Tsunami Propagation Information for Increasing Small Islands Community Preparedness around the Sumatra Island

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Alicia Allison
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1 Tsunami Propagation Information for Increasing Small Islands Community Preparedness around the Sumatra Island S YA M S ID IK F IR D A U S TEU K U M. R A S YIF TS U N A M I A N D D IS A S TER M ITIG A T I O N R E S E A R C H C E N TER S YIA H K U A LA U N IV E R S I T Y, B A N D A A C E H - IN D O N E S IA Presented at the 6th South China Sea Tsunami Workshop Nanyang Technology University, Singapore, 6-8 November 2013

2 Background Number of small islands that are also prone to tsunami disaster in the world is large. Some of the small islands are used also for economy centers for the region. On the other hand, some of the small islands are far from sufficient tsunami mitigation infrastructures. Among the examples of tsunami prone small islands are Banyak Islands and Weh Islands. The Banyak Islands are a group of small islands that has about 7,000 people live in the islands. These islands are considered rather remote from mainland infrastructures. The Weh Island, on the other hand, is a relatively well-developed small island located at the tip of the Sumatra Island. The population of the Weh Island is about 23,000 people. This study elaborates the use of the tsunami propagation information for tsunami evacuation procedure by considering the local community knowledge, landscape challenges, and information challenges for the islands.

3 Objectives of the research To investigate the use of tsunami propagation information for increasing local-community s preparedness at small islands, especially to see chances for increasing tsunami evacuation effectiveness at the small islands. Research Key Questions: Are there any alternative to use the tsunami propagation information for tsunami mitigation at the tsunami prone small islands? What contributions can be made by increasing tsunami evacuation infrastructures in the islands?

4 Study Area Weh Island Banyak Islands Nias Islands Mentawai Islands Enggano Island

5 Methodology This study used numerical simulation to produce Tsunami Estimated Time of Arrival (ETA) to the small islands by the advantage of COMCOT. The simulation was based on estimated based on 9 tsunami genic events, i.e.: 9.2 Mw in 2004 (Indian Ocean Tsunami) 8.6 Mw in 2005 (Nias Islands) 7.9 Mw in 2007 (around Bengkulu) 7.8 Mw in 2010 (two times, i.e. One around Mentawai Islands and another one was around Simeulue Island) 7.2 Mw in 2012 (around Simeulue) 8.5 and 8.2 Mw in 2012 (Western off shore area of Simeulue Island) 8.9 Mw which is based on a seismic gap around Mentawai Islands as suggested by Natawidjaja (2011).

6 The Scenario of Earthquake

7 Seismic Gap Indicated by Danny Hilman Natawidjaja (2013)

8 Bathymetric data were obtained from recent data collected by Geospatial Information Agency of Indonesia and GEBCO. Simultaneously, a set of questionnaires were distributed to about 150 respondents at two islands in Pulau Banyak islands to investigate the community preparedness to the tsunami. The questionnaires consist of 25 questions to know about: The community s knowledge on the tsunami hazard The community s response on emergency condition Tsunami risk reduction efforts conducted by the community. Comparisons were made to see the difference of the small islands s community preparedness.

9 The Islands: Flat and small Discussion with Local Community

10 Numerical Simulation Domain Extents of Grid Coordinate Grid Spacing Grid Numbers longitude Latitude System SWE min 1851 m 1731*1938 Spherical Linear min m 1558*1400 Spherical Linear min m 1622*1516 Spherical Linear min m 1174*742 Spherical Linear

Producing Tsunami Estimated Time of Arrival for the Small Islands Tsunami Estimated Time Arrivals (ETAs) were produced by setting 17 observation points around populated small islands.

11 Producing Tsunami Estimated Time of Arrival for the Small Islands Tsunami Estimated Time Arrivals (ETAs) were produced by setting 17 observation points around populated small islands. The ETAs were based on Indonesian Climatology, Geophysical and Meteorological Agency (BMKG) who determines that tsunami warning will be released when the tsunami wave reach 0.5 m. The shortest ETA is used for tsunami mitigation for the tsunami prone small islands.

12 Initial Waves (1) 2010 (Mentawai) From A Seismic Gap around Mentawai Island

13 Maximum Tsunami Waves Propagation 2004 Aceh 2005 Nias 2007 Bengkulu 2010 Mentawai From A seismic gap around Mentawai

14 ETA for the Small Islands The shortest ETA were found to be critical for evacuation process for the small islands community Mentawai Islands are grouped as the shortest ETA among the small islands around Sumatra Name of Island ETA (min) Weh 34 Aceh Islands 16 Simeulue 26 Banyak Islands 40 Nias 26 Siberut 20 Sipora 15 North Pagai 12 South Pagai 9 Enggano 9

15 ETA vs Warning Time Warning 1 Warning 2 Enggano South Pagai North Pagai Sipora Siberut Nias Banyak Islands Simeulue Aceh Islands Weh Time inminutes People have 1 to 35 minutes to evacuate from tsunami. But, where and how can they evacuate?

16 Evacuation Supports Time to evacuate Higher Place Inland Cellular Phone Name of Island (min) to Evacuate TEWS Connection Weh 29 Yes Yes Yes Aceh Islands 11 Yes No No Simeulue 21 Yes Yes Yes Banyak Islands 35 Some islands NO NO Yes Nias 21 Yes No Yes Siberut 15 Yes No Mostly No Sipora 10 Some villages No No Mostly No North Pagai 5 Some villages No No No South Pagai 1 Yes No No Enggano 4 No No No

17 Community s Knowledge on Tsunami (Banyak Islands case) Yes No

18 Potential Communication supports during emergency Yes No

19 Tsunami Evacuation for the Small Islands Community A good practice was shown by Balai Island s of Aceh people during Nias Earthquake and tsunami in March People decided to go to offshore area using their boats. A similar experience was also learned at another case in Maldives during Indian Ocean Tsunami in A good advocacy was seen to be effective to deliver the lesson to the community.

20 TEWS for the small Islands? At some cases, especially when Mentawai tsunami in 2010, the TEWS did not function properly due to some reasons. Another campaign to put TEWS equipments around Sumatra is underway. Learning from Mentawai case and Nias case, the TEWS will give a slim chance for the islands community to receive the warning. The TEWS was found effective when the tsunami occurred far from the islands. But in most of the cases, the tsunami source was near.

21 Measures for Tsunami Mitigation at the small islands Vertical evacuation Building Routine Drills Evacuation transportation: Boats Effective Tsunami Evacuation for small islands

22 Conclusions and Recommendations Tsunami waves propagation, in terms of estimated time of arrivals are essential for increasing effectiveness of tsunami evacuation. Mentawai Islands are among the most tsunami prone small islands due to their short time of the tsunami propagation. With a narrow window time for evacuation, inland facilities to disseminate tsunami warning are important. Furthermore, routine evacuation drills to the sea at some small islands are recommended.

23 Thank you T E R IMAKA S IH

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