1 The 2004 tsunami in Aceh

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Key points

  • The Indian Ocean tsunami was triggered by a 9.1 magnitude earthquake off the coast of Aceh, affecting 92,000 ha of agricultural land and plantations in Aceh, and farmland and farmers’ livelihoods in Thailand, India and Sri Lanka.
  • Wave heights were up to 30 m along Aceh’s west coast and up to 5 m along the east coast.
  • More than 330,000 people in Aceh required food and financial assistance as a result of loss of farming and fishing livelihoods.
  • The Food and Agriculture Organization of the United Nations has developed a post-tsunami damage classification system to help determine where to concentrate land rehabilitation efforts.
A low-lying coastal area on the west coast of Aceh, showing flooded land and deposition of debris in the foreground with tropical jungle and forested mountains in the background.

Photo: Food and Agriculture Organization of the United Nations

Indonesia is prone to seismic activity, which can cause earthquakes, volcanic eruptions and destructive tsunamis. A large proportion of Indonesia’s coastline is at risk from tsunamis (Horspool et al. 2013).

An assessment of tsunami hazards in Indonesia (Horspool et al. 2013) estimates at least a 1-in-100-year probability of a significant tsunami of 5–10 m for parts of west and central Indonesia, and a tsunami of 2–3 m for eastern Indonesia. The chance in any one year of a major tsunami (over 3 m) occurring somewhere in Indonesia is greater than 10%. Parts of the Indonesian coastline are particularly vulnerable, as a result of a focusing effect that magnifies tsunami height (Futurity 2013). Regions with the greatest chance of experiencing a major tsunami include the west coast of Sumatra and nearby islands, and parts of the south coasts of Java, Bali and West Nusa Tenggara (Figure 1).

The probability of a tsunami greater than 3 metres occurring in Indonesia in a year is highest (at a probability of 1 in 50 to 1 in 10) for the west coast of Sumatra and nearby islands, and parts of the south coasts of Java, Bali and West Nusa Tenggara. Areas with a low probability (less than 1 in 1000) for a tsunami in a year include the west coasts of Sumatra and Java, the south and east coasts of Kalimantan, the south coast of Sulewesi and the west coast of Irian Jaya.

Source: Horspool et al. (2013)

Figure 1 Risk map for a tsunami greater than 3 m in Indonesia

On Sunday 26 December 2004, at 7.58 am, an undersea earthquake measuring 9.1 on the Richter scale occurred off the west coast of Sumatra. The earthquake occurred in the subduction zone between the Indian and Indo-Australian tectonic plates (described in some texts as the Indian and Burma plates) and led to a massive 1,200-km-long fault slip along the boundary of the plates. The earthquake’s upward thrust displaced enormous volumes of water, generating a tsunami that reached Aceh about 20 minutes after the earthquake.

The tsunami affected both the west and east coasts of Aceh (Figure 2). An International Tsunami Survey Team (Tsuji et al. 2005) documented wave heights of 5–12 m around Banda Aceh at the island’s north-west end and found evidence that wave heights may have ranged from 15 to 30 m along a stretch of at least 100 km on the west coast. The United States Geological Survey measured wave heights of more than 30 m along the Lampuuk to Leupeung stretch of the west coast (USGC 2005a, b). A United Nations Educational, Scientific and Cultural Organization (UNESCO) team found wave heights of 10–15 m at Meulaboh on the west coast (Yalciner et al. 2005). Wave heights on the east coast were lower—around 5 m at Sigli and 2 m at Bireuen (USGC 2005a).

Land inundated by the tsunami in 2004 included an area of around 10 km to the east and west of Banda Aceh in the north-west, stretching several kilometres inland; and, on the west coast, most coastal areas from around Meulaboh (in the south) to the most north-western tip of Aceh Province (Deudap and Weh islands). On the north coast, an area from the north of Sigli to Bireuen was inundated.

Source: Adapted from Dartmouth Flood Observatory

Figure 2 Coastal areas of Aceh inundated by the December 2004 tsunami

The earthquake raised the sea floor in front of the fault rupture and caused subsidence behind the rupture along Aceh’s west coast. Observations of trees with their roots and lower trunks submerged in sea water indicated that coastal land subsided 1–2 m in some areas (USGC 2005a), while large sandy areas indicated significant uplift in other areas (Figure 3). Similar observations of uplift and subsidence of more than 1 m were made on the Andaman and Nicobar islands (Chand et al. 2013; Intergovernmental Oceanographic Commission 2013) and the island of Nias off the coast of Sumatra.

Before the earthquake, a narrow strip of sand separated vegetation from the water on islands off the west coast of Aceh. After the earthquake, uplift has resulted in the water receding, leaving a sandy expanse between pockets of vegetation.

Photos: Kerry Sieh, Singapore Earth Observatory

Figure 3 Before and after photographs showing the dramatic uplift (right) caused by the 2004 earthquake on offshore islands along Aceh’s west coast, contrasting with long-term subsidence before the earthquake (left)

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