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

2005年3月苏门答腊大地震期间，巽他巨型逆冲断层上的同震变形为探索俯冲楔的非永久性（弹性）变形和永久性变形之间的关系提供了一个难得的机会。2005年，作为与海沟平行的隆起带的一部分，尼亚斯岛上升了3米。这种隆起可以通过岛下巨型逆冲带上大约10米的弹性错位来模拟，这与2005年的破裂没有造成次生断层的明显滑动的观察结果一致。由于地震期间上升的地点在前几十年下沉，因此Nias的同震和震间变形总和为零是可能的。然而，同震隆起模式模拟了该岛的形状及其东北倾的地质结构，这一事实表明，2005年的一些隆起将作为永久变形保留下来。岛上凸起的珊瑚礁梯田的存在助长了这种怀疑。该项目将实地测绘和U-Th测年相结合，以确定尼亚斯岛和巴图群岛珊瑚平台隆起的几何形状和速率。目标是通过量化Nias上的同震变形如何划分为弹性和永久分量，将长期隆起与同震隆起联系起来。对巽他大逆冲断层地震和地震部分（尼亚斯岛和更南边的巴图群岛）上方珊瑚台地的变形程度进行比较，为评估大逆冲断层破坏模式影响覆岩块变形的可能性提供了基础。这项工作将允许对大型逆冲应变积累和释放的简单弹性模型的有效性进行测试。它还将有助于理解巨型逆冲构造物理学，并评估沿着这个和其他俯冲带（如卡斯卡迪亚）的地震危险性。这项工作正在与印度尼西亚科学研究所的科学家密切合作进行，并涉及在印度尼西亚和美国开展重要的外联活动，以教育人们了解地震和海啸的危害。

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.