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

2005年3月苏门答腊岛大地震期间顺他巨型逆冲之上的同震形变为探索俯冲楔体的非永久性(弹性)和永久性变形之间的关系提供了一个难得的机会。2005年，作为海沟平行隆起带的一部分，尼亚斯岛上升了3米。这种抬升可以用岛下巨型逆冲带上约10米的弹性错动来模拟，这与2005年的破裂没有在次级断层上造成重大滑动的观察一致。由于在地震中上升的场地在前几十年都在下沉，尼亚斯的同震和震间变形可能总和为零。然而，同震隆起模式既模拟了岛屿的形状，也模拟了其东北向倾斜的地质结构，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的几何形状和抬升速度。其目标是通过量化尼亚斯的同震形变如何被划分为弹性和永久性分量，将长期隆起与同震隆起联系起来。对舜达巨型逆冲(尼亚斯岛与更南端的巴图群岛)地震和非地震部分上方珊瑚阶地的变形程度的比较，为评估巨型逆冲破坏模式影响上覆块体变形的可能性提供了依据。这项工作将对巨型锈菌应变累积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，并评估该俯冲带和其他俯冲带(如卡斯卡迪亚)的地震危害。这项工作是与印度尼西亚科学研究所的科学家密切合作开展的，涉及在印度尼西亚和美国开展的大量宣传活动，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.