Coseismic Uplift of Nias Island, Sumatra: Permanent versus Recoverable Components

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Coseismic deformation above the Sunda megathrust during the giant Sumatran earthquake of March 2005 presents an exceptional opportunity to explore the relationship between impermanent (elastic) and permanent deformation of the subduction wedge. In 2005, Nias Island rose up to 3 meters as part of a trench-parallel belt of uplift. This uplift can be modeled by an elastic dislocation of about 10 meters on a strip of the megathrust beneath the island, consistent with the observation that the 2005 rupture did not cause significant slip on secondary faults. Since sites that rose during the earthquake were subsiding in prior decades, it is possible that coseismic and interseismic deformations of Nias sum to zero. However, the fact that the coseismic uplift pattern mimics both the shape of the island and its northeast-dipping geologic structure suggests that some of the 2005 uplift will be retained as permanent deformation. The presence of raised coral-reef terraces on the island encourages this suspicion. This project is combining field mapping and U-Th dating to characterize the geometry and rates of uplift of coral platforms on Nias Island and the Batu Islands. The goal is to relate long-term uplift to coseismic uplift by quantifying how coseismic deformation on Nias is partitioned into elastic and permanent components. A comparison of the extent of deformation of coral terraces above seismic and aseismic parts of the Sunda megathrust (Nias Island versus the Batu Islands, farther south) provides a basis for evaluating the possibility that the mode of megathrust failure influences deformation of the overriding block. This work will allow a test of the validity of simple elastic models of megathrust strain accumulation and release. It will also contribute to understanding megathrust tectonophysics and to evaluating seismic hazards along this and other subduction zones such as Cascadia. This work is being carried out in close collaboration with scientists at the Indonesian Institute of Sciences and involves significant outreach activities in Indonesia and United States to educate people about earthquake and tsunami hazards.