Hazard Analysis of Landslides Triggered by 

Typhoon Chata’an on July 2, 2002, in Chuuk State, 

Federated States of Micronesia

By Edwin L. Harp, Mark E. Reid, and John A. Michael

U.S. Geological Survey Open-File Report 2004-1348

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .................................................................................................................................................................................1

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

Overview of the Landslide Event..................................................................................................................................................1

Geology and Physiographic Setting..............................................................................................................................................4

Landslides Triggered by Chata’an ................................................................................................................................................5

Types of Landslides................................................................................................................................................................5

Landslide Inventory................................................................................................................................................................6

Debris-Flow Runout...............................................................................................................................................................6

Significant Landslides...................................................................................................................................................................7

Weno (Moen) Island...............................................................................................................................................................7

 Nepukos Slump/Debris Flow—Location 1 ....................................................................................................................7

 Mwan Slump/Debris Flow—Location 2 ........................................................................................................................7

Tonoas (Dublon) Island..........................................................................................................................................................8

 Nechap Area Slump/Debris Flow—Location 3..............................................................................................................8

 Kuchua Slump/Debris Flow—Location 4 ......................................................................................................................8

 Roro Debris Slide/Flow—Location 5...........................................................................................................................11

Fefan Island..........................................................................................................................................................................11

 Pwene Slump/Debris Flows—Map Location 8............................................................................................................11

 Sapore Slump/Debris Flow and Incipient Landslide—Map Location 10 ....................................................................11

Uman Island .........................................................................................................................................................................15

 Sanuk Slump/Debris Flow—Map Location 12 ............................................................................................................15

Udot Island...........................................................................................................................................................................15

 Penia Slump/Debris Flow—Map Location 14 .............................................................................................................15

Fanapanges Island ................................................................................................................................................................15

 Faia Slumps and Incipient Slumps—Map Location 16................................................................................................15

Assessment of Remaining and Future Landslide Hazards..........................................................................................................17

Continuing Landslide Hazards.............................................................................................................................................17

Future Landslide Hazards ....................................................................................................................................................17

 Slope-Stability Analysis...............................................................................................................................................17

 Slope-Stability Analysis Variables...............................................................................................................................18

 Results: Factor of Safety (Susceptibility) Map ............................................................................................................19

 Results: Debris-Flow Hazard Map ...............................................................................................................................20

 Practical Use of the Debris-Flow Hazard Map.............................................................................................................21

Summary and Conclusions .........................................................................................................................................................21

Acknowledgements.....................................................................................................................................................................22

References Cited .........................................................................................................................................................................22

Cover Photo: Oblique aerial view of debris flows on the island of Tonoas in 

Chuuk State, Federated States of Micronesia triggered by Typhoon Chata’an.

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1

ABSTRACT

More than 250 landslides were triggered across the eastern 

volcanic islands of Chuuk State in the Federated States of 

Micronesia by torrential rainfall from tropical storm Chata’an 

on July 2, 2002. Landslides triggered during nearly 20 inches 

of rainfall in less than 24 hours caused 43 fatalities and the 

destruction or damage of 231 structures, including homes, 

schools, community centers, and medical dispensaries. 

Landslides also buried roads, crops, and water supplies. The 

landslides ranged in volume from a few cubic meters to more 

than 1 million cubic meters. Most of the failures began as 

slumps and transformed into debris flows, some of which 

traveled several hundred meters across coastal flatlands into 

populated areas. A landslide-inventory map produced after 

the storm shows that the island of Tonoas had the largest 

area affected by landslides, although the islands of Weno, 

Fefan, Etten, Uman, Siis, Udot, Eot, and Fanapanges also had 

significant landslides. Based on observations since the storm, 

we estimate the continuing hazard from landslides triggered 

by Chata’an to be relatively low. However, tropical storms and 

typhoons similar to Chata’an frequently develop in Micronesia 

and are likely to affect the islands of Chuuk in the future. 

To assess the landslide hazard from future tropical storms, 

we produced a hazard map that identifies landslide-source 

areas of high, moderate, and low hazard. This map can be 

used to identify relatively safe areas for relocating structures 

or establishing areas where people could gather for shelter 

in relative safety during future typhoons or tropical storms 

similar to Chata’an.

INTRODUCTION

On July 2, 2002, heavy rains from tropical storm Chata’an 

(subsequently Super Typhoon Chata’an) triggered several 

hundred landslides across various islands of Chuuk State in 

the Federated States of Micronesia (figs. 1 and 2), resulting in 

43 fatalities and the destruction or damage of 231 structures 

(FEMA Internal Report). Landslide specialists from the U.S. 

Geological Survey (USGS) were requested to perform a 

reconnaissance of the landslides and to evaluate the continuing 

landslide hazards to residents and property on the affected 

islands of Chuuk State. This work was conducted under the 

Federal Emergency Management Agency (FEMA) mission 

assignment subtask 1427-DR-FM-COE-POD-05 with the U.S. 

Army Corps of Engineers (USACE).

Between July 27 and August 7, 2002, USGS landslide 

specialists and USACE geotechnical engineers conducted 

a field reconnaissance of the Chuuk islands affected by 

landslides triggered by tropical storm Chata’an. USGS 

scientists (Harp and Reid) obtained a broad overview of the 

landslide distribution by means of an aerial overflight and 

performed detailed field investigations of selected highimpact landslides to assess failure processes, to analyze the 

distribution of failures, and to evaluate the remaining hazard 

to people and property in future rainfall events. We provided 

preliminary information about continuing landslide hazards 

in Chuuk State to FEMA and the USACE in August 2002. 

Following this field reconnaissance, we prepared a detailed 

inventory map of the landslides triggered by Chata’an. Using 

Geographical Information System (GIS) technology, we then 

applied a slope-stability analysis to the islands affected by 

landslides from Chata’an and created a hazard map portraying 

the likely locations of future debris flows. In September 2003, 

we returned to Chuuk State to present these hazard maps to 

local officials and to examine the status of the previous year’s 

landslides.

This report first presents an overview of the landslide event, 

the types and distribution of landsliding triggered by Chata’an, 

and individual descriptions of some of the significant 

landslides. The report then discusses continuing hazards from 

landslides for the islands affected by Chata’an. To address 

landslide hazard, the report presents a landslide (debris-flow) 

hazard map and discusses possible uses of this map.

OVERVIEW OF THE LANDSLIDE EVENT

Intense rainfall from tropical storm Chata’an pounded the 

islands of Chuuk State on July 1 and 2, 2002. This deluge 

triggered at least 269 landslides on steeper slopes of the 

islands of Weno (Moen), Tonoas (Dublon), Fefan, Etten, 

Uman, Siis, Udot, Eot, and Fanapanges (Falabeguets), all 

located within Chuuk (Truk) Lagoon (table 1 and fig. 2). 

During the storm, 43 people were killed, and 231 structures 

were destroyed by landslides (FEMA Internal Report). 

Landslides affected the greatest area on Tonoas. Aerial 

reconnaissance identified only a few, relatively small recent 

landslides on the island of Tol and nearby small islands. 

According to local residents, most of the landslides 

occurred between 7 a.m. and 2 p.m. on July 2. At the airport 

on the island of Weno, 19.74 inches of rain fell on July 2, with 

several rain bursts having hourly intensities greater than 3 

inches per hour (fig. 3). Although the timing and amounts of 

rainfall likely varied between islands, many of the landslides 

appear to have been triggered during or immediately following 

intense rain bursts (fig. 3). Residents experienced prolonged 

thunder and lightening during the storms, often during the time 

when landsliding occurred. Hundreds of people evacuated 

their homes in fear of future landsliding, and many remained 

evacuated more than a month later.

The landslides triggered by Chata’an range in volume 

from a few cubic meters to more than 1 million m3 and 

occurred in deeply-weathered volcanic rocks. Most of the 

landslides that caused fatalities and property damage were 

slumps (coherent failure masses whose downslope movement 

commonly has a rotational component) that transformed 

into debris flows. After starting to move downslope, many 

of the landslides transformed into debris flows, slurry-like 

mixtures of water, clay, sand, silt, and boulders. The larger 

and particularly destructive debris flows appeared to be aided 

in this transformation by water impounded by the slump, 
2 

Figure 1. Chuuk State, Federated States of Micronesia.

0

0 500 KILOMETERS

300 MILES

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140 E 160

Yap State Chuuk State

Pohnpei State

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OF PALAU

COMMONWEALTH

OF THE NORTHERN

MARIANA ISLANDS REPUBLIC

OF THE

MARSHALL

ISLANDS

FEDERATED

STATES OF

MICRONESIA

Guam

Saipan

Majuro

Kwajalein

Wake

Pohnpei

Kosrae

Nauru

PACIFIC OCEAN

INSET MAP

20N

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140 E 160 180 160 W

HAWAII

INDONESIA

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OF MICRONESIA

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Tutuila

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OF THE NORTHERN

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REPUBLIC OF THE

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0 500 MILES

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Area of inset map

2 Water-Quality Reconnaissance of Ground Water in the Inhabited Outer Islands of Chuuk State, Federated States of Micronesia

Area of

Figure 2

Figure 1. Map of the western Pacific Ocean showing locations of the Federated States of Micronesia and of Chuuk State (after 

Hamlin and Takasaki, 1984-85).
3

Figure 2. Map of Chuuk islands within Chuuk (Truk) Lagoon (after Hamlin and Takasaki, 1984-85).

Table 1. Amount of landsliding on the islands of Chuuk State, Federat ed States of Micronesia, triggered by tropical storm Chata'an

Percent of island Percent of coastal lowlands

Number of slides Area of island (m2) Area of slides (m2) affect ed by landslides affect ed by landslides

Weno 64 18,735,300 327,000 1.7 1.2

Tonoas 57 8,674,700 663,900 7.7 5.1

Fefan 96 12,051 ,400 468,900 3.9 2.4

Uman 23 3,985,300 252,400 6.3 6.7

Etten 5 525,300 17,500 3.3 1.6

Udot 20 4,546,400 145,700 3.2 0.9

Eot 1 363,200 2,300 0.6

Fanapanges 3 1,656,200 24,200 1.5 1.6

Tot al 269 50,537,800 1,901,900 3.8 2.5
4 

either directly behind the rotated slump mass or upstream of a 

temporary landslide dam. Debris flows can travel rapidly (as 

fast as 50 km/hr) and can flow long distances over relatively 

gentle slopes. Most of the debris flows formed during 

Chata’an raced down steeper slopes, coming to rest only after 

traveling across flat coastal lowlands. The largest debris flow, 

which occurred on Fefan, traveled more than 700 m, whereas 

smaller flows typically traveled about 100 m or less across 

the coastal lowlands. These debris flows buried people and 

crushed many homes and other structures in their paths (see 

table 2).

In addition to these slump/debris flows, many incipient 

landslides currently exist on the islands. These are areas that 

began to slide during the storm but did not fully fail. They 

are characterized by fresh tensional cracking at their upslope 

margins; typically cracks are open or the downslope side has 

dropped. Lateral and toe areas of these incipient slides may 

not be well defined. These features range in length from a few 

meters to more than 300 m; many are slightly arcuate in plan 

view and suggest rotational movement. 

Landslides triggered by strong tropical storms are a 

recurring phenomenon in the Chuuk islands. Geomorphic 

evidence of previous landslides is present on many steeper 

slopes in the Chuuk islands. In 1976, Typhoon Pamela 

initiated a landslide that killed people on the island of Weno, 

and in February of 2002, a rainstorm triggered a large 

landslide visible on the eastern slope of the island of Tol 

(Johnson Elimo, personal commun., 2002). In recent years, 

landslides and debris flows triggered by tropical storms 

have occurred on other volcanic islands with steep slopes 

throughout the Pacific Ocean, such as Tonga and Vanuatu. 

Reports document extensive storm-related landslide activity 

on Pohnpei, Federated States of Micronesia (Harp and Savage, 

1997) and on the island of Oahu, Hawaii (Ellen and others, 

1991, Peterson and others, 1993). The greatest loss of life 

from a landslide in North American history was caused by a 

landslide within the Mameyes district in the city of Ponce, 

Puerto Rico triggered by Tropical Storm Isabel in October 

1985 ( Jibson, 1989).

GEOLOGY AND PHYSIOGRAPHIC 

SETTING

The Chuuk Islands are the erosional remnants of a 

partly submerged large shield volcano, long since inactive. 

These remnants largely consist of basaltic, andesitic, and 

trachytic lava flows, however, pyroclastic deposits are locally 

interbedded with the flows. The volcano extends from the 

ocean floor which is about 5,000 m below, to the water surface 

and was formerly several thousand feet above sea level. Coral 

reefs now form a surrounding barrier at the volcano’s margins 

at the water surface while fringing reefs border the volcanic 

islands within the lagoon. No traces exist of crater or caldera 

walls marking a central volcanic vent. However, geologic 

evidence indicates a central crater that produced large 

volumes of pyroclastic ejecta that now appears as volcanic 

breccia (Stark and Hay, 1963).

The Chuuk Islands comprise small volcanic and reef islands 

in a lagoon (formerly called Truk Lagoon) formed by a coral 

reef (fig. 2). They are divided into an eastern group (Weno, 

Tonoas, Fefan, and Uman), a central group (Udot and Eot), 

and a western group (Tol, Fanapanges, and Romonum). The 

high volcanic islands range in size from about 30 km2 (Tol-8 

km x 3.2 km; Weno-6.4 km x 4.8 km) to the smallest islands 

which are less than 0.5 km in diameter. The highest elevation 

on Tol is 446 m; the highest elevations on Weno and Tonoas 

are 369 m and 344 m, respectively. Slopes, in general, average 

15o with slopes of 30o common at the higher elevations. Most 

Figure 3. Bar graph of hourly rainfall from tropical 

storm Chata’an at Weno airport (WSO) station. Time 

of occurrence of significant landslides obtained from 

local residents. Table 2 contains more information 

about the significant landslides (by number).
5

of the volcanic islands have small streams originating in the 

steeper uplands and have flat coastal lowlands around their 

margins. These lowlands are commonly narrow, but on some 

islands they are wider than 0.5 km. In addition, 24 low coralreef islands are within the lagoon. The surrounding barrier reef 

is capped by by 41 low coral islands. The largest is slightly 

longer than 3 km and about 0.5 km wide with most being 

much smaller (Stark and Hay, 1963). Landslides triggered 

during tropical storm Chata’an occurred predominantly on the 

steeper parts of the volcanic islands.

LANDSLIDES TRIGGERED BY 

CHATA’AN

Types of Landslides

Many types of landslides were triggered during tropical 

storm Chata’an. The landslide nomenclature used in this report 

is from Varnes (1978). Debris flows caused the most death 

and destruction. Most debris flows on the islands of Chuuk 

State originated from rotational slumps; a few originated 

from shallow translational landslides. These landslides were 

triggered by rainfall infiltrating into the ground, increasing 

ground-water pressures, and weakening the slopes. Most of 

the initial landslides occurred on slopes steeper than 20o. 

Many saturated or nearly saturated slumps then transformed 

into debris flows, raced down steep slopes, traveled across 

the coastal flats, and came to rest leaving thick deposits of 

saturated mud, boulders, and debris. Figure 4 shows a slump 

(landslide source), debris-flow track, and debris deposit at 

Mwan (no. 2, pl. 1) on the island of Weno.

Rotational slumps triggered during the storm varied in 

depth from 1 to 40 m, and were composed primarily of deeply 

weathered basaltic- and andesitic-volcanic rocks with some 

surficial colluvium. Sliding surfaces commonly occurred near 

the contact between weathered and unweathered volcanic 

rocks. Slides were not present in hard, unweathered bedrock. 

Some of the landslides, generally those on steeper slopes with 

shallow colluvium underlain by stronger bedrock, originated 

as thin, slab-like translational slides. Most of the damaging 

debris flows originated as deep rotational slumps in highly 

weathered bedrock. The most damaging significant landslides 

are discussed in the section on significant landslides below.

At several sites, the initial slump or slide block dammed 

either a stream course or springs emanating from the slide 

Table 2. S ignificant landslides in Chuuk State, Federat ed States of Micronesia, triggered by t ropical storm

Chata'an

Map

number Locat ion Island

Number of

Fatalit ies Other S ignificance

Est imated Volume

(m3)

Description in

text

1 Nepukos Weno 3 Destroyed houses 20,000 *

2 Mwan Weno 1 Destroyed houses 10,000 *

3 Nechap Tonoas 14 Destroyed houses 100,000 *

4 Kuchua Tonoas 5 Destroyed houses 200,000 *

5 Roro Tonoas

Destroyed wat er supply

reservoir 100,000 *

6 Enin Tonoas 5 Destroyed houses 1000

7 Chun Tonoas Destroyed dispensary 3000

8 Pwene Fefan 2 28,000 *

9 Mese iku Fefan 2 Destroyed house 100

10 Sapore Fefan 4 Destroyed houses 10,000 *

11 Mochon Uman Very large volume 1,500,000

12 Sanuk Uman 1 Destroyed houses 10,000 *

13 Fanai Siis 1

14 Penia Udot 4 Destroyed houses 10- 15 ,000 *

15 Mwalitiw Udot 1 Destroyed houses

16 Faia Fanapanges

Destroyed houses and

dispensary 8000 *

Tot al

Fatalit ies 43
6 

scarp and then mobilized as a debris flow as the landslide dam 

was overtopped by the impounded water. This was the case in 

the large and particularly destructive debris flows at Nechap, 

Kuchua, and Roro (nos. 3, 4, and 5, pl. 1) on the island of 

Tonoas where the impounded water appeared to be a major 

factor in the mobilization of the landslide masses into highspeed slurries. 

Landslide Inventory

We produced an inventory map showing the size and 

distribution of landslides triggered by tropical storm Chata’an 

on the islands of Weno, Tonoas, Fefan, Uman, Etten, Udot, 

Eot, and Fanapanges (pl. 1). The few landslides on the islands 

of Siis and Tol were not mapped. With two exceptions, on 

southern Fefan Island and on Fanapanges Island, incipient 

landslide cracks also were not mapped. Landslide-source areas 

and their deposits were located and plotted using surveys 

from small aircraft, boat, foot, and automobile. Hand-held 

photographs taken during a 2.5 hour overflight of the volcanic 

islands on August 6, 2002, were particularly useful; these 

photographs were used to map landslide boundaries onto 1:

25,000-scale topographic maps of the islands. Although 

these oblique aerial photographs were useful in locating the 

landslides, the lack of complete stereographic-photographic 

coverage limits the location accuracy to about 10-100 m. In 

addition, some of the smaller landslides were undoubtedly 

missed because of the lack of vertical view of the entire land 

surface. Plate 1 illustrates the extensive landsliding produced 

by Chata’an; 269 landslides are shown.

We compiled statistics on the landslides shown in plate 

1 using Geographical Information System (GIS) analysis. 

Table 1 shows the number of mapped landslides on each 

island, the area affected by landslides, and the amount of the 

coastal lowlands affected by landslides. We defined the coastal 

lowlands as encompassing relatively flat land lower than 10 m 

in elevation; these coastal flats are where most residents live. 

The greatest number of mapped landslides occurred on Fefan 

(96), followed by Weno (64) and Tonoas (57). However, 

Tonoas had the greatest area affected by landslides (almost 8 

percent of the island) and the majority of fatalities (24 of 43 

total) (see “Significant Landslides” section below). 

Debris-Flow Runout

The distance debris flows travelled across gently sloping 

ground and the coastal flats was extremely important in 

causing damage to people and property during Chata’an. 

Most of the population of the volcanic islands reside on the 

flat coastal shorelines of the islands. The degree of damage 

wrought by landslides was linked directly to the runout 

length of debris flows that invaded these areas of highest 

population density. Runout distances depended on the volume 

of the source landslide and the slope steepness below the 

source areas; large initial volumes and steep slopes resulted 

in longer runouts. Using the landslide-inventory map (pl. 1), 

we measured and compiled runout distances for all debris 

flows that had significant runout. Runout was defined as the 

distance a debris flow traveled after it reached a distinctive 

break in slope below its source, identifiable on the topographic 

base of plate 1. For longer flows, part of this runout is across 

the coastal flats. Figure 5 shows a bar graph of the runout 

distances of 72 debris flows that had a flow path that could be 

measured. The range of runout distances varied from a low 

of 10 m to a maximum of about 700 m. The average runout 

distance was 89 m.

Figure 4. A. Oblique aerial photograph of Mwan slump/debris flow showing the location of the landslide source, debris-flow 

track, and debris-flow deposit. B. Landslide source (slump) for the Mwan debris flow. Yellow circle locates person for scale.
7

SIGNIFICANT LANDSLIDES

Although at least 269 landslides were triggered by 

Chata’an, only a few of these landslides resulted in death and 

extensive property damage. Table 2 lists significant landslides 

caused by Chata’an that resulted in human fatalities, were 

particularly large, or destroyed public facilities such as water 

reservoirs, or medical dispensaries. Many of these slides 

also crushed homes and community centers, damaged local 

water supplies (commonly springs), downed power lines, 

buried agricultural crops, and blocked roads or trails. All of 

these sites were visited on the ground by USGS scientists, 

except for the Fanai landslide on the island of Siis and the 

Mwalitiw landslide on Udot, which are not identified on 

plate 1. The Fanai landslide on the island of Siis was not 

mapped because we did not visit this island, and the Mwalitiw 

landslide on the island of Udot was not identified because 

of imprecise information from local sources. Landslide 

volumes were estimated in the field. Fatality numbers were 

obtained from the Chuuk State Office of Vital Statistics as 

of August 6, 2002, and were later confirmed in consultation 

with Dr. Carlos Sanchez of the U.S. Centers for Disease 

Control and Prevention. Both large landslides (such as the 

Kuchua landslide on the island of Tonoas, no. 4, pl. 1) and 

rather small landslides (such as the Meseiku landslide on the 

island of Fefan, # 9, Plate 1) caused fatalities. However, some 

extremely large landslides (such as the Mochon landslide 

on Uman, no. 11, pl. 1) did not cause any deaths. Fatalities 

occurred where people were sheltered or traveling directly in 

the path of oncoming large or small debris flows. Estimates 

of homes and structures damaged or destroyed were obtained 

from local residents. Other landslides, not described below, 

also damaged or destroyed homes, water supplies, power lines, 

crops, and roads. The following sections describe some of the 

significant landslides and also discuss potential short-term 

hazards posed at these landslide sites. Location numbers refer 

to landslides listed in table 2 and shown on plate 1.

Weno (Moen) Island

Nepukos Slump/Debris Flow—Map Location 1

Above Nepukos village on the western side of Weno Island, 

Chata’an triggered a slump that transformed into a debris flow; 

this flow overran five houses, a road, and powerline at the base 

of the slope, caused three fatalities, and continued about 70 m 

out onto coastal flats and into mangrove swamps. The initial 

slump of about 20,000 m3 occurred on a knoll with a slope of 

about 35° at the end of a ridge (fig. 6). The headscarp of the 

slump was about 10 m high and 30 m wide at its base. The 

slump occurred in weathered, clay-rich volcanic rocks. The 

landslide scar and remaining debris are now well drained and 

eroded so that water emanating from the headscarp flows in a 

well-developed channel. Future rainfall could generate some 

smaller failures from the headscarp; however, such failures are 

not likely to travel beyond the immediate headscarp area.

Mwan Slump/Debris Flow—Map Location 2

This slump/debris flow occurred inland from the village 

of Mwan on the island of Weno (fig. 4) and resulted in one 

fatality. As with the Nepukos landslide, the Mwan landslide 

began as a slump in weathered volcanic rocks on a 35° slope 

and transformed into a rapidly moving debris flow. The flow 

traveled about 100 m downslope along a stream channel and 

buried a road and water well at the base of the slope. The 

debris flow also may have incorporated additional material 

from the stream channel. It then flowed onto flatter ground, 

destroyed a house, and stopped about 60 m downslope of the 

road. The initial slump, about 50 m high, had an approximate 

volume of 10,000 m3. Clay-, sand-, and boulder-rich landslide 

deposits remain along the sides of the stream channel and in 

the depositional area downslope of the buried road. Drainage 

channels have been eroded into remnants of the landslide 

debris in the slide scar. Two small incipient slumps remain 

adjacent to each side of the initial headscarp; they have 

volumes of about 1,000-2,000 m3 each. Both incipient slumps 

have undergone about 3 to 4 m of displacement. Future heavy 

rainfall may reactivate these incipient slumps; however, it is 

doubtful that they will cause additional damage in the short 

term. Any newly mobilized materials likely will be small and 

will be impeded by the rough, irregular slide surface.

Figure 5. Bar graph showing horizontal runout distances of 72 

individual debris flows triggered by tropical storm Chata’an. 

Distances were determined from the topographic base map 

used for Plate 1.
8 

Tonoas (Dublon) Island

Nechap Area Slump/Debris Flow—Map Location 3

One of the largest landslide disasters transpired in the 

village of Nechap on the island of Tonoas. Here, residents 

reported that debris flows caused 14 fatalities and destroyed 

31 dwellings, a water-supply system, and powerlines (fig. 7). 

Concrete water tanks were transported more than 100 m out 

to the ocean’s edge by the flows. The damaging landsliding 

began as two slumps in weathered volcanic rocks, with a 

combined volume of more than 100,000 m3. These slumps 

mobilized into rapid debris flows; deposits from these flows 

cover a road and extend across the flat coastal lowland for 

about 175 m and into the ocean (figs. 8 and 9). The lower 

slump of approximately 75,000 m3 has a slope width of about 

90 m, a scarp height of 120 m, and an average thickness of 

7 m. The upper slump mass has a width of about 70 m, a 

scarp height of 50 m, and a thickness of about 7 m. The upper 

slump mass initially impounded water draining from the 

landslide scarp and formed a rapidly moving slurry of soil and 

weathered rock (debris flow) as the impounded water breached 

the slump mass. The debris flow from the upper slump 

evidently occurred after the debris flow from the lower slump 

mass, because it forms a lobate deposit that overlies the debris 

deposit from the lower slump.

Both of the slumps failed completely, meaning that most 

or all, of the slide material within the source areas underwent 

many meters of displacement. Much of the slump material 

was remobilized into a debris flow, and the scarp area now is 

well drained. The likelihood of additional large failures from 

these scarps is low. Remaining debris-flow material, which 

extends to the ocean, likely will be eroded by future rainfall 

and surface runoff. The continuing hazard to people living 

near these deposits is low. Nearby, two smaller slumps (one 

about 200 m to the east and another about 100 m to the west) 

are each about one-fifth of the combined volume of the larger 

flows. These slumps also failed completely, but they only 

partially mobilized into debris flows. The continuing hazard 

to people living on the coastal lowlands near the toes of these 

landslides also is relatively low.

Kuchua Slump/Debris Flow—Map Location 4

This landslide is one of the largest triggered by tropical 

storm Chata’an (fig. 10). The landslide and resulting debris 

flows resulted in five fatalities and six homes destroyed. The 

headscarp of the source slide is about 150 m wide, 150 m long 

in the downslope direction, and 10 m in average thickness, 

resulting in a total volume of approximately 200,000 m3. The 

slope of the headscarp area, prior to failure, was approximately 

30°-40°. The failure surface is arcuate in cross section, and 

initial movement of this mass apparently resulted in the 

temporary impoundment of a large volume of water draining 

from the scarp and possibly surface gullies. In addition to 

impounded water infiltrating the slump mass, catastrophic 

failure likely resulted from overtopping of the slump mass 

by the impounded water, and/or from piping and subsequent 

rapid failure. The catastrophic failure mobilized much of the 

slump material, and the resulting flows cut a deep erosion 

channel through the slump. This channel exits at the east side 

of the source area and indicates where mixed water and debris 

flowed out of the source area. According to Chester Gustav, 

who lives near the distal end of the debris flow, the first wave 

of debris and water arrived at 10:00 a.m. on July 2, followed 

by two more waves of debris. The entire debris-flow activity 

took place over 26 minutes. The distal end of the debris flow 

Figure 6. Oblique aerial 

photograph of Nepukos slump/

debris flow on the island of Weno.
9

Figure 7. Oblique aerial view of 

Nechap slump/debris flow on the 

island of Tonoas.

Figure 8. Deposits from Nechap 

debris flow showing destroyed 

homes and displaced concrete 

water tanks (upper left in photo 

near shoreline).
10 

Figure 9. Distal part of Nechap 

debris flow, near the ocean, 

showing debris from destroyed 

houses and closeup of water tank.

Figure 10. Oblique aerial view of 

Kuchua slump/debris flow (right) 

and Roro debris slide/flow (left) on 

the island of Tonoas.
11

extended about 500 m into the taro field beyond the break in 

slope at the base of the flow path (figs. 10 and 11). The overall 

length of the flow is greater than 700 m. The headscarp area 

has relatively low continuing landslide hazard. Farther down 

the flow path, considerable debris remains along the west side 

of the main channel of flow. These slopes are relatively gentle 

(~ 15°), however, and are unlikely to remobilize into massive 

debris flows.

Roro Debris Slide/Flow—Map Location 5

Another large landslide occurred approximately 500 m to 

the southwest of the Kuchua landslide. Here, the headwaters 

of a stream catchment basin were denuded of vegetation and 

surficial material by a complex of several thin (1-2 m thick), 

planar debris slides totaling about 100,000 m3 (fig. 10). The 

upper part of this large debris-slide complex transformed into 

a flow or multiple flows and traveled down two main channels. 

These two channels coalesced downstream, shortly upstream 

of a 3-m-high water-catchment structure. Debris flows overran 

and filled this structure, burying water-supply pipes (fig. 

12); flows then traveled downstream into a small community 

of houses. The largest (western) lobe of the source slide 

shows evidence of having briefly dammed a small stream; 

catastrophic breaching of this debris dam likely resulted in one 

of the coalescing debris flows.

Fefan Island

Pwene Slump/Debris Flows—Map Location 8

Near the school in the village of Pwene (Wininis), two 

children were killed by a debris flow on the afternoon of July 

2. Here, two slumps failed from a 30° slope above the school 

and coalesced to produce a 28,000 m3 debris flow (figs. 13 and 

14). A 50-m-wide tree covered ridge that did not fail during 

the storm separates the two slump sources. The debris flow 

spread out across a nearly flat field behind the school. The 

slumps underwent extensive mobilization into debris flows, 

and little material remains in the headscarp area. 

Sapore Slump/Debris Flow and Incipient 

Landslide—Map Location 10

On the southern end of the island of Fefan, a slump/debris 

flow buried a road, destroyed nine homes, and killed four 

people during Chata’an (figs. 15 and 16). The slump originated 

on slopes of 35° and moved about 10,000 m3 of weathered 

volcanic rock before transforming into a debris flow. Most of 

the slump material has undergone 5-10 m of displacement, and 

any additional movement likely will be small and confined 

to the source area. The headscarp of a large incipient slide 

extends from this slump scar northward along the hillslope 

for at least 500 m. This scarp has recent vertical displacement 

of 2-3 m (Figure 17), although older roots near the slump 

scar suggest that at least part of the scarp was preexisting. A 

15 m-long toe is present below this scarp about 300 m from 

the slump where the slump mass has overthrust the ground 

surface by about 0.5 m. Several houses are located near and 

downslope of the toe. Future behavior of this large incipient 

landslide is uncertain. It may undergo renewed movement 

with future rainfall. However, the depth of this slump is 

estimated to be 15-25 m, and such a deep-seated landslide of 

this volume would be unlikely to move in response to light or 

moderate rainfall. Reactivation of this landslide may require 

abnormally high rainfall intensities and longer durations such 

as that provided by tropical storm Chata’an.

Figure 11. Downslope view of 

Kuchua slump/debris-flow path. 

Deposits from this flow covered 

taro fields at the distal end of the 

deposit.
12 

Figure 12. Water catchment basin 

inundated by debris from the Roro 

debris slide/flow. Part of the basin 

wall (in the small pond) has been 

exposed by digging after the debris 

flow event.

Figure 13. Aerial view of Pwene 

slump/debris flow on the island of 

Fefan. This site shows a debris-flow 

doublet (center of photo) with a 

downslope line of trees located on a 

ridge between the two debris flows 

that coalesced.
13

Figure 14. Head scarp of the 

northern Pwene slump/debris 

flow.

Figure 15. Oblique aerial view of Sapore slump/debris flow (left center) on the island of Fefan. Dashed line 

indicates approximate position of reactivated head scarp of preexisting large incipient slump. Preexisting 

slump mass is below and to the right of dashed line.
14 

Figure 16. Downslope view of 

Sapore debris flow showing area 

where nine houses were buried or 

destroyed by the deposit.

Figure 17. Scarp of reactivated large incipient slump (see 

fig. 15) near Sapore produced by rainfall from tropical storm 

Chata’an.
15

Uman Island

Sanuk Slump/Debris Flow—Map Location 12

On the northeastern coast of the island of Uman in the 

village of Sanuk, another slump that transformed into a debris 

flow traveled across relatively narrow coastal lowlands. The 

debris flow buried 13 homes, a community center, a road, and 

caused one fatality. The 50-m high, 6,000 m3- slump initiated 

on slopes of about 30° in highly weathered volcanic rocks (fig. 

18). After transforming into a debris flow, it traveled about 

140 m downslope to the coastal lowlands and incorporated 

about 4,000 m3 of colluvium and weathered bedrock along its 

travel path. The resulting total volume of the flow deposit is 

about 10,000 m3. Some of the slump debris still remains in the 

slide source area; however, this material is unlikely to travel 

beyond the original source area due to small source volumes 

and roughness of the existing flow path.

Udot Island

Penia Slump/Debris Flow—Map Location 14

A slump located on the northwestern side of the island 

of Udot mobilized into a debris flow, traveled about 220 m 

downslope to the coastal flats, and caused four fatalities. 

The initial slump source had an 80 m-high scar and an 

estimated volume of 10-15,000 m3 (fig. 19). As with most of 

the other larger landslides triggered by Chata’an, the source 

area is located in deeply weathered, basaltic-volcanic rocks. 

According to local residents, this event occurred at about 

7:00 or 8:00 a.m. on July 2. Small remnants of the original 

mass may mobilize in future intense rains but are unlikely to 

travel beyond the original source area.

Fanapanges Island

Faia Slumps and Incipient Slumps—

Map Location 16

On the eastern coast of the island of Fanapanges, a 

continuous series of slumps and incipient slumps were 

triggered during Chata’an, again within highly weathered, 

basaltic-volcanic rocks. The entire complex affects at least 

250 m of trail along the coastline. The northernmost of these 

slumps, 50 m in height, with a volume of about 8,000 m3, 

transformed into a debris flow and traveled about 100 m to 

the shoreline (fig. 20). This flow destroyed 15 homes, a water 

basin, and a dispensary in the village of Faia. According to 

local residents, this event occurred about 10:00 a.m. on July 2. 

Incipient cracking, with a vertical displacement of about 0.5-

1.5 m, extends southward from the headscarp of this slump 

(A, fig. 21). Adjacent to this slump and below the incipient 

cracking is a relatively intact, wedge-shaped incipient slide, 

60 m wide along the trail (B, fig. 21). South of this, shallow 

blocky failures from the toe of a larger slump extend for 

about 90 m along the trail and reach the coastline (C, fig. 21). 

According to residents, this slump occurred a few minutes 

after the destructive northern debris flow. To the south of this 

feature, incipient cracks curve and step shoreward, crossing 

the trail about 70 m south of recent slumps (D, fig. 21). During 

very wet periods, this entire area (about 250 m long) may be 

subject to renewed slumping and possibly debris-flow activity. 

This extensive area remains potentially hazardous and would 

be a relatively poor site for relocating a dispensary.

Figure 18. Lower portion of a 

slump/debris flow at the village 

of Sanuk on the island of Uman 

that destroyed 13 homes and a 

community center.
16 

Figure 19. Aerial oblique view of 

slump/debris flow at the village of 

Penia on the island of Udot.

Figure 20. Faia slump/debris flow 

and incipient slumps that destroyed 

15 homes and a dispensary on the 

island of Fanapanges.
17

ASSESSMENT OF REMAINING AND 

FUTURE LANDSLIDE HAZARDS

Continuing Landslide Hazards

Most of the landslides triggered by tropical storm Chata’an 

have undergone extensive failure and downslope movement, 

and the hazard at these landslide sites has decreased to fairly 

low levels. For those landslides where most of the landslide 

mass transformed into debris flows, the hazard to areas below 

the toe of the landslide now is relatively low because groundwater levels have decreased significantly from what they were 

when the failures were triggered, and because the movement 

of additional smaller failures from headscarp areas will be 

impeded by the presence of landslide material, which creates 

a rough, irregular surface within the flow path. Normal nontyphoon rainfall conditions are unlikely to generate additional 

landslides other than small failures from the oversteepened 

headscarps of the existing landslides. Small slumps and soil 

falls generated from these headscarps are unlikely to travel far 

from the base of the initial headscarp as their movement will 

be impeded by irregularities of the slide surface and previous 

landslide debris within the failure path. When we returned 

to the islands of Chuuk in September 2003, about 1 year 

after Chata’an, little additional movement had occurred at 

the landslide sites we visited. Most deposits were becoming 

heavily revegetated, and minor surface erosion had occurred 

on the landslide scars.

Future Landslide Hazards

 The islands of Chuuk State eventually will be deluged 

by future severe tropical storms or typhoons that will, in 

turn, trigger new landslides. During times of fair weather 

and normal rainfall patterns, the slopes of the Chuuk islands 

probably will remain stable, because they are adapted to this 

level of rainfall. As discussed above, most of the landslide 

material from the Chata’an landslides already has been 

transported downslope by debris flows. New landslides 

from a future storm are likely to initiate from other hillslope 

areas; these include areas adjacent to the landslide scars from 

Chata’an, incipient landslides, or parts of hillslopes with 

geologic and hydrologic conditions similar to those where 

landslides have occurred in the past. We performed a slopestability analysis of the Chuuk islands affected by Chata’an to 

identify hillslope areas susceptible to landslides from a future 

large storm. We then developed a hazard map showing the 

likelihood of future landsliding on the slopes of these islands.

 Slope-Stability Analysis

We used a geotechnical analysis method to evaluate 

the slope stability of each 10-m by 10-m area (grid cell) 

throughout the islands of Weno, Tonoas, Fefan, Etten, Uman, 

Udot, Eot, and Fanapanges affected by a future storm similar 

to Chata’an. Grid cells were defined by a 10-m digital 

elevation model (DEM) prepared by the USGS from 1:25,000-

scale topographic maps of the islands. The stability analysis 

compares the ratio of the resisting forces to the driving forces 

operating within each slope cell according to the equation:

A

B

C

D

Figure 21. Aerial view of the 

Faia landslide and incipient 

slumps. Letters denote 

locations described in text.
18 

where FS is the static factor of safety, c’ is the effective 

cohesion, φ’ is the effective friction angle, α is the slope 

angle, γ is the material unit weight, γw is the unit weight of 

water, t is the slope-normal thickness of the slope element, 

and m is the proportion of the element thickness that is 

saturated with ground water flowing parallel to the ground 

surface (Jibson and others, 2000; Harp and others, 2002). This 

method, commonly referred to as an infinite-slope analysis, 

approximates each slope cell as a rigid sliding block and 

assumes that the stability of each cell is independent of the 

cells surrounding it. High values of factor of safety reflect 

more stable slopes, whereas low values reflect relatively 

unstable slopes; values less than 1.0 indicate statically 

unstable slopes. Using equation 1, we calculated the FS for 

each slope element (grid cell); ranges of FS were then colored 

to produce a landslide-susceptibility map.

Slope-Stability Analysis Variables

To calculate FS in a GIS-based analysis, we needed values 

for the variables listed in equation 1 for each 10-m grid cell of 

the DEM. Insufficient information exists to determine all these 

variables for each cell, therefore some values were estimated. 

Ground- surface slope (α) was computed from the DEM grid 

cells using ArcGrid software (ESRI, 1998). An example of 

a slope map for the island of Tonoas is shown in figure 22. 

The thickness of the potential sliding block (t in equation 

1) at each cell was estimated at a constant 3.5 m, reflecting 

an average thickness of Chata’an-triggered landslides. For 

ground-water conditions during intense rainfall similar to 

tropical storm Chata’an, we assumed a fully saturated soil 

column with slope-parallel ground-water flow within all cells 

(m = 1 in equation 1). Ground-water conditions undoubtedly 

varied from place to place and over time during Chata’an; 

nevertheless this assumption provides a relative “worst case” 

condition for computing slope stability.

Where no geotechnical data on strength properties and 

unit weights of geologic units are available, as in the case 

of Chuuk State, reasonable estimates can be made based on 

the type of materials present and the use of typical values 

associated with those types of materials (Harp and others, 

2002; Jibson and others, 2000). Most steep slopes on the 

islands are mapped as “undivided volcanic rocks” described 

as “chiefly lava flows, basaltic and andesitic in composition” 

with lesser units of volcanic breccia (Stark and Hay, 1963). 

All of the large landslides that resulted in significant damage 

or loss of life were located within these materials. From 

our site observations, we noted the similarity between the 

landslide materials and those exposed in the landslide scarps. 

Because of this similarity throughout the islands that we 

visited and the landslide sites observed, we assigned a single 

value of unit weight and shear strength to all of the slope 

Tonoas

Etten

Slope, degrees

0 - 10

10 - 20

20 - 30

30 - 40

0 1 KILOMETER >40

Figure 22. Slope map of the 

island of Tonoas prepared from 

a 10-m digital-elevation model 

(DEM).
19

materials of the islands. We assigned the unit weight of soils 

(γ in equation 1) to be a constant 15.7 kN/m3 (100 lbs/ft3). 

The shear strength of water-saturated soil or weathered 

rock consists of two components—effective cohesion (c’) 

and effective angle of friction (φ’). We calibrated spatially 

uniform values of these variables to capture the maximum 

amount of Chata’an landsliding in grid cells with the lowest 

computed stability while still using reasonable estimates of c’

and φ’ (see discussion in following section). This calibration 

process resulted in an estimated uniform c’ of 38 kilopascals 

(kPa) (800 lbs/ft2) and φ’ of 24°. These values are reasonable 

for weak, weathered volcanic materials, and again provide 

a relative “worst case” condition. Thus, slope is the primary 

model variable that changed between 10-m grid cells.

Results: Factor-of-Safety (Susceptibility) Map

Using equation 1, we computed the static factor of safety, 

FS, for each 10-m grid cell of the affected Chuuk islands. 

The resulting slope-stability (factor-of-safety) map was then 

used to construct the debris-flow hazard map described in 

the next section. An example of the factor-of-safety map for 

the island of Tonoas is shown in figure 23. Factor-of-safety 

categories are portrayed on the map as different colors, 

ranging from magenta and red for the lowest values or least 

stable slopes to green for the highest values or most stable 

slopes. These colors indicate the general susceptibility of 

a given slope to landsliding in a storm similar to Chata’an. 

Overall, most of the debris flows initiated within slopes 

shown as magenta, red, or orange colors in figure 23, 

indicating a generally good agreement between the actual 

locations of landslide-source areas and those slopes predicted 

to be highly susceptible. There are, however, debris flows 

that initiated on slopes containing a significant number of 

cells of intermediate susceptibility. Notably, some of the 

larger debris flows on Tonoas, including those at Nechap, 

Kuchua, and Roro, initiated on slopes such as these. These 

debris flows began as complex deep-seated failures (greater 

than 10-m thick and much larger than a 10-m grid cell) 

with complex failure mechanisms (see discussions above). 

They are poorly represented by the slope-stability analysis 

described by equation 1. In general, the smaller, thinner, and 

more numerous debris flows more closely approximate the 

assumptions of our stability analysis, for example, a simple 

sliding block of constant thickness. For these more common 

debris flows, our analysis performed well.

Theoretically, an FS value of less than 1.0 means slope 

failure. To calibrate the hillslope strength in our slope stability 

analysis, we adjusted values of c’ and φ’ so that many of 

the grid cells containing debris-flow sources produced by 

Chata’an had a FS near 1. Because the mapped landslides 

shown in plate 1 contain both the source landslide and the 

debris flow track and deposit, we defined the debris-flow 

(landslide) source as the upslope quarter of each mapped 

landslide area. The value FS=1.0 portrayed on the map should 

not be considered as a precise threshold for slope failure 

for each specific site location because the data used in this 

Tonoas

Etten

Landslides triggered by

Tropical Storm Chata'an

Factor

of Safety

- 4.0

- 

- 3.0

- 

- 2.0

- 

- 1.0

0 1 KILOMETER

Figure 23. Factor-of-safety 

(susceptibility) map of the 

island of Tonoas using the 

slope-stability analysis 

described in text.
20 

relatively simple slope-stability analysis were generalized on a 

regional basis. Nevertheless, the clustering of mapped debrisflow sources within the lowest FS categories indicates that this 

model is useful for establishing the relative susceptibility of 

various hillslopes. Comparing the computed FS for each grid 

cell with the source locations of actual debris flows caused by 

Chata’an provides a tool to stratify or rank debris-flow source 

susceptibility.

The effectiveness of these factor-of-safety categories as 

predictors of landslide occurrence can be analyzed using the 

ratio of the number of 10-m landslide cells in each FS category 

to the total number of 10-m cells in each of the respective 

categories. This ratio for all the Chuuk islands analyzed is 

shown in figure 24 as a percentage. This percentage also 

expresses the spatial probability of a mapped landslide 

occurring in that FS category. The lowest FS categories have 

the highest spatial probabilities and are the best predictors 

of a landslide occurring within the respective categories. For 

example, if a parcel of land has an FS value of between 0.5 

and 1.0, figure 24 shows that there was about a 4.1-percent 

chance that a debris flow would have been triggered there by 

precipitation from tropical storm Chata’an. Similarly, areas 

having FS values of between 1.0 and 1.5 had a 2.3 percent 

probability of sliding, and the pattern of decreasing percent 

probability continues with increasing FS (fig. 24). With debrisflow source probability plotted versus FS in figure 24, we were 

able to establish debris-flow hazard categories in terms of FS.

Results: Debris-flow Hazard Map

Using the results from our factor-of-safety analysis and our 

observations of debris-flow runout, we created a debris-flow 

hazard map for the Chuuk islands affected by Chata’an (pl. 

2). This map portrays the hazard from two aspects of debris 

flows—their source landslides (usually on steeper slopes) and 

their runout paths (often on flat coastal lowlands). To portray 

the debris-flow source hazard, we divided the FS categories, 

shown in figure 24, from our slope-stability analysis into 

three groups. Areas with FS=0.0-2.5 contain landslide sources 

that make up 2 percent of the cells within this FS group and 

are termed “high”, FS=2.5-4.0 contain sources that make up 

1 percent of the cells within this FS group and are termed 

“moderate”, and FS> 4.0 contain landslide sources that 

make up ~ 0.1 percent of the cells within this FS group and 

are termed “low”. These groups are assigned the colors red, 

Chuuk Probability Phi-24 C-800 T-11.5 Amended

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0-6.5 6.5-7.0 7.0-7.5 >7.5

FACTOR OF SAFETY 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0-6.5 6.5-7.0 7.0-7.5 >7.5

FACTOR OF SAFETY

PROBABILITY IN PERCENT

Figure 24. Histogram of debris-flow source probability versus factor-of-safety based on slope stability analysis of the islands 

of Weno, Tonoas, Fefan, Etten, Uman, Udot, Eot, and Fanapanges. Larger probabilities indicate an increased likelihood that a 

Chata’an-triggered landslide (debris-flow source) occurred on hillslopes with the corresponding factor-of-safety.
21

yellow, and green, respectively, on plate 2. Figure 24 shows 

that FS=0.5-1.0 stands out noticeably from the rest of the data 

(highest probability), but the areal extent of this category is 

small. For this reason, we combined this FS category with the 

three adjacent bins, FS=1.0-2.5, that have similar values, to 

form the “high” hazard category. The limits of the “moderate” 

category are somewhat more arbitrary (FS=2.5-4.0), although 

the probabilities in these two bins are significantly higher 

than in the rest of the bins (FS > 4.0). The bins with low 

probabilities form the “low” hazard category. In summary, 

areas of “high” hazard have a 2 percent probability, areas of 

“moderate” hazard have a 1 percent probability, and areas 

of “low” hazard have a 0.1 percent probability of producing 

slump/debris flows during rainfall conditions similar to those 

of tropical storm Chata’an in the eastern islands of Chuuk 

State.

The debris-flow hazard map shown in plate 2 illustrates 

that most of the debris flows triggered by Chata’an originated 

within “high” hazard areas. Only a few small landslides have 

source areas located in “moderate” or “low” hazard categories. 

This map, derived from the documented occurrence of debris 

flows from tropical storm Chata’an, predicts where “high,” 

“moderate,” and “low” debris-flow hazard is likely to occur 

from future extreme storms similar to Chata’an. For lesser 

rainfall conditions where total saturation of the soil may not 

occur, the susceptibility of the slopes to debris-flow initiation 

is lower. However, the relative susceptibility between hazard 

categories likely will be the same, that is, high hazard areas 

likely will contain the most landslides (debris-flow sources), 

although a storm smaller than Chata’an may produce fewer 

and smaller slides.

In addition to debris-flow hazard posed by the source areas, 

plate 2 also shows the expected runout areas of typical debris 

flows. An important question to most of the population living 

on the volcanic islands of Chuuk State is how far debris flows 

are likely to travel from the steep slopes out onto the coastal 

flats where most people live. To define this area of possible 

debris-flow inundation, we used the runout distances measured 

from debris flows triggered by Chata’an. These runout 

distances (discussed above) cover a broad range (fig. 5) but 

average 89 m beyond a significant break in topographic slope. 

Runout distances across the coastal flats commonly were less 

than this amount. Plate 2 shows potential runout areas onto 

the coastal flats (tan color) 100 m from the edge of steeper 

landslide-susceptible slopes. Areas beyond this 100-m line 

(farther from steep slopes) have a high probability of being 

unaffected by most debris flows; they are still at some risk 

from the relatively few, extremely large, long-runout debris 

flows. We chose 100 m as a relatively conservative (safer) 

limit; nevertheless, on most islands it identifies some relatively 

safe ground beyond the limit.

Our debris-flow hazard map was constructed using limited 

information and simplistic analytical methods; therefore, it has 

practical limitations. The assumptions inherent in our slope 

stability analysis are a significant simplification of natural 

hillslope conditions, where geologic and hydrologic conditions 

generally are quite variable and difficult to predict. Moreover, 

our analyses of stability and runout distances are based 

on observations from only one storm event, Chata’an. As 

discussed above, the locations of individual landslides from 

this event, shown on plate 1, are approximate with varying 

accuracy. In future storms, not all of the high-hazard areas 

designated on plate 2 will be affected by landslides (Chata’an 

only affected a small percentage of the mapped high-hazard 

areas), and some slides are likely to occur outside these areas. 

Nevertheless, based on the Chata’an experience, high and 

moderate hazard areas identified on the map are more likely 

to be affected by future landslides and debris flows than the 

low hazard areas. Thus, the map can be used for planning 

purposes.

Practical Use of the Debris-Flow Hazard Map

Another tropical storm or typhoon, similar to Chata’an, 

likely will trigger abundant new debris flows on the volcanic 

islands. The debris-flow hazard map shown on plate 2 can be 

used to help reduce the impact from future storm-triggered 

debris flows. Because little public land exists in the Chuuk 

Islands, relocation of homes and other structures away from 

high hazard areas may be impractical. Nevertheless, important 

new or replacement structures, such as schools, community 

centers, and medical dispensaries, could be located away from 

moderate or high hazard areas and outside the 100-m debrisflow runout zone to reduce future risk from debris flows. 

Many people were killed during Chata’an by gathering in 

buildings in the direct path of debris flows. The hazard map 

identifies relatively low-risk safe zones (beyond the 100-m 

debris-flow runout limit) where residents could stay in public 

or private shelters during a future tropical storm similar to 

Chata’an. In the event of typhoon conditions and predicted 

heavy rainfall (similar to the almost 20 inches of rain in less 

than 24 hours that was recorded at the Weno airport during 

Chata’an), residents could travel to lower-risk shelters located 

beyond the runout areas identified by the debris-flow hazard 

map. To be effective, such shelter areas would need to be 

identified and well publicized prior to future storms. If such 

shelters had existed during Chata’an, undoubtedly some, but 

probably not all, of the debris-flow fatalities could have been 

avoided.

SUMMARY AND CONCLUSIONS

The intense rainfall accompanying tropical storm Chata’an 

in July 2002 triggered more than 250 landslides on the islands 

of Weno, Tonoas, Fefan, Uman, Etten, Udot, Fanapanges, 

and Siis in Chuuk State, Federated States of Micronesia. We 

visited these islands and many of the landslides a few weeks 

after the storm. The landslides ranged in volume from a few 

cubic meters to more than 1 million cubic meters; the majority 

mobilized into debris flows. The most damaging landslides 

were larger slumps near the coastal flats that transformed 

into rapidly moving debris flows; these debris flows traveled 
22

downslope and across the coastal flats causing 43 deaths and 

extensive property damage. The greatest area affected by 

landslides and the greatest damage done by landslides was on 

Tonoas. We mapped the landslides triggered by Chata’an (both 

source area and deposits) on 1:25,000-scale topographic maps 

to produce a landslide inventory map (pl. 1).

The continuing hazard posed by landslides triggered by 

Chata’an is low. However, future tropical storms or typhoons 

that produce intense rain, similar to Chata’an, likely will 

trigger new landslides and debris flows. To evaluate the hazard 

posed by future landslides to the islands of Chuuk State, 

we used GIS technology to perform a simple slope-stability 

analysis of each 10-m cell (from a DEM) of the islands 

listed above. This analysis computed a factor-of-safety, FS, 

(a measure of slope stability) for each 10-m grid cell using 

ground-surface slope angle and shear strength of geologic 

materials, assuming an average landslide thickness based on 

field observations, fully saturated slope conditions, and slopeparallel ground-water flow. We calibrated the analysis by 

varying uniform slope strength until the majority of observed 

source landslides triggered by Chata’an were located in cells 

with low FS. We then computed the spatial probabilities of 

landslides occurring during Chata’an for each range of FS. 

Using this information, we constructed a debris-flow hazard 

map (pl. 2). This map portrays the expected relative landslide 

hazard (high, moderate, and low) on steep slopes during a 

future storm similar to Chata’an. It also portrays areas that 

may be inundated by large debris flows during such a storm, as 

well as areas that are relatively safe from future debris flows.

The inventory map of landslides and debris flows triggered 

by tropical storm Chata’an (pl. 1) serves as a record to 

compare with any future occurrence of landslides in the 

islands of Chuuk State. The debris-flow hazard map (pl. 2) 

can be used to select locations that are relatively safe from 

future landslides and debris flows. This map may aid the siting 

of new public facilities, such as schools, community centers, 

and medical dispensaries. It also may be used to help select 

and designate evacuation areas where residents can gather 

in relative safety when future tropical storms and typhoons 

trigger landslides in these islands.

ACKNOWLEDGMENTS

The authors express their appreciation to the Federal 

Emergency Management Agency (FEMA) for funding the 

reconnaissance and analysis presented here and to the U.S. 

Army Corps of Engineers (USACE) for providing logistical 

support and direction as a subtasking agency. Individual 

thanks go to Nick Russo, FEMA Federal Coordinating Officer 

on Chuuk, to Ken Suiso, USACE, for help with transportation 

logistics and the reconnaissance flight, to Ray Kong and 

Ed Flint for assistance in the fieldwork and its logistics, to 

Rusty Witwer and Tom Atkin of the U. S. Forest Service for 

assistance with follow up investigations in 2003, to Johnson 

Elimo of the U. S. National Weather Service for providing 

rainfall data, and to Dianne Brien of the USGS for help 

with digital maps. We especially thank Marcelino Elias and 

Innocente Penno for their guide services and assistance in 

contacting local residents and gathering information.

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