig. 15) and Ingram Creek 

(fig. 16A). Linear patterns in the observed sand boils at the Little 

Susitna River suggest liquefaction of paleochannels or recent 

channels, similar to paleochannel liquefaction observed following 

the 2011 Tohoku, Japan (Pradel and others, 2014), and 2016 

Kaikoura, New Zealand (Bastin and others, 2018), earthquakes. 

A second dense field of sand boils was observed within the tidal 

mudflats of Ingram Creek near Portage during our overflight 

on December 5. Sand boils below Ingram Creek were spaced 

more irregularly than those observed at the Little Susitna River. 

An example of the ephemeral nature of much of the evidence 

of ground failure following earthquakes, these sand boils were 

unrecognizable during our ground-based reconnaissance of the 

Ingram Creek mudflats just 4 days later. However, we documented 

soft irregular depressions that likely represent relict sand-boil 

vents (fig. 16B) remaining after tidal erosion of loose sand ejecta. 

Additional possible relict sand boils were observed near Portage 

and along the north shore of the Knik Arm (figs. 16C, D). Features 
Observations 13

Figure 14. Photographs of minor cracking along 1964 landslide grabens in Anchorage. A, Fresh crack along the 

main scarp of 1964 Government Hill landslide at Sunset Park. The crack that extends upward from the bottom center 

of the photograph is continuous for about 30 meters along the old scarp; it has 1–2 centimeters (cm) of extensional 

separation. No net downslope movement of the 1964 landslide mass is apparent. The 1964 landslide moved from right 

to left in the photograph. B, Crack extending along the edge of a graben that formed in the 1964 Turnagain Heights 

landslide. The crack is visible extending upward from the bottom left part of the photograph. This and several similar 

graben-bounding cracks on this landslide had about 1 cm of extensional separation. No evidence of net downslope 

movement of the 1964 landslide mass is apparent. Locations shown in figure 4.

Figure 15. Photograph of a complex of sand boils along the mouth of the Little Susitna River north of Anchorage.

Linear patterns of sand boils may be related to liquefaction of paleochannels. Location shown in figure 4.
14 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 16. Photographs of sand 

boils and possible relict sand 

boils observed following the 

Anchorage earthquake. A, Welldefined sand boils near the outlet 

of Ingram Creek in Turnagain 

Arm on December 5, 2018. B, 

Possible relict sand boil vents 

at the mouth of Ingram Creek 

outlined by footprints. C, Possible 

relict sand boils near Portage. 

These steep-sided, water-filled 

voids in otherwise hard-packed 

riverbanks below high tide are 

believed to be the remnant vents 

from liquefaction during the 

earthquake. D, Close-up view of 

possible relict ~60-centimeterdiameter sand boil. Locations 

shown in figure 4.

interpreted as relict sand boils were all steep-sided circular 

depressions. The depressions were filled with disturbed, loose soil 

surrounded by stiffer and partly frozen soil. All noted relict sand 

boils were below high tide level, near zones of lateral spreading or 

previous observations of sand boils where sand ejecta could have 

been eroded by tidal action. 

Scattered reports of ejected sand in crawl spaces and 

foundations cracked by settlement and a bridge abutment in 

downtown Anchorage (fig. 17A) indicate liquefaction caused 

significant damage in some Anchorage locations and residential 

dwellings. In southern Anchorage near Sand Lake, a sand boil and 

small fissure were observed following the earthquake (fig. 17B).

Extensional Cracks with Associated Sand Ejecta

Extensional cracks with ejected sands where liquefactioninduced lateral spreading could be positively identified were 

observed near the north abutment of the Old Glenn Highway 

crossing of the Knik River (fig. 18). Extensional cracks within 

point-bar sands extended semicontinuously along a ~175 m 

stretch of the Knik River just downriver from a nearby highway 

bridge. Interestingly, we observed no displacements or damage to 

the bridge or lateral spreading just upriver. Typical crack widths 

downriver from the bridge were 10 cm, reaching a maximum of 

~50 cm. Cracking was observed continuously through surficial 

sand and ice. Sand ejecta was observed frozen to the surface of a 

~15-cm-thick sheet of ice (fig. 18). At the nearby Eklutna Tailrace, 

we observed additional extensional cracks (generally 7.5–10 cm 

wide but extending to 30 cm in places) with clean sand within the 

freshly exposed surfaces. Observations made by local residents prior 

to our arrival reported larger extensional (lateral spreading) cracks 

near the Eklutna Tailrace site within river gravels as wide as ~75 cm. 

However, these were obscured by snow during our reconnaissance. 

Extensional Cracks without Associated Sand Ejecta

Most evidence of liquefaction from the 2018 Anchorage 

earthquake involved extensional cracking we associated with 

lateral spreading without clear evidence of sand ejecta. Notable 

examples include highly disrupted cracking and displacement on 

the banks of the Susitna River (fig. 19A) and along the shoreline of 

the Knik Arm (figs. 19B, C). Extensional cracks commonly were 

highly angular (for example, figs. 19A, B) and were preserved 

in a seasonally frozen crust of soils. Although we do not expect 

frozen surficial soils to have liquefied, we interpret their blocky and 

irregular displacements to reflect deeper subsurface deformation, 

likely caused by liquefaction of susceptible and unfrozen soils. 

Lateral spreading was also observed at the northern edge of the Port 

of Alaska (fig. 19D) as well as sand boils in the tidal flats at a nearby 

public boat launch.
Observations 15

Figure 17. Photographs of evidence of liquefaction in Anchorage. A, Sand ejecta along the Chester Creek trail in 

Anchorage near the C Street overpass (photograph by Janet Curran, U.S. Geological Survey). Minor tilting of the 

bridge abutment was observed at this site. B, Sand boil in backyard of a house in southern Anchorage near Sand 

Lake. Sand boil is situated along a small fissure that propagated tens of meters through a house (located toward 

the top of the photograph). Settlement of part of the structure occurred close to the fissure (photograph by Roger D. 

Jones, resident). Locations shown in figure 4.

Figure 18. Photograph of extensional lateral-spread cracks on the north bank of the Knik River near the Old Glenn

Highway. Sand ejecta are frozen on top of ~15 centimeters (cm) of clear ice that rests on clean river sands. Tape 

extended 30.5 cm. Location shown in figure 4.
16 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Figure 19. Photographs of 

extensional lateral-spread cracks 

without obvious sand ejecta. A, 

Angular extensional cracks within 

a point bar island along the lower 

Susitna River with an associated 

set of compressional ice cracks 

caused by lateral displacement. 

B, Disrupted extensional 

cracks caused by liquefaction. 

Individual cracks are as wide as 

60 centimeters (cm). Cumulative 

lateral displacement at this site 

was ~1.5 meters. C, Arcuate 

extensional cracks above mean 

high tide in Knik Arm. Individual 

cracks are 2.5–15 cm wide 

extended semicontinuously for 

several hundred meters parallel 

to the shore. D, Extensional 

lateral-spread cracks at the Port 

of Alaska. 

Other Observations 

The most abundant and impactful style of ground failure 

triggered by this earthquake was extensional cracking not clearly 

associated with either landsliding or liquefaction. Many instances 

occurred adjacent to roads and residential structures near the 

edges of steep natural and fill slopes (fig. 20). In some instances, 

cracks propagated through existing buildings, or fill material 

moved downslope away from an existing structure on grade. This 

behavior was most common near the edges of graded building 

pads bounded by steep slopes. We noted widespread extensional 

cracking in the hilly residential areas of Peters Creek, Eagle 

River, and Chugiak (for example, fig. 20B). We did not make a 

comprehensive assessment, so other areas also may have been 

affected. Total horizontal and vertical offsets were typically tens 

of centimeters or less. The vast majority of cases we observed 

appeared to involve artificial fill and could be due simply to minor 

downslope slumping or settling, but we also observed cracking 

patterns in some locations that indicated potential incipient 

landsliding in both natural and fill slopes. Seismically generated 

cracking along the edges of slopes has been documented for other 

earthquakes (for example, Keefer and Manson, 1998; Jibson and 

others, 2018). 

We also documented several river-bank collapses with 

vertical displacements as much as about 2 m. Bank collapse was 

typically observed along small creeks and streams at or near 

Figure 20. Photographs of extensional cracking. A, Extensional 

cracking in overpass embankment along the Glenn Highway. B, 

Extensional cracking and settling around a private residence in 

Chugiak. Locations shown in figure 4.
Observations 17

mean high tide. Notable examples of this failure mode were 

along Campbell Creek (fig. 21A) within Anchorage and along 

Turnagain Arm below Girdwood (fig. 21B) in the remains of a 

forest that died because of tectonic subsidence in the 1964 Alaska 

earthquake. Marsh grasses covered in freshly deposited, unfrozen 

mud below the high tide line were the most common evidence 

of recent bank collapse. The bank collapses also were generally 

associated with recent extensional cracks. We did not observe 

ejecta of liquefiable materials at any bank-collapse sites. We 

suggest these bank collapses could either be lateral spreads caused 

by liquefaction that did not vent to the surface or simply slumping 

and (or) settlement of nonliquefiable material. 

One unusual site of extensional cracking was the island in 

the center of Lower Fire Lake (fig. 22). The island appears to 

be composed of glacial till. Its south end was crisscrossed by 

extensional cracks that propagated across its summit in an “X” 

pattern and then into the water. Blocks of ice adjacent to the south 

end of the island had been ripped up, thrust over one another, and 

then refrozen in place, suggesting a back-and-forth motion of the 

island during the earthquake that disturbed the surrounding ice. 

Additionally, we noted patches of dark ice around the island and 

along linear fissures in lake ice north of the island. These dark 

patches of ice suggest that warmer water could have been released 

from localized sections of lake sediment during the earthquake 

and could have partly melted lake ice directly above. Similar 

observations were made in a lake on the Kenai Peninsula located 

south of Anchorage and Turnagain Arm.

We observed high outflows from numerous springs in the 

steep valleys along the north side of Turnagain Arm. Though 

we lack pre-earthquake baseline data to determine if this 

apparent uptick was a result of the earthquake, we do have 

confirmation from one resident of Eagle River valley that a 

spring near his house had dramatically increased its output 

since the earthquake. This suggests the earthquake might have 

changed the subsurface hydrology, which could influence 

future landslide hazard. This phenomenon also has been 

observed in other earthquakes (for example, Rojstaczer and 

Wolf, 1992; Esposito and others, 2001).

Figure 21. Photographs of examples of bank collapse following 

the Anchorage earthquake. A, Possible lateral spreading leading 

to partial bank collapse along lower Campbell Creek, Anchorage. 

Marsh grasses along the creeksides were freshly covered in soft, 

unfrozen muds. B, Bank collapse near Girdwood, Turnagain Arm. 

Multiple lateral spreads or bank collapses were observed within 

the tidal zone at this location. Locations shown in figure 4.

Figure 22. Photographs from Lower Fire Lake. A, Extensional 

cracks along a small island within Lower Fire Lake. B, Cracks in 

ice adjacent to the island showing clear evidence of movement 

and refreezing. Location shown in figure 4.
18 Ground Failure Triggered by Shaking during the 2018 Anchorage Earthquake

Discussion

Previous research has related earthquake magnitude to 

number of triggered landslides. For the 2018 Anchorage, Alaska, 

earthquake investigated here (M7.1), the relation of Malamud 

and others (2004) predicts about 3,200 landslides; the relation 

of Keefer (2002) predicts about 8,200 landslides. A complete 

inventory of triggered landslides for this earthquake is unlikely to 

be produced because tree and snow cover and incomplete image 

availability impair visibility. Our field investigations, however, 

suggest that the number of landslides triggered is significantly 

less than the range predicted by global models, which were drawn 

primarily from shallow crustal earthquakes near plate boundaries 

that have strike-slip and reverse focal mechanisms. The relative 

paucity of observed ground failures following the 2018 Anchorage 

earthquake and similar deep, normal-fault, earthquakes without 

significant ground failure triggering—for example, the 2001 

M6.8 Nisqually, Washington (Highland, 2003), and 2001 M7.7 El 

Salvador (Bommer and others, 2002) earthquakes—point to the 

significance of focal depth and mechanism not currently accounted 

for in current magnitude-landslide relations. Further consideration 

of earthquake depth, mechanism, and regional setting may provide 

improved rapid estimates of the magnitude of potential landslides 

in the future. 

The USGS ground-failure models implicitly account for 

focal depth and mechanism through the ShakeMap groundmotion model, which uses ground-motion measurements, 

felt reports, and other factors that contribute to shaking. But 

the performance of the near-real-time ground-failure models 

was mixed. Overall, the alert levels (after the earthquake 

magnitude was refined from the initial M6.7 to the final M7.1) 

indicating that significant areas were exposed to landslide 

and liquefaction hazard (orange alert) agreed with our field 

observations. Additionally, the landslide model correctly shows 

the area of most concentrated landsliding around Eagle River 

and Peters Creek, as well as areas far up those valleys in the 

Chugach Mountains where we observed numerous rockfalls. 

The landslide model predicted scattered areas of high hazard 

farther south in the Chugach Mountains; what we saw was 

even more sparse than the model estimated, but snow cover 

prevented a comprehensive assessment. Landslides were 

triggered along the coastal bluffs surrounding Anchorage; the 

near real-time model did not show high hazard there, possibly 

related to the coarseness of the DEM (~250 m) or a lack of 

constraint of slope strength on a global scale. Although many 

of the areas that were affected by extensional cracking and 

incipient landsliding in Anchorage, Eagle River, and Peters 

Creek did have elevated (but low, 0.2–5 percent) probabilities 

of landslide occurrence, the landslide model was not trained on 

this type of ground failure and underestimated the probabilities. 

The physically based map of Jibson and Michael (2009), which 

uses a much finer resolution DEM (~6 m) but was not specific 

to this earthquake, did show these areas as having high seismic 

landslide hazard.

The near-real-time liquefaction model accurately indicated 

that liquefaction was likely to occur in many areas, such as 

along the Little Susitna River, the tidal flats of Matanuska 

Valley, McArthur Flats, the mouth of the Knik River, and 

Ingram Creek. However, we did not observe evidence of 

liquefaction in some areas where the model predicted high 

probabilities, such as the Susitna River and Chickaloon Bay, 

possibly because tidal erosion and snow cover hid surface 

evidence of liquefaction. Within Anchorage, evidence of 

liquefaction was observed in both locations of high predicted 

liquefaction hazard (probabilities of liquefaction as much as 

about 10 percent), such as along Chester Creek (fig. 17A) and 

the Port of Alaska (fig. 19D), and zones of low predicted hazard, 

for example, around Sand Lake (fig. 17B). Other locations, such 

as Campbell Creek (fig. 21A), experienced ground deformations 

but we were not able to conclusively determine that liquefaction 

occurred. Overprediction of the near-real-time models has 

been noted in other recent earthquakes (Allstadt and others, 

2018), but underprediction in locations like Sand Lake is a new 

observation that merits analysis.

The location and magnitude of the minor cracking 

at Government Hill and Turnagain Heights indicates that 

the 2018 ground motion triggered minor deformation 

associated with 1964 landslide failure surfaces. These 1964 

landslides might have amplified the shaking, as has been 

observed elsewhere (Hartzell and others, 2017), but no 

observable downslope movement of the landslide masses or 

remobilization occurred. 

The landslides at Potter Hill (fig. 13), on the other hand, 

involved significant downslope movement of new landslide 

material as well as reactivation of parts of the 1964 landslide 

deposits. This area is prone to such landslides, as indicated 

by repeated failures caused by both earthquakes and rainfall 

(Hansen, 1965).

Much of the damage from this earthquake was from 

extensional cracking (fig. 20) that could be related to incipient 

landsliding, possible impedance contrasts between cut and fill 

parts of slopes (Stewart and others, 2001), and high levels of 

ground motion near the edges of steep slopes (Ashford and 

others, 1997). Our models do not take these phenomena into 

account, which clearly merit additional study.

Summary

We developed an initial inventory of ground failure 

features from the November 30, 2018, M7.1 Anchorage 

earthquake. This inventory of 153 features is from groundbased observations soon after the earthquake (December 5–10) 

that include the presence or absence of liquefaction, landslides, 

and individual crack traces of lateral spreads and incipient 

landslides. This is not a complete inventory and simply shows 

general trends and examples of types and distribution of ground 

failures documented. Overflight observations (December1–6) 

documented landslide and liquefaction presence or absence 

within the Chugach Mountains and along Cook Inlet in 

regions inaccessible to vehicles, which greatly expanded 

the geographic scope of this reconnaissance. Field-mapped 
References Cited 19

ground-failure observations have been augmented with a set of 

565 georeferenced and annotated images from both field and 

overflight reconnaissance cataloging additional ground-failure 

presence or absence from the Anchorage earthquake (Grant and 

others, 2020).

Tidal erosion, fresh snowfall, limited daylight, and 

adverse flying conditions contributed significantly to the 

uncertainty and incompleteness of ground-failure observations 

during this reconnaissance. Notably, substantial liquefaction 

features at the mouths of the Little Susitna River (December 1) 

and Ingram Creek (December 5) were absent during subsequent 

overlapping missions (December 6 and 9, respectively) because 

of tidal action. A large rockfall observed on December 1 

on Rainbow Peak was not observed during a subsequent 

December 5 overflight because of snow cover, which suggests 

that the general lack of observations of landsliding within the 

Chugach Mountains is uncertain.

Key observations include the following:

• Landslides triggered by the 1964 Alaska earthquake 

responded to the shaking but did not reactivate with 

significant downslope movement during the recent 

earthquake. Observed deformation is limited to 

extensional cracks 1–2 cm wide within the Turnagain 

Heights and Government Hill landslides.

• Most observed ground failure (landslides and 

liquefaction) exhibited permanent deformations of less 

than ~20 cm.

• A few flow slides within highway fill (most notably, 

Vine Road and Minnesota Drive) and glacial-marine 

sediment near Potter Hill produced long runout deposits 

and significant damage to roadways in the case of fill 

landslides. Slides occurred in the same location as the 

Potter Hill landslides that were triggered by shaking in the 

1964 M9.2 Alaska earthquake.

• Widespread lateral spreading and (or) riverbank slumping 

was observed along lower stretches of multiple rivers and 

creeks along the Turnagain and Knik Arms of Cook Inlet.

• Multiple extensional cracks and possible incipient 

landslides that have coseismic displacements of several 

centimeters to a few meters occurred along Peters Creek, 

Eagle River, and southern Anchorage near residential 

dwellings. 

• The 2018 earthquake produced less ground failure over 

a more limited area than would be predicted by present 

models that depend on magnitude alone. Other factors 

such as focal depth and mechanism probably played an 

important role in the relative paucity of ground failure 

caused by this earthquake.

• The near-real-time ground-failure models had a mixed 

performance. They correctly showed ground-failure 

concentrations in some areas, but over- or underpredicted 

hazard in other areas.

• Some of the locations of observed slides along the 

coastal bluffs were not mapped as having high landslide 

probability but were identified as having high liquefaction 

probability by ground-failure models because the resolution 

of the DEM could not identify these slopes. This is a good 

motivation to use high-resolution DEMs for the landslide 

model. Slope is used as a proxy for the shear-wave velocity 

in the upper 30 m of the crust (VS30) for liquefaction, and 

higher resolution slopes perform more poorly as predictors 

of VS30, but a high slope at higher resolution should be 

effective at precluding the presence of liquefaction. 

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Menlo Park Publishing Service Center

Manuscript approved April 29, 2020

Edited by Monica Erdman

Layout and design by Cory Hurd
Grant and others—Ground Failure Triggered by Shaking during the November 30, 2018, Magnitude 7.1 Anchorage, Alaska, Earthquake—Open-File Report 2020–1043

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20201043