Tsunami Vulnerability Evaluation In The Mentawai Islands Based On The Field Survey Of The 2010 Tsunami

Nat Hazards (2014) 71:851–870 DOI 10.1007/s11069-013-0936-z ORIGINAL PAPER

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 Ó Springer Science+Business Media Dordrecht 2013

Abstract On October 25, 2010, a large earthquake 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. 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), Graduate School 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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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 Re´union island, located 5,000 km southwest of the Mentawai islands (Sahal and Morin 2012). According to Indonesian National Disaster Management Agency [Badan Nasional Penanggulangan 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 international 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 an effective 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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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)

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.

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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 tsunami threats, a structured questionnaire was carried out in some of the villages. The questionnaire 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 suffering serious 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 each village 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

123

Bosua

Bosua

Bosua

Bosua

Bosua

Bosua

Old-Gobik

Old-Gobik

Old-Gobik

Old-Gobik

Masokut

Masokut

Masokut

Masokut

Masokut

Masokut

Bere-Berilou

Bere-Berilou

Bere-Berilou

Bere-Berilou

Bere-Berilou

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

b

E 99°48.1370

E 99°48.1570

E 99°48.1590

E 99°48.2050

E 99°48.2300

E 99°48.2330

E 99°48.2430

E 99°49.2720

E 99°49.2700

E 99°49.2800

E 99°49.3010

E 99°47.3480

E 99°47.3420

E 99°47.3500

E 99°47.2830

E 99°47.2920

E 99°47.2960

E 99°43.7480

E 99°43.7360

E 99°43.7410

E 99°43.7460

E 99°43.7460

S 2°21.9990

0

S 2°21.9850

S 2°21.9500

S 2°21.9420

0

S 2°21.9260

S 2°22.1230

S 2°22.0780

S 2°22.0710

S 2°22.0530

S 2°20.8610

0

S 2°20.8160

S 2°20.7700

S 2°20.7800

S 2°20.7760

S 2°19.9820

S 2°19.9420

S 2°19.9270

S 2°19.8530

0

S 2°19.798

S 2°20.845

S 2°21.930

S 2°21.990

Longitude

Latitude

0.35

0.96

1.26

3.18

2.61

4.42

5.67

6.96

2.43

5.01

3.43

2.45

3.36

3.57

5.69

2.55

2.70

2.86

2.95

4.69

3.34

4.81

Height (m)

0.24

0.94

0.30

2.71

2.35

2.89

4.62

5.85

0.43

2.57

0.97

1.70

2.80

3.40

5.23

0.00

0.64

0.74

1.97

4.40

3.10

4.80

Inundation depth (m)

BB broken branch, DB debris, MI mud line inside building, EW eyewitness account

If the tsunami trace was located along or near the road from the coast, the estimated distance from the coast is given

Bosua

1

a

Location

No.

Table 1 Tsunami trace heights measured in Sipora island

354

256

120

74

13

–

–

–

–

–

–

160

81

61

33

283

257

229

197

–

93

52

Distance from the coast (m)a

EW

EW

EW & MI

EW

BB

DB

BB

DB

EW

EW

EW

BB

BB

DB

BB

EW (run-up)

EW

EW

BB

DB

DB

DB

Watermarkb

Nat Hazards (2014) 71:851–870 855

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856

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(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)

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

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857

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

0

Masokut

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 mistake

b

According to the information obtained in the authors’ field survey

(a)

(b) profile 1

profile 2

(c)

(d)

profile 3

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

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 were destroyed (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.

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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

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 their lives 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 (‘‘NewGobik’’) to the east side of the former village.

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859

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

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 damage were 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.

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

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

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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)

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 collected answers 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).

123

123

Farmer

40–49

Age

(no answer)

Do kids carry out tsunami simulations in the school?

TV, Radio Surfers, surf organizations

After the first tsunami wave

High terrain in the vicinity

The fact that neighbors were evacuating the area

Walking/running

Where did you obtain information about the tsunami?

When did you evacuate?

Where did you go?

What made you decide to evacuate?

How did you evacuate the area?

[evacuation]

Only once

Have you taken part in a tsunami evacuation simulation in the last 5 years?

[drills and education]

Bosua

Occupation

1

Village

[basic information]

Respondent number

Table 3 Results of questionnaires

Walking/ running

Walking/running

Walking/running

Walking/ running

Information from neighbor or family

The fact of living in an area of tsunami risk The fact of living in an area of tsunami risk

The effects of the first tsunami wave

Information from neighbor or family Walking/ running

High terrain in the vicinity

High terrain in the vicinity

After the first tsunami wave

After the first tsunami wave

After receiving information that a tsunami could arrive High terrain in the vicinity

TV, Radio

No

We do not have evacuation simulations

50–59

Farmer

Bere-Barilou

6

Deduced by himself (after a major earthquake)

No

We do not have evacuation simulations

50–59

Farmer

Bere-Barilou

5

Deduced by himself (after a major earthquake)

Yes

Only once

40–49

Other (not specified)

Masokut

4

Tall building

After the first tsunami wave

Surfers, surf organizations

Yes

Only once

30–39

Farmer

Masokut

3

High terrain in the vicinity

After the earthquake

Surfers, surf organizations

Only one time

Once every few years

20–29

Housewife

Bosua

2

Walking/running

Information from neighbor or family

High terrain in the vicinity

After the first tsunami wave

TV, Radio Deduced by himself (after a major earthquake)

No

We do not have evacuation simulations

50–59

Village leader

Bere-Barilou

7

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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 resident evacuated 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 generates 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 a severe 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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864 Fig. 8 Important points in tsunami vulnerability evaluation of the Mentawai islands

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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)

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 areas would 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 an engineering and financial point of view to construct and maintain tsunami shelters which are high and strong enough against such tsunamis. Thus, securing and expanding evacuation 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 paved would 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

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

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 concrete bricks 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 little lateral 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 religious 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.

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

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

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

deformation using the formula of Mansinha and Smylie (1971) and the tsunami propagation 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 make use 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 corals suggested 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

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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 close to 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 education 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 does not, 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 evacuation strategy. 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 residents’ 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, and suggestions 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 contemporary 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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