Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami

Low-lying coral islands are fragile landforms susceptible to long-term sea-level rise and extreme events, such as hurricanes and tsunamis. The Sumatran earthquake of 26 December 2004 generated waves that reached the Maldives 2500 km away. Observations of the effects of the tsunami are presented here, based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited Maldivian islands. The surveys showed there was no substantial island erosion and no significant reduction in island area. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localized retreat of exposed island scarps by up to 6 m, deposition of cuspate spits to leeward, and vertical island building through overwash deposition of sand sheets up to 0.3 m thick, covering up to 17% of island area. These results have implications for island stability indicating that low-lying reef islands are physically robust and the geological signature of tsunamis on atoll island development is minor.