U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3005

January 2007

Landslide Hazards in the Seattle, Washington, Area 

Printed on recycled paper

Seattle’s geology and climate contribute to landslides

The Seattle area was glaciated during the last ice age. The 

arrangement of glacial deposits and other weak rocks, along 

with the effects of coastal erosion, define where most landslides 

occur. Seattle has warm dry summers and cool moist winters. 

Average annual rainfall is about 37 inches and most landslides 

occur during the rainy season, which begins approximately in 

October or November and lasts through the following April or 

May. In general, landslides are common on coastal bluffs and 

other steep slopes. Glacial deposits consisting of layered silt 

and clay, sand, or sand and gravel, as well as 

other weak rocks of similar composition, occur 

throughout the area and are prone to landslides. Ground water tends to accumulate in 

the sand layers and weaken them by elevating 

pore pressures, especially where thick sand 

deposits rest on low permeability silt beds. 

Thus, many historical landslides have occurred 

in the thick deposits of glacial outwash sand 

found throughout the area. Deposits from these 

landslides form a topographic bench on many 

bluffs. Many recent landslides have occurred 

in the deposits and headscarps of previous 

landslides. Wave action constantly wears away 

the base of bluffs, which helps to maintain 

their steepness and contributes to recurrent 

landslide movement on the bluffs. Most slides 

occur after heavy winter rainfall soaks the 

ground and further weakens the soil. Major 

earthquakes in 1949, 1965, and 2001 have also 

caused landslides.

Four main types of landslides are common in 

the Seattle area, including (1) shallow translational earth slides, (2) debris flows and rapid 

earth flows, (3) soil topples and associated soil and rock falls, and 

(4) large, translational and rotational earth slides that are several 

meters to tens of meters deep. Shallow, rapidly moving translational slides are the most common type. They usually occur in 

loose sandy deposits that mantle steep slopes and on the headscarps and toes of large, deep-seated landslides. Most debris and 

earth flows begin as shallow translational slides; once the slides 

start moving, they transform to flows if enough water is present. 

The rapid movement of shallow slides and flows imparts significant destructive power. Soil topples and rock or soil falls occur 

mainly from steep outcroppings of till or other glacial deposits. 

Soil blocks topple (forward rotation out of the slope, below the 

center of mass of the soil block) from bluffs after cracks open 

within a few meters of the face of the bluff. Locally, soil topples 

pose a serious hazard, but few have caused major damage. Large 

deep landslides occur mainly on bluffs that overlook Puget Sound 

or Lake Washington, and they have caused severe damage to 

homes and infrastructure. These slides commonly occur in thick 

deposits of glacial outwash sand and till or in deposits of previous 

landslides. Most move slowly for periods of weeks to years, but 

some can also move rapidly. 

USGS landslide hazard studies for the Seattle area

Following major landslides in 1996 and 1997, Seattle and 

other Puget Sound area communities resolved to reduce future 

landslide losses. Seattle commissioned Shannon & Wilson, Inc., 

to compile a database of historical landslides from information 

These houses overlook the Magnolia Bridge area of Seattle. The white house on the right 

side hangs precariously over the headscarp of a large landslide (photo by Hugh Shipman, 

Washington Department of Ecology).

The Seattle, Washington, area is known for 

its livability and its magnificent natural setting. The city and nearby communities are 

surrounded by an abundance of rivers and lakes 

and by the bays of Puget Sound. Two majestic 

mountain ranges, the Olympics and the Cascades, 

rim the region. These dramatic natural features 

are products of dynamic forces—landslides, 

earthquakes, tsunamis, glaciers, volcanoes, and 

floods. The same processes that formed this beautiful landscape pose hazards to the ever-growing 

population of the region. Landslides long have 

been a major cause of damage and destruction to 

people and property in the Seattle area.
that the city and other groups had been collecting for about a 

century (see http://www.seattle.gov/DPD/Landslide/Study/default.

asp). In 1998, the Federal Emergency Management Agency 

(FEMA) designated Seattle as a “Project Impact” city with the 

goal of encouraging the city to become more disaster-resistant 

to the effects of natural hazards. A major recommendation of the 

Project Impact council was that Seattle and the U.S. Geological 

Survey (USGS) collaborate to produce a landslide hazard map 

of the city. Related efforts to acquire high-resolution topographic 

data and to update geologic mapping of the city contributed 

greatly to development of the landslide maps. Cooperation of 

the city and the wealth of data that became available enabled the 

USGS to produce a suite of landslide maps and related products 

that include: (1) a shallow-landslide hazard map, (2) a landslideterrain map, (3) a landslide recurrence and probability map, (4) a 

hazard map for deep-seated landslides, (5) a landslide hazard map 

linked to rainfall intensity, and (6) rainfall thresholds for forecasting landslides. The Seattle landslide database allowed comparison 

of actual landslide locations and times with those predicted by 

modeling and analysis.

The following paragraphs briefly describe and illustrate some 

of the maps. Please see our website (http://landslides.usgs.gov) 

for the complete maps and for more detailed information.

Shallow landslide hazard map

The “Shallow Landslide Hazard map” (a portion shown in 

fig. 1) shows the susceptibility and relative hazard of Seattle 

slopes to shallow landslides, the most common type of rainfallinduced slope failure in Seattle. Shallow landslides are particularly hazardous to people and property because they often transform into debris flows, slurries of soil and rock that can travel 

rapidly (35–40 mph) downslope. Recent geologic mapping, 

soil strength data, and high-resolution topographic data were 

used to evaluate the stability of each slope segment of the map. 

The stability was indexed and the results were compared with 

the landslide data set. The resulting map indicates categories of 

relative landslide hazard for the entire city of Seattle. 

Landslide terrain map using LIDAR imagery

The “Landslide Terrain map” (fig. 2) shows the geomorphology of the Seattle area for three kinds of landslide features—landslide headscarps, landslide deposits, and denuded 

slopes (slopes formed by mass wasting and erosion since 

retreat of glacial ice). This mapping effort involved the use of 

Light Distance and Ranging (LIDAR) data to expose landslide 

features in the many heavily vegetated areas in Seattle. Nearly 

all of the landslides mapped from this data are complexes consisting of numerous individual landslides. Mapped landslides, 

headscarps, and denuded slopes delineate areas of historical 

landslide activity more thoroughly than previous Seattle-wide 

landslide mapping efforts. More than 93% of about 1,300 

reported historical landslides occurred within the boundaries 

of the LIDAR-mapped landforms. Virtually all (99.7%) natural 

historical landslides occurred within these boundaries.

Map of landslide probability and recurrence 

The “Landslide Probability and Recurrence map” (fig. 3) 

indicates annual probabilities for climatically induced landslides 

in Seattle. Seattle’s historical landslide database was used to 

apply a time-dependent probabilistic approach to the evaluation of future landslides. Landslide probability values were 

established using the landslide database. Statistical analysis of 

the database showed that the annual probability of one or more 

landslides is 1% or greater for about 9% of the area of Seattle. 

Maximum annual probability of future individual landslides is 

25% on a hillslope in west Seattle. 

Deep landslide hazards

Deep landslides on the coastal bluffs of Puget Sound can 

cause extensive property damage. These slides typically are 

induced by ground-water conditions resulting from extended or 

successive wet winters rather than individual storms. Because 

they are large, their stability is controlled by three-dimensional 

Figure 1. Portion of Shallow Landslide Hazard map showing part of the 

Magnolia area of Seattle. 

Figure 2. Portion of Landslide Terrain map showing part of the Magnolia 

area of Seattle. LIDAR imagery revealed landslide and erosion features 

in the yellow areas of the map and the features of large old landslides in 

the red and orange areas. Most historical landslides have also occurred 

in these areas.
(3-D) effects, including complex 3-D ground-water flow as well 

as 3-D variations in strength and topography. A deep landslide 

hazard map (fig. 4) indicates the potential for these large landslides by showing the relative stability and instability of slopes 

on the bluffs of west Seattle.

The resulting map demonstrates that when slopes are dry, 

steepness and strength control potential instability. However, 

where ground water perches on lower permeability clay layers, 

extended wet winter conditions can increase the water table 

near the bluff face. Elevated ground-water pressures can lower 

slope stability. Given extended wet conditions, the 3-D analyses 

identify low stability areas in and adjacent to historically active 

deep-seated landslides.

Forecasting landslides

The exact time and location of landslides are very hard to 

predict even with sophisticated monitoring and models. Nevertheless, most landslides around Puget Sound occur during wet 

weather between October and April, so it is possible to know the 

general time when landslides are more likely to occur. Scientists 

at the USGS have developed methods and formulas based on 

past rainfall amounts to identify when landslides are likely. These 

formulas are called “precipitation thresholds.” Two thresholds 

have been developed for the Seattle area. The first one, called 

the Cumulative Precipitation Threshold (fig. 5), tracks precipitation over the last 18 days and indicates when the ground is wet 

enough to be susceptible to landslides. Between 3.5 and 5.3 

inches of rain are required to exceed this threshold, depending on 

how much rain has fallen in the last 3 days. The second, called 

the Intensity Duration Threshold (fig. 6), tracks rainfall during 

a storm and indicates when it is raining hard enough to cause 

multiple landslides if the ground is already wet. 

Figure 3. Portion of Landslide Probability and Recurrence map showing 

part of the Magnolia area of Seattle. Probabilities shown on this map 

were determined based on the frequency of historical landslides in 

each grid cell of the map.

 

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Figure 4. Potential for deep-seated landslides during wet winter conditions in the Duwamish Head area of Seattle (oblique aerial view toward 

south). Red and pink areas are considered most likely to produce large, 

deep-seated landslides.

Figure 5. Cumulative Precipitation Threshold. When rainfall totals for the 

last 3 days (vertical axis) and the previous 15 days (horizontal axis) plot 

in the blue area of the graph (requires a combined total between 3.5 and 

5.3 inches of rain over the 18-day period), the probability of landslides 

occurring is about 10%. 

0

1

2

3

4

5

6

P

0 1 2 3 4 5 6 7 8 9 10 3, 3-DAY CUMULATIVE PRECIPITATION, INCHES

P15, 15-DAY CUMULATIVE PRECIPITATION BEFORE P3, INCHES

Cumulative Precipitation Threshold

P3=3.5-0.67P15

Landslides likely

Landslides

unlikely

J

JJ

J

J

J J

JJ

J

J J

J

HH

H

H

H

H

H

H

H

H H

H

FF F

F

F

F

F

F

F

F

F

F

FF

F

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

20 25 30 35 40 45 50 55

MEAN RAINFALL INTENSITY, I, INCHES PER HOUR

RAINFALL DURATION, D, HOURS

Threshold

J March 1997 landslides

H January 1986 landslides

F Other landslides

I=3.257D-1.13

Landslides unlikely

Landslides very likely

Figure 6. Intensity Duration Theshold. The graph, based on analysis of 

historical landslides, shows that a storm producing at least 2–3 inches 

of rain in 1–2 days is very likely to cause many landslides if the ground is 

already wet from recent rain. 
By comparing recent and forecast rainfall amounts to the 

thresholds, meteorologists, geologists, and city officials can 

help people know when to be prepared for landslides. The 

thresholds are accurate, but imperfect indicators of when landslides may occur. For example, statistical analysis of landslides 

that occurred between 1978 and 2003 showed that 85% of them 

occurred when the Cumulative Precipitation Threshold was 

exceeded. Also, the thresholds do not predict landslides caused 

by earthquakes or by construction. The thresholds work best 

in areas along the east side of Puget Sound, from Tacoma to 

Everett, but they can also give preliminary guidance in nearby 

areas, including Kitsap and Island Counties, the northern part of 

Thurston County, and the eastern part of Mason County. Additional data and analyses are needed to refine the thresholds for 

these areas and other parts of Washington State. 

How to access the maps and how to use them

Access

Most of the landslide maps and information about the rainfall 

thresholds are available on-line. The Internet addresses of maps 

shown in figures 1, 2, and 3 and a report describing the rainfall 

thresholds (figs. 4 and 5) are listed below: 

Chleborad, A.F., Baum, R.L., and Godt. J.W., 2006, Rainfall 

thresholds for forecastng landslides in the Seattle, Washington, 

area—Exceedance and probability: U.S. Geological Survey 

Open-File Report 2006-1064, accessed December 12, 2006, at 

URL http://pubs.usgs.gov/of/2006/1064/

Coe, J.A., Michael, J.A., Crovelli, R.A., and Savage, W.Z., 2000, 

Preliminary map showing landslide densities, mean recurrence 

intervals, and exceedance probabilities as determined from 

historic records, Seattle, Washington: U.S. Geological Survey 

Open-File Report 2000-303, accessed December 12, 2006, at 

URL http://pubs.usgs.gov/of/2000/ofr-00-0303/. Map available at 

http://pubs.usgs.gov/of/2000/ofr-00-0303/plate1.html

Harp, E.L., Michael, J.A., and Laprade, W.T., 2006, Shallowlandslide hazard map of Seattle, Washington: U.S. Geological 

Survey Open-File Report 2006-1139, accessed December 12, 

2006, at URL http://pubs.usgs.gov/of/2006/1139/

Schulz, W.H., 2005, Landslide susceptibility estimated from 

mapping using light detection and ranging (LIDAR) imagery 

and historical landslide records, Seattle, Washington: U.S. Geological Survey Open-File Report 2005-1405, accessed December 12, 2006, at URL http://pubs.usgs.gov/of/2005/1405/

See http://landslides.usgs.gov/statelocalinfo/seattle/ for links to 

additional reports and information. A link to the deep landslide 

map will be added there once it is available. 

Use of the maps

These USGS maps are intended as a general guide to 

landslide occurrence, not as a predictor of landslide hazard at 

specific sites in Seattle. These maps depict the potential for 

landslides from hillside source areas, but they do not depict 

landslide travel paths or areas of landslide deposition. The maps 

(figs. 1, 2, 3, and 4) do not take the place of an on-site survey 

Where can I get more information?

http://landslides.usgs.gov/

http://landslides.usgs.gov/statelocalinfo/seattle/

To see current Outlooks,

http://landslides.usgs.gov/advisories/

Puget Sound area landslides and local weather,

http://www.ecy.wa.gov/programs/sea/landslides/

http://www.wrh.noaa.gov/sew/

How can I prepare for landslides?

http://landslides.usgs.gov/statelocalinfo/seattle

Also contact:

Lynn Highland, U.S. Geological Survey Landslide

Information Officer, phone: 1-800-654-4966,

e-mail: highland@usgs.gov

Seattle Geology

http://geology.wr.usgs.gov/wgmt/pacnw/pnwmap.html 

http://geomapnw.ess.washington.edu/index.php 

http://pubs.usgs.gov/of/2005/1252/

http://www.seattle.gov/projectimpact/pdfs/Sea_geo_map.pdf

Authors: Rex Baum, Ed Harp, and Lynn Highland

Publication Layout: Margo Johnson, Denver Publishing 

 Service Center

or the professional judgment of a geologist or a geotechnical engineer. Appropriate uses of the maps include (1) storm 

preparedness planning for emergency access and response, 

(2) planning for development or redevelopment of hillside areas, 

and (3) municipal facility planning. 

The maps shown in figures 1, 2, and 3 represent different but complementary ways of depicting potential landslide 

hazards in Seattle. Figure 1, based on simplified hydrologic 

and mechanical models of slope stability, depicts areas that are 

potentially unstable. Relative slope stability is classified on the 

basis of slope, a static surface water table, and average engineering properties of the geologic units, rather than historical 

landslide occurrence or landforms. This approach is somewhat 

less subjective than methods based on geomorphic interpretation and is not limited to areas of reported landslide occurrence. 

The geomorphic approach used to create the map shown in 

figure 2 depicts areas of past landslide occurrence over a longer 

time scale (since the last glacial retreat) than the historically 

based map in figure 3. Consequently, figure 2 gives a more 

complete picture than figure 3 of what areas have been affected 

by landslides and have potential for future landslides, provided 

that factors causing past slope instability continue to operate. 

The historically based probabilistic map shown in figure 3 is 

useful for identifying areas of frequent landslide damage so that 

city officials can set priorities in landslide mitigation efforts. 

However, historical landslides have been reported mainly in 

inhabited areas, and, thus, figure 3 gives little information on 

landslide occurrence or susceptibility of undeveloped hillsides.