Coastline changes to Aceh from the great 2004 Sumatra-Andaman earthquake

Location

Continent: Asia
Country: Indonesia
State/Province: Aceh
City: Banda Aceh
UTM coordinates and datum: none

Setting

Climate Setting: Tropical
Tectonic setting: Island Arc
Type: Process

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Trees dropped into the surf zone by coseismic subsidence associated with the 2004 Sumatra-Andaman earthquake near Lhok Nga, Aceh. Andrew Moore, Earlham College.

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Coral reef lifted out of the water by the 2004 Sumatra-Andaman earthquake near Simelue Island. Andrew Moore, Earlham College.

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Map of uplift and subsidence from the 2004 Sumatra-Anadaman earthquake (Chlieh et al., 2007). Chlieh et al., 2007, DOI: 10.1785/0120050631

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Saltwater inundated rice fields with new shoreline forming at lower left. Andrew Moore, Earlham College.

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Drainage widened by tsunami and blocked by beach retreat into river mouth. Andrew Moore, Earlham College.

Description

The 2004 Sumatra-Andaman earthquake, one of the largest earthquakes ever recorded, did more than shake northern Indonesia. In addition to generating a devastating tsunami, it permanently dropped the Acehenese coastline by up to 1 m (Figure 1), and uplifted nearby islands by up to 2 m (Figure 2, 3). This sudden change in relative sea level caused new coastline along most of the southwest coast of Aceh, resulting in rapid adjustment by the shore to these new conditions, and creating new hazards for an already devastated populace.

Where the shoreline dropped, river valleys and farm fields were drowned, and a new shoreline developed quickly by wave action (Figure 4, 5). The loss of farms makes local people more reliant on food aid from outside, but the new shoreline increases the problem by changing local drainage patterns as river mouths are blocked by the new beach. The drowned river valleys can permanently enlarge coastal drainages, requiring new bridges before contact can be reestablished with the rest of Indonesia.

Where the shoreline uplifted, coral reefs which commonly protect the shoreline from large waves were destroyed, leaving uplifted coastal regions vulnerable to coastal retreat. Reefs and mangroves also provide a haven for fish used as food by locals, so that the loss of these habitats also results in a decline in available food. Sediment flux from these shorelines tends to increase, and harbors previously accessible to fishing boats may be uplifted to the point that they are no longer usable.

Damage from the 2004 Sumatra-Andaman earthquake resulted from more than earthquake shaking and the tsunami that accompanied the earthquake. Tectonic uplift and subsidence caused coastal changes that can be considered permanent on human timescales. Understanding the nature and distribution of these changes is the work of geomorphologists, and such an understanding will be necessary to any sustained rebuilding effort in this region.

Associated References:

• Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y. and Galetzka, J., 2007, Coseismic slip and afterslip of the Great (Mw 9.15) Sumatra—Andaman Earthquake of 2004, Bulletin of the Seismological Society of America, v. 97, no. 1A, p. S152-S173, DOI: 10.1785/0120050631.
• Jaffe, B.E., Borrero, J.C., Prasetya, G.S., Peters, R., McAdoo, B., Gelfenbaum, G., Morton, R., Ruggiero, P., Higman, B., Dengler, L., Hidayat, R., Kingsley, E., Kongko, W., Lukiyanto, Moore, A., Titov, V., and Yulianto, E., 2006, Northwest Sumatra and Offshore Islands Field Survey after the December 2004 Indian Ocean Tsunami, Earthquake Spectra, v. 22, n. S3, p. S105-S135.
• Moore, A., Nishimura, Y., Gelfenbaum, G., and Kamataki, T., 2006, Sedimentary deposits of the 26 December 2004 tsunami on the northwest coast of Aceh, Indonesia, Earth, Planets, and Space, v. 58, p. 253-258.

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