Tsunami Vulnerability Evaluation in the Mentawai Islands--Mikami, Takahito; Shibayama, Tomoya;...

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O R I G I N A L P A P E R

Tsunami vulnerability evaluation in the Mentawai

islands based on the field survey of the 2010 tsunami

Takahito Mikami • Tomoya Shibayama • Miguel Esteban •

Koichiro Ohira • Jun Sasaki • Takayuki Suzuki • Hendra Achiari •

Teguh Widodo

Received: 15 February 2013 / Accepted: 29 October 2013 / Published online: 9 November 2013 Springer Science+Business Media Dordrecht 2013

Abstract On October 25, 2010, a large earthquake occurred off the coast of the Men-

tawai islands in Indonesia, generating a tsunami that caused damage to the coastal area of

North Pagai, South Pagai, and Sipora islands. Field surveys were conducted soon after the

event by several international survey teams, including the authors’. These surveys clarified

the tsunami height distribution, the damage that took place, and residents’ awareness of

tsunamis in the affected islands. Heights of over 5 m were recorded on the coastal area of

the Indian Ocean side of North and South Pagai islands and the south part of Sipora island.In some villages, it was difficult to evacuate immediately after the earthquake because of

the lack of routes to higher ground or the presence of rivers. Residents in some villages had

T. Mikami (&) T. Shibayama

Department of Civil and Environmental Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku,

Tokyo 169-8555, Japan

e-mail: [email protected]

M. Esteban

Graduate Program in Sustainability Science-Global Leadership Initiative (GPSS-GLI), GraduateSchool of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, Japan

K. Ohira

Chubu Electric Power Company, 5-5 Mikura, Ibigawa-cho, Ibi-gun, Gifu 501-0704, Japan

J. Sasaki

Department of Socio-Cultural Environmental Studies, Graduate School of Frontier Sciences, The

University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, Japan

T. Suzuki

Faculty of Urban Innovation, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku,

Yokohama, Kanagawa 240-8501, Japan

H. Achiari

Bandung Institute of Technology, Bandung, Indonesia

T. Widodo

Graduate Program in Sekolah Tinggi Ilmu Administrasi BNM Pariaman, Sumatra, West Sumatera,

Indonesia

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Nat Hazards (2014) 71:851–870

DOI 10.1007/s11069-013-0936-z


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taken part in tsunami drills or education; however, not all villages shared awareness of

tsunami threats. In the present paper, based on the results of these field surveys, the

vulnerability of these islands with regards to future tsunami threats was analyzed. Three

important aspects of this tsunami disaster, namely the geographic disadvantage of the

islands, the resilience of buildings and other infrastructure, and people’s awareness of tsunamis, are discussed in detail, and corresponding tsunami mitigation strategies are

explained.

Keywords Tsunami vulnerability Field survey Remote islands 2010

Mentawai Islands Tsunami Sipora island

1 Introduction

At 21:42 local time on October 25 (14:42 UTC on October 25), 2010, a large earthquake of

magnitude 7.7 occurred off the coast of the Mentawai islands in Indonesia, generating a

tsunami that caused damage to the coastal area of North Pagai, South Pagai, and Sipora

islands. The tsunami propagated through the Indian Ocean and also reached La Réunion

island, located 5,000 km southwest of the Mentawai islands (Sahal and Morin 2012).

According to Indonesian National Disaster Management Agency [Badan Nasional Pen-

anggulangan Bencana (BNPB) in Indonesian], this earthquake and tsunami caused 509

casualties (with a further 21 people missing) and heavily damaged 550 houses in the

Mentawai islands, as of November 22, 2010.To measure tsunami trace heights and to gather information from residents of the

Mentawai islands, field surveys were conducted soon after the event by several interna-

tional survey teams (Tomita et al. 2011; Satake et al. 2012; Hill et al. 2012). These teams

mainly focused on North and South Pagai islands, and thus, the authors surveyed Sipora

island, so that all areas damaged due to this tsunami could be accurately recorded.

The southwest coast of Sumatra island and the remote islands off its coast are one of the

most active areas in the world regarding earthquakes and tsunamis. In recent years, four

tsunami disasters have occurred in this area: the 2004 Indian Ocean Tsunami (Jaffe et al.

2006), the 2005 Nias Island Tsunami (Borrero et al. 2011), the 2007 Bengkulu Tsunami

(Borrero et al. 2009), and the 2010 Mentawai Islands Tsunami (see Fig. 1). Among these

events, the 2004, 2005, and 2010 tsunamis caused damage to these remote islands. The

2004 and 2005 tsunamis affected Simeulue and Nias islands, and the 2010 tsunami affected

the Mentawai islands.

In these islands, compared to the large cities in Sumatra island (e.g., Padang or Banda

Aceh), a tsunami arrives quickly due to the short distance between the epicenter and the

islands, leaving little time for the residents to evacuate. Tsunami mitigation infrastructure,

such as a warning system or tsunami shelters, is generally not well developed in this

region, especially in the islands off the coast of Sumatra. In addition, it should be noted that

because some of these islands are good for surfing (Buckley (2002) reported that aneffective surfing season was about 30 weeks per year for the Mentawai islands), many

foreign people visit them throughout the year. It thus appears imperative that coastal risk

management in these islands should take into account these specific factors (remoteness,

short evacuation time, and a very specific tourist type) to formulate adequate tsunami

mitigation strategies.

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Based on these considerations, the present paper aims to offer some insights on how to

improve preparedness against future tsunami threats in these remote islands. First, the

results of the field surveys conducted after the 2010 Mentawai Islands Tsunami are

summarized based on the literature produced by other teams and the results of the authors’

own surveys. Then, some particular vulnerabilities found in the surveys are highlighted,

and possible tsunami mitigation strategies are discussed.

2 Tsunami field survey

2.1 Method

The authors’ own field survey was conducted on the 19 and 20th of November 2010,

around one month after the event. By this time, the other survey teams had already

surveyed North and South Pagai islands, so in order to obtain comprehensive view of the

entire event, the authors visited the south coast of Sipora island, including the four main

villages affected by the tsunami: Bosua, Old-Gobik, Masokut, and Bere-Berilou.The aim of the survey was to record the distribution of tsunami trace heights as well as

to understand the situation of the damaged area. At each surveyed tsunami trace, the

precise location of the point was first recorded using a GPS instrument. Then, the height

was measured using a laser ranging instrument (IMPULSE, Laser Technology Inc.), a

prism, and staffs. The tsunami traces were identified by broken branches, debris on trees,

watermarks left on structures, and eyewitness accounts.

Fig. 1 Map around Sumatra island and locations of recent earthquakes generating a tsunami (The locations

and magnitudes of earthquakes were based on information from the United States Geological Survey)

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Table 1 shows the results of measurement surveys. The tsunami trace heights on Sipora

island were converted to the heights above the estimated tide level according to WXtide32

(available at http://www.wxtide32.com/ ) at the tsunami arrival time in Siberut island,

located to the north of Sipora island.

To understand residents’ behavior during the event and their awareness of tsunamithreats, a structured questionnaire was carried out in some of the villages. The question-

naire was translated into Indonesian and conducted by some of the authors (native speakers

of Indonesian).

2.2 Tsunami height distribution

The tsunami trace heights measured by the four survey teams (Tomita et al. 2011, Satake

et al. 2012, Hill et al. 2012, and the authors) were compiled in Fig. 2a. The datasets of

Tomita et al. (2011), Satake et al. (2012), the authors, and Hill et al. (2012) covered South

Pagai island, North and South Pagai islands, Sipora island, and all the three islands

(especially the small islands near the coast), respectively. These datasets covered a wide

coastal area of the Indian Ocean side of the Mentawai islands. To the authors’ knowledge,

this series is one of the densest datasets obtained in this chain of remote islands off the

southwest coast of Sumatra island.

Inundation heights of over 5 m were measured from the south part of Sipora island to

the south part of South Pagai island. Run-up heights were measured in Tuapejat, the

northernmost village of Sipora island, and Sanding island located 20 km away from the

southern tip of South Pagai island; however, the heights were only around 2 m, and the

tsunami was not destructive in these areas (Hill et al. 2012). Therefore, the area sufferingserious damage was only the coastal area of the Indian Ocean side of North and South

Pagai islands and the south part of Sipora island. The largest tsunami height (16.9 m) was

recorded in Sibigau island, located off the west coast of South Pagai island, which resulted

in extensive damage to the palm tree forest (Hill et al. 2012). Such an extreme run-up was

recorded only in one area, indicating that a large co-seismic deformation probably occurred

off the coast of South Pagai island.

2.3 Damage to each village

The location of the villages surveyed by the authors and the tsunami trace heights in eachvillage are shown in Fig. 2b. Inundation heights exceeded 3 m in all villages surveyed and

were over 6 m in some locations. The inundation heights measured in Bere-Berilou (the

maximum inundation height was 3.18 m) were relatively smaller than those measured in

the other villages. According to the dataset of Hill et al. (2012), inundation heights of 9.05

and 5.55 m were measured in Siruamata island, 3 km offshore from Bere-Berilou. This

small offshore island could have thus help to decrease the tsunami height in the shade of

the island.

The situation map issued by the United Nations Office for the Coordination of

Humanitarian Affairs (OCHA 2010) shows the damage (Table 2) and the satellite images(Fig. 3) of villages in Sipora island. Table 2 indicates that Gobik suffered much more

serious damage when compared to other villages. This was part of the reason why its

inhabitants had relocated to another place after the tsunami took place (as will be explained

later). Figure 3 shows that all villages had around a 100-m-wide strip of coastal vegetations

between the residential area and the shoreline. Why residents do not choose to build their

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http://www.wxtide32.com/http://www.wxtide32.com/


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T a b l e 1

T s u n a m i t r a

c e h e i g h t s m e a s u r e d i n S i p o r a i s l a n d

N o .

L o c a t i o n

L a t i t u d e

L o n g i t u d e

H e i g h t ( m )

I n u n d a t i o n d e p t h ( m )

D

i s t a n c e f r o m t h e c o a s t ( m ) a

W a t e r m a r k b

1

B o s u a

S 2 2 1 . 9 9 9 0

E 9

9 4 8 . 1 3 7 0

4 . 8 1

4 . 8 0

5

2

D B

2

B o s u a

S 2 2 1 . 9 9 0 0

E 9

9 4 8 . 1 5 7 0

3 . 3 4

3 . 1 0

9

3

D B

3

B o s u a

S 2 2 1 . 9 8 5 0

E 9

9 4 8 . 1 5 9 0

4 . 6 9

4 . 4 0

–

D B

4

B o s u a

S 2 2 1 . 9 5 0 0

E 9

9 4 8 . 2 0 5 0

2 . 9 5

1 . 9 7

1

9 7

B B

5

B o s u a

S 2 2 1 . 9 4 2 0

E 9

9 4 8 . 2 3 0 0

2 . 8 6

0 . 7 4

2

2 9

E W

6

B o s u a

S 2 2 1 . 9 3 0 0

E 9

9 4 8 . 2 3 3 0

2 . 7 0

0 . 6 4

2

5 7

E W

7

B o s u a

S 2 2 1 . 9 2 6 0

E 9

9 4 8 . 2 4 3 0

2 . 5 5

0 . 0 0

2

8 3

E W

( r u n - u p )

8

O l d - G o b i k

S 2 2 2 . 1 2 3 0

E 9

9 4 9 . 2 7 2 0

5 . 6 9

5 . 2 3

3

3

B B

9

O l d - G o b i k

S 2 2 2 . 0 7 8 0

E 9

9 4 9 . 2 7 0 0

3 . 5 7

3 . 4 0

6

1

D B

1 0

O l d - G o b i k

S 2 2 2 . 0 7 1 0

E 9

9 4 9 . 2 8 0 0

3 . 3 6

2 . 8 0

8

1

B B

1 1

O l d - G o b i k

S 2 2 2 . 0 5 3 0

E 9

9 4 9 . 3 0 1 0

2 . 4 5

1 . 7 0

1

6 0

B B

1 2

M a s o k u t

S 2 2 0 . 8 6 1 0

E 9

9 4 7 . 3 4 8 0

3 . 4 3

0 . 9 7

–

E W

1 3

M a s o k u t

S 2 2 0 . 8 4 5 0

E 9

9 4 7 . 3 4 2 0

5 . 0 1

2 . 5 7

–

E W

1 4

M a s o k u t

S 2 2 0 . 8 1 6 0

E 9

9 4 7 . 3 5 0 0

2 . 4 3

0 . 4 3

–

E W

1 5

M a s o k u t

S 2 2 0 . 7 7 0 0

E 9

9 4 7 . 2 8 3 0

6 . 9 6

5 . 8 5

–

D B

1 6

M a s o k u t

S 2 2 0 . 7 8 0 0

E 9

9 4 7 . 2 9 2 0

5 . 6 7

4 . 6 2

–

B B

1 7

M a s o k u t

S 2 2 0 . 7 7 6 0

E 9

9 4 7 . 2 9 6 0

4 . 4 2

2 . 8 9

–

D B

1 8

B e r e - B e r i l o u

S 2 1 9 . 9 8 2 0

E 9

9 4 3 . 7 4 8 0

2 . 6 1

2 . 3 5

1

3

B B

1 9


S 2 1 9 . 9 4 2 0

E 9

9 4 3 . 7 3 6 0

3 . 1 8

2 . 7 1

7

4

E W

2 0


S 2 1 9 . 9 2 7 0

E 9

9 4 3 . 7 4 1 0

1 . 2 6

0 . 3 0

1

2 0

E W

& M I

2 1


S 2 1 9 . 8 5 3 0

E 9

9 4 3 . 7 4 6 0

0 . 9 6

0 . 9 4

2

5 6

E W

2 2


S 2 1 9 . 7 9 8 0

E 9

9 4 3 . 7 4 6 0

0 . 3 5

0 . 2 4

3

5 4

E W

a

I f t h e t s u n a m i t r a c e

w a s l o c a t e d a l o n g o r n e a r t h e r o

a d f r o m t h e c o a s t , t h e e s t i m a t e d d

i s t a n c e f r o m t h e c o a s t i s g i v e n

b

B B b r o k e n b r a n c h ,

D B d e b r i s , M I m u d l i n e i n s i d e b u

i l d i n g ,

E W e y e w i t n e s s a c c o u n t

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houses closer to the shoreline is speculative, but could relate to previous events destroying

houses located too close to the sea.

2.3.1 Bosua

The tsunami reached around 300 m inland, propagating along a road that started from the

coast and ran inland. The inundation height of 4.81 m measured near the shoreline was the

largest recorded for this village, gradually decreasing as the wave travelled inland, and the

(a)

(b)

Fig. 2 Map of the Mentawai islands and distribution of tsunami trace heights measured by four survey

teams (Tomita et al. 2011; Satake et al. 2012; Hill et al. 2012 and the authors): a Mentawai islands, b south

part of Sipora island (The area surrounded by a dotted line in a indicates the area shown in b)

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run-up height was 2.55 m. The coastal trees located around the shoreline might have

provided some limited protection, though almost all houses in the inundated area weredestroyed (Fig. 4a).

People evacuated from the coastal area when the tsunami arrived, and thus no casualties

were reported, though there was no tall building or high ground (like a hill) around the

village. However, no damage was observed along the road that runs through the center of

the village and communicates it with the inland area of the island (Fig. 4b), so people

could easily escape the wave and refuge around there.

(a) (b)

(c) (d)

profile 1

profile 2

profile 3

Fig. 3 Satellite images of the surveyed villages in Sipora island issued by OCHA (2010): a Bosua, b Old-

Gobik, c Masokut, d Bere-Berilou (The areas highlighted by a dotted line show the surveyed section in

Fig. 9. Circles show the 5th administrative capitals.)

Table 2 Summary of the damage in surveyed villages in Sipora island (OCHA 2010)

Population Affected population Death Injured Missing

Bosua 435 312 0 6 0

Gobik 84 250a

10 40 0Masokut 287 – 8 0 0

Bere-Berilou 182 – (5)b – –

a The affected population in Gobik could indicate there were some people from outside of the village at the

time or could be a mistakeb

According to the information obtained in the authors’ field survey

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2.3.2 Old-Gobik

Old-Gobik is located to the east of Bosua, and the two villages are connected by a road.

There was a swamp with mangroves between the residential area and the coast (Fig. 5a),

which the tsunami traversed to inundate the whole village. An inundation height of 5.69 m

was measured in the swamp area, and inundation heights of 2.5–3.5 m were recorded

inside the residential area. Almost all houses were washed away, leaving only their

foundations behind (Fig. 5b).

This was the most damaged village in the authors’ survey, with ten people losing theirlives due to the tsunami. The road inside the village ran parallel to the shoreline, and there

was no road connecting the village with the higher inland area, and thus residents could not

effectively escape the incoming tsunami.

Due to the serious damage caused by the tsunami, residents decided to relocate to higher

ground (which is why the place is called ‘‘Old-Gobik’’) and start a new village (‘‘New-

Gobik’’) to the east side of the former village.

Fig. 4 Photographs of Bosua: a destruction of houses, b inland area along a road which did not suffer any

damage due to the tsunami

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2.3.3 Masokut

In this case, a river ran through the north side of the village (Fig. 6a), while a sand dune

protected its shoreline (the height of the top of the sand dune was around ?1.5 m from the

sea level at the tsunami arrival). Inundation heights were measured at six points in this

village: 4.42, 5.67, and 6.96 m in the north side, 2.43 m in the middle of the village, and

3.43 and 5.01 m in the south side. This distribution of heights indicates that the tsunami

penetrated the village from the river mouth in the north and from a gap in the sand dune in

the south.

Many houses were severely damaged, and the few which did not suffer clear damagewere located at ground levels that were higher than those of the other houses. However,

even for the case of some of the houses located in higher ground, up to 1-m deep scour was

recorded at some locations (Fig. 6b).

Eight people lost their lives in this village. Again, there was no route connecting the

residential area to higher ground, as per the case of Old-Gobik, and thus residents had

difficulty to evacuate.

Fig. 5 Photographs of Old-Gobik: a swamp area near the coast, b destroyed houses

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2.3.4 Bere-Berilou

In this village, a road connected the coastline with the inner side of the village (Fig. 7a),

running downhill toward the inland area and explaining why the tsunami reached over

300 m from the shoreline. Houses were heavily damaged in an area up to 100 m from the

shoreline, while few houses were damaged further inland from that point. The damage to

the houses was especially severe at the seaward side (Fig. 7b). Inundation heights of 2.61

and 3.18 m were measured inside the heavily damaged area and became progressively

smaller as the tsunami propagated inland, with a height of 0.35 m measured 350 m away

from the shoreline.

The inundation heights measured in this village were relatively small in comparison

with those measured in the other three villages. However, five people lost their lives.

Though there was a road which could bring people inland, there was no higher ground or a

tall building located close to the shoreline. Thus, it could be difficult for people to find a

Fig. 6 Photographs of Masokut: a destruction of houses on the riverside, b tsunami-induced scour around a

house

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place to evacuate in a short period of time, especially in the case where a larger tsunami

attacked the area.

2.4 Tsunami questionnaire

Structured questionnaires about the preparedness of local inhabitants and their behavior

during the tsunami were also carried out during the authors’ survey. Due to time constraints

(it took up to 1 h for each questionnaire to be carried out, as residents often went into great

detail to recount their experiences during and after the tsunami), the authors collectedanswers from only seven residents, which did not cover all the villages surveyed. Strictly

speaking, it is difficult to claim that these seven answers provide an accurate idea of

behavior of residents in the island during the tsunami. Nevertheless, the collected answers

provide valuable information about the general level of disaster preparedness in the region

and some insights into the behavior of residents during the event (Table 3).

Fig. 7 Photographs of Bere-Berilou: a flat road from the coast to inland, b damaged building (the seaward

side of the building suffered heavy damage)

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T a b l e 3

R e s u l t s o f q

u e s t i o n n a i r e s

R e s p o n d e n t n u m b e r

1

2

3

4

5

6

7

[ b a s i c i n f o r m a t i o n ]

V i l l a g e

B o s u a

B

o s u a

M a s o k u t

M a s o k u t

B e r e - B a r

i l o u

B e r e - B a r i l o u

B e r

e - B a r i l o u

O c c u p a t i o n

F a r m e r

H

o u s e w i f e

F a r m e r

O t h e r ( n o t s p e c i fi e d )

F a r m e r

F a r m e r

V i l l a g e l e a d e r

A g e

4 0 – 4 9

2

0 – 2 9

3 0 – 3 9

4 0 – 4 9

5 0 – 5 9

5 0 – 5 9

5 0 –

5 9

[ d r i l l s a n d e d u c a t i o n ]

H a v e y o u t a k e n p a r t i n a

t s u n a m i e v a c u a t i o n

s i m u l a t i o n i n t h e l a

s t

5 y e a r s ?

O n l y o n c e

O

n c e e v e r y

f e w y e a r s

O n l y o n c e

O n l y o n c e

W e d o n o t h a v e

e v a c u a t i o n

s i m u l a t i o n s

W e d o n o t h a v e

e v a c u a t i o n

s i m u l a t i o n s

W e

d o n o t h a v e

e v a c u a t i o n

s i

m u l a t i o n s

D o k i d s c a r r y o u t t s u

n a m i

s i m u l a t i o n s i n t h e s

c h o o l ?

( n o a n s w e r )

O

n l y o n e t i m e

Y e s

Y e s

N o

N o

N o

[ e v a c u a t i o n ]

W h e r e d i d y o u o b t a i n

i n f o r m a t i o n a b o u t t h e

t s u n a m i ?

T V , R a d i o

S u r f e r s , s u r f

o r g a n i z a t i o n s

S

u r f e r s , s u r f


S u r f e r s , s u r f


D e d u c e d b y h i m s e l f

( a f t e r a m a j o r

e a r t h q u a k e )

D e d u c e d

b y

h i m s e l f ( a f t e r a

m a j o r

e a r t h q u

a k e )

T V , R a d i o

T V , R a d i o

D e d u c e d b y

h i m s e l f ( a f t e r a

m

a j o r

e a r t h q u a k e )

W h e n d i d y o u e v a c u a t e ?

A f t e r t h e fi r s t

t s u n a m i w a v e

A

f t e r t h e

e a r t h q u a k e


t s u n a m i

w a v e

A f t e r r e c e i v i n g

i n f o r m a t i o n t h a t a

t s u n a m i c o u l d

a r r i v e

A f t e r t h e

fi r s t

t s u n a m

i w a v e


t s u n a m i w a v e


t s

u n a m i w a v e

W h e r e d i d y o u g o ?

H i g h t e r r a i n i n t h e

v i c i n i t y

H

i g h t e r r a i n i n

t h e v i c i n i t y

T a l l b u i l d i n g

H i g h t e r r a i n i n t h e

v i c i n i t y

H i g h t e r r

a i n i n t h e

v i c i n i t y

H i g h t e r r a i n i n

t h e v i c i n i t y

H i g

h t e r r a i n i n t h e

v i c i n i t y

W h a t m a d e y o u d e c i d e t o

e v a c u a t e ?

T h e f a c t t h a t

n e i g h b o r s w e r e

e v a c u a t i n g t h e

a r e a

I n f o r m a t i o n

f r o m n e i g h b o r o r

f a m i l y

T h e e f f e c t s o f

t h e fi r s t

t s u n a m i

w a v e

T h e f a c t o f l i v i n g i n

a n a r e a o f t s u n a m i

r i s k

T h e f a c t

o f l i v i n g

i n a n a r e a o f

t s u n a m

i r i s k

I n f o r m a t i o n

f r o m n e i g h b o r o r

f a m i l y

I n f o r m a t i o n f r o m

n e i g h b o r o r

f a

m i l y

H o w d i d y o u e v a c u a t e t h e

a r e a ?

W a l k i n g / r u n n i n g

W

a l k i n g /

r u n n i n g

W a l k i n g /

r u n n i n g



W a l k i n g /

r u n n i n g


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Education regarding tsunamis and evacuation drills had been conducted in Bosua and

Masokut, but not in Bere-Berilou. This indicates that awareness regarding tsunami events

was not shared among the entire population of the island, and disaster preparedness was

clearly provided only at the village level.

Five of the respondents evacuated after the first tsunami wave, and only one residentevacuated after the earthquake itself. This indicates that some residents did not consider

seriously the possibility of a tsunami after the earthquake, or ignored the potential dangers.

This also indicates some degree of confusion on the part of local inhabitants about how

strong an earthquake has to be before they should evacuate. However, the authors did not

pose the question of whether respondents felt the earthquake, as conversations with local

inhabitants and government officials in the area indicated that the earthquake had been felt

throughout the islands. It also should be noted that all the other survey reports (Tomita

et al. 2011; Satake et al. 2012; Hill et al. 2012) described that residents evacuated after

hearing the sound of the incoming wave. Because the tsunami attacked the villages at night

and ground shaking was not strong (this event was a ‘‘tsunami earthquake,’’ which gen-

erates large tsunamis relative to the seismic magnitude (Satake et al. 2012; Hill et al.

2012)), many residents appeared to only respond to the event after seeing inundation or

hearing the incoming wave.

Most of the residents who answered the questionnaires said they walked or ran to high

terrain to save their lives. Residents in these villages did not use a car or a bike for

evacuation, a fact that should be considered by future tsunami mitigation strategies. Part of

the reason for this appears to lie in the relative lack of means of many inhabitants, who did

not appear able to afford such means of transport. Also, local roads are very narrow

concrete paths, wide enough to be traversed only by motorbikes, though many are in asevere state of disrepair. No cars were seen in any of the villages surveyed by the authors,

as it was clear that it was impossible for them to gain access along the roads. In fact, much

of the local communication appeared to be by motorboats along the coastline.

It is important to note that many of the inhabitants surveyed in Sipora failed to evacuate

correctly, and only did so after feeling the effects of the tsunami. Casualty rates were not

higher due to the relative low height of the tsunami in many villages. Thus, while in many

cases people were aware of the threat of the tsunamis, the lack of more clear information

on the side of authorities and of when to evacuate prevented a more adequate response.

3 Discussion

Based on the results of the field surveys when considering the risk management strategy to

protect against future tsunamis in the Mentawai islands, the following three points should

be carefully considered: the geographic disadvantage of the islands, the resilience of

buildings and other infrastructure, and people’s awareness of tsunamis (Fig. 8). The

present situations regarding these three points and possible tsunami mitigation strategies in

the Mentawai islands are described below.

3.1 Geographic disadvantage of the islands

To consider tsunami mitigation strategies, first it is important to understand the geographic

conditions of the place concerned. In all villages surveyed by the authors, there was coastal

vegetation or sand dunes between the shoreline and residential areas, which were generally

located in low-lying ground 2–3 m above the sea level. Figure 9 shows the profiles of the

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tsunami trace height and the ground level in three villages. Though the inundation heights

gradually decreased as the tsunami progressed inland, almost all houses were washed away

in the inundated areas of Bosua and Old-Gobik.

The most effective way to mitigate future tsunami damage in these low-lying areaswould be to relocate houses to higher ground, as the villagers did in Gobik. However, in

some of these villages, this does not appear to be a realistic prospect, either because there is

no appropriate high ground near the residential area or because residents want to continue

living near the coast (for example, as these are advantageous for fishing). In such a case,

residents could explore the possibility of the construction of tsunami shelters or evacuation

routes to higher ground, which can carry people to safe locations in a short period of time.

Tsunami shelters or evacuation buildings, based on the experience of the 2011 Tohoku

Earthquake and Tsunami in Japan (Mikami et al. 2012), should be at least over 20 m high

(six story or more), especially as they should be designed with level 2 tsunamis in mind

(Shibayama et al. 2013). However, in small impoverished islands, it is difficult from anengineering and financial point of view to construct and maintain tsunami shelters which

are high and strong enough against such tsunamis. Thus, securing and expanding evacu-

ation routes should be considered as the most immediate evacuation strategy. Where there

is a road connecting the coastal and inland areas, like in the case of Bosua, residents can

use it for other purposes, and thus, this represents a ‘‘no regrets’’ strategy, as even if a

tsunami does not take place for a long period of time residents can benefit from improved

communications. If a road does not exist, like in the case of Old-Gobik, local inhabitants

would have to find an appropriate way to reach higher ground. In such cases, it could be

possible for residents to create small footpaths along the forest, which even if not pavedwould allow them to quickly access high ground in the event of an emergency.

It should be noted that small rivers running across roads were found in Muntei Barubaru

and Sabeugukgung in North Pagai island. Tomita et al. (2011) reported that in Muntei

Barubaru, people crossing a bridge over a river were swept away by the incoming tsunami

propagating along the river. Hill et al. (2012) reported that in Sabeugukgung, residents

could not cross a river before the tsunami struck and thus many were killed. Local residents

Tsunami Vulnerability Evaluation

Geography

• Safe high ground

• Evacuation route• River / Coastal vegetation• Closeness to rupture area

Infrastructure

• Buildings / Houses

• Coastal structures

• Tsunami warning system

People’s awareness• Drills• Education to children

• Indication (map / sign)

Fig. 8 Important points in

tsunami vulnerability evaluation

of the Mentawai islands

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must therefore prepare evacuation routes before the next tsunami occurs, avoiding rivers

when possible, and should also distribute this information to visitors. In this sense, a large

tsunami evacuation map was found in Padang (a major city located on the south coast of

Sumatra island, Fig. 10a), and tsunami evacuation signs were found in Pangandaran(located on the south coast of Java island, Fig. 10b, c), showcasing how such measures

have been implemented elsewhere in Indonesia. The Mentawai islands should also prepare

maps or at least small tsunami evacuation signs for people to understand easily what to do

when an earthquake occurs. This type of maps or signs can also contribute to sustaining

people’s awareness of tsunamis.

3.2 Resilience of buildings and other infrastructure

The buildings in the islands were mainly constructed with wooden materials or concretebricks and were generally only one- or two-stories high. These low-lying constructions

typically stood along the main road of each village, and throughout the survey, the authors

could not find a building which was tall and strong enough for residents to evacuate in case

of a major tsunami.

Most buildings were totally destroyed inside the inundated areas with the exception of

Bere-Berilou, where the inundation heights were smaller than in the other villages.

Whatever buildings survived inside the inundation areas were due to them being located in

grounds slightly higher than adjacent houses.

Generally speaking, the quality of the constructions in the area was low, with littlelateral stability and thus easily swept by the current generated by the tsunami. Many of

them did not use reinforced beams or columns, and buildings that did have these structural

elements generally fared better (Fig. 7b). These ‘‘better buildings’’ often served for reli-

gious purposes, typically built to a higher standard than the adjacent houses. Some of these

also benefited from not having lateral walls, allowing the water to flow through the

building with little hindrance and probably contributing to their survival.

Fig. 9 Profiles of the tsunami trace height and the ground level

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Although it is unlikely that many buildings located close to the sea would survive a

major tsunami, the construction of houses on elevated columns (stilts) could protect

property against the more frequent tsunami events (Tomita et al. 2008). However, it is not

clear whether the local population currently has the financial resources to build such

houses, given the very modest quality of much of the construction.

Coastal structures to protect against tsunamis were not found in these islands. However,

after the 2004 Indian Ocean Tsunami, some tsunami warning systems have been developed

in Indonesia, such as the GITEWS (German Indonesian Tsunami Early Warning System)project (Munch et al. 2011).

However, in the case of the 2010 event, the rupture area was so close to the islands that

there was almost no time for information to be disseminated throughout the islands.

Figure 11 shows a simulation of the tsunami propagation up to 20 min after the earthquake

using the fault parameters proposed by Satake et al. (2012). In this numerical simulation

carried by the authors, the model was composed of a calculation of the initial seafloor

Fig. 10 Tsunami evacuation map and signs: a map in Padang, b, c signs in Pangandaran

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deformation using the formula of Mansinha and Smylie (1971) and the tsunami propa-

gation using linear shallow water equations which were solved using a finite-difference

method with a leap-frog sheme. The bathymetric data were obtained from the General

Bathymetric Chart of the Oceans (GEBCO). These data are organized with a grid size of

30 s (about 900 m). Fig. 11 shows that on the Indian Ocean side of the Mentawai islands,

at first the sea level lowered (something that the residents would have not noticed as the

tsunami took place at night) and then the first tsunami wave arrived 10–20 min after the

earthquake occurred.

In such a case and given the relative poverty and underdeveloped tsunami mitigation

measures, it is very difficult to effectively disseminate the tsunami warning throughout

every village in the islands. In the Mentawai islands, it is thus necessary to actively makeuse of drills and education (as will be explained next) as well as to develop and maintain

tsunami warning systems.

3.3 People’s awareness of tsunamis

The southwest coast of Sumatra island and the islands off the coast have been attacked by

tsunamis many times (Hamzah et al. 2000). However, when focusing on a specific location,

the recurrence intervals of large events are too long for residents to remember the previous

event. For example, Sieh et al. (2008) reported that sea-level changes extracted from coralssuggested four series of earthquakes occurred in the Mentawai islands in the past

700 years, and Monecke et al. (2008) reported that three layers were found in 1,000-year

sediment records in the north coast of Sumatra island. These results indicate that the

recurrence intervals of large earthquakes or tsunamis were longer than a century.

In addition, there were large earthquakes that did not generate a large tsunami (e.g., the

2009 Padang Earthquake). If there is no tsunami despite strong ground shaking, the local

Fig. 11 Spatial distribution of the water surface level around Mentawai islands 0–20 min after the

earthquake occurred

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population can start to think that not all large earthquakes require evacuation from the

coast.

Figure 12 shows the tsunami height distributions in the islands during the 2004, 2005,

and 2010 tsunamis, with each tsunami attacked only two or three of the small islands off

Sumatra. The faults that cause earthquakes off Sumatra island stretch parallel to and closeto Sumatra island, and thus, the areas affected by a potential tsunami are generally limited

to a few hundred km along the faults.

All of these issues may result in weakening people’s awareness of tsunamis (a decrease

in their ‘‘tsunami culture’’) and fail to evacuate promptly after an earthquake. To prepare

against future tsunamis, it is important to have a plan to sustain people’s awareness of

tsunamis. The plan has to encompass three main issues: frequent tsunami drills, the edu-

cation of children, and providing relevant and correct information.

During the authors’ survey, residents in some villages explained that they had taken part

in tsunami drills, though these drills were not conducted in all villages and were infrequent.

Tsunami drill is recognized as an effective way to make residents understand what to do

during such events (e.g., Dengler 2005). Residents have to continue taking part in drills in

places where they have been established, and residents should be encouraged to start drills

where they do not have them yet.

Even if tsunamis only attack an area infrequently, residents can mitigate tsunami

damage if they manage to preserve the past experience in their community (what could be

described as a ‘‘tsunami culture’’). It has been reported that a tradition of oral histories

about the past tsunami events saved some residents in the 2004 Indian Ocean Tsunami

(McAdoo et al. 2006) and in the 2007 Solomon Islands Tsunami (Fritz and Kalligeris

2008). To maintain this traditional knowledge where it exists, and create it where it doesnot, it is important to educate the younger generations.

In the case of having tsunami drill or education, particular attention should be paid to

misunderstandings extracted from a few experiences. Hill et al. (2012) reported that some

Fig. 12 Distribution of tsunami trace heights of the 2004 Indian Ocean Tsunami (Jaffe et al. 2006), the

2005 Nias Island Tsunami (Borrero et al. 2011), and the 2010 Mentawai Islands Tsunami (Tomita et al.

2011; Satake et al. 2012; Hill et al. 2012 and the authors) in the remote islands off Sumatra island

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residents in the Mentawai islands thought that a tsunami would not attack their village

because of the relatively gentle and slow ground shaking. This attitude came from the

experience in the 2007 Bengkulu earthquake, which caused large ground shaking but did

not result in a large tsunami in the Mentawai islands. Thus, giving correct information or

knowledge about tsunami and earthquake is a key factor for the success of any evacuationstrategy. To eliminate this kind of misunderstandings, it is important to teach residents to

evacuate immediately after feeling any ground motion, or when instructed to do so by local

authorities.

4 Conclusion

The authors summarized the results of the field surveys conducted after the 2010 Mentawai

Islands Tsunami to clarify the tsunami height distribution, the actual damage, and resi-

dents’ awareness of these events in the affected islands. Heights of over 5 m were recorded

on the coastal area of the Indian Ocean side of North and South Pagai islands and the south

part of Sipora island. In some villages, it was difficult to evacuate immediately after the

earthquake because of the lack of routes to higher ground or the presence of rivers.

Residents in some villages had taken part in tsunami drills or education; however, not all

villages shared the same level of tsunami awareness.

Based on the results, a discussion was made on the vulnerability of these islands with

regards to future tsunami threats. Three important aspects (the geographic disadvantage of

the islands, the resilience of buildings and other infrastructure, and people’s awareness of

tsunamis) of any future risk management strategy in these islands were discussed, andsuggestions were made to improve tsunami mitigation strategies. However, due to the

relative poverty and lack of financial means of the islanders, it appears difficult that much

can be improved in the short term and the prospects of these coastal villages appear

particularly worrisome in case a larger tsunami event takes place.

After the 2004 Indian Ocean Tsunami, many people have paid more attention to

earthquakes and tsunamis in the vicinity of Sumatra island. Much work has been carried

out regarding historical (e.g., Natawidjaja et al. 2006; Kanamori et al. 2010) and con-

temporary tsunamis, though tsunami records are still limited and it is thus difficult to

understand the real potential risks facing coastal communities. Further study of tsunamis in

the area should be encouraged, alongside with investment by the Indonesian government

and international aid agencies to improve disaster preparedness and increase the resilience

of local communities.

Acknowledgments The present work was supported by the Grants-in-Aid for Scientific Research (B) No.

22404011 from the Japan Society for the Promotion of Science (JSPS). Some of the figures were generated

using the Generic Mapping Tools (Wessel and Smith 1998).

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