Collaborative Research: Forearc Cracks and the Rupture Segments of Great Earthquakes, N. Chile and S. Peru

沿着南美洲西部边缘的板块边界沿着产生了地球上任何地方最大的地震。这些地震的规模基本上与破裂带的长度有关。该项目的目的是利用智利北方阿塔卡马沙漠中暴露和保存良好的开放式张裂组确定长期平均破裂长度。为了完成这一任务，由非构造过程形成的裂缝与板块边界地震产生的裂缝区分开来。由康奈尔大学和迈阿密大学的科学家领导的一个研究小组，在结构地质学、干旱土壤形成、表面暴露年代测定和土壤物理学方面具有专长，他们正在访问智利北方的主要地区，以制定区分不同起源的裂缝的客观标准。通过测定不同但相邻表面具有不同裂缝密度的年代，该团队正在确定裂缝产生的速率，并通过研究裂缝沿着潮湿土壤和裂缝内形成的石膏脉的同位素组成，确定地下水在裂缝形成中的作用。最后，雷达干涉测量法提供了在实际地震影响前弧期间打开的裂缝的图像。与客观的标准在手，裂纹人口确定2米分辨率的图像在整个北方智利和南部秘鲁进行评估，以确定哪些形成或已被激活的大板块边界地震。通过模拟与假设和真实的板块边界破裂相关的静态和动态应力，这些裂缝的方向可以与地震破裂区域相关。这项工作的主要科学成果将是首次对跨越数千至数万次事件的特征地震概念进行长期评估，而不是对可能用古地震学技术解决的少数事件进行评估，这项工作将直接导致对影响北方智利和秘鲁南部的大地震的长期时间平均性质进行评估。通过反演平均滑动分布的裂缝方向，这项工作可以帮助灾害响应规划人员预测哪些地区可能受到大地震的影响最大。这一信息与伊基克和阿里卡两个城市（每个城市都涉及数十万人）直接相关，自19世纪末以来，这两个城市还没有经历过大地震。这一部分的重复时间为100至150年，是目前整个南美洲边缘最危险的部分。之前沿着智利边缘的地震已经产生了海啸，在太平洋边缘造成了破坏。例如，1960年智利地震引发的海啸摧毁了夏威夷大岛上的希洛镇。了解伊基克阿里卡段的平均滑动分布将有助于在这些地区建立更准确的海啸生成模型。该项目由NSF地球科学部构造计划和NSF国际科学与工程办公室支持。

The plate boundary along the western edge of South America produces some of the largest earthquakes anywhere on Earth. The size of those earthquakes is fundamentally related to the length of the rupture zone. The purpose of this project is to determine the long term average rupture length using suites of open tension cracks that are well exposed and preserved in the Atacama Desert of northern Chile, the driest place on Earth. To accomplish this task, cracks formed by non-tectonic processes are differentiated from those produced by plate boundary earthquakes. A research team, headed by scientists from Cornell University and Miami University, with expertise in structural geology, arid soil formation, surface exposure dating, and geophysics are visiting key localities in northern Chile to develop objective criteria by which cracks of different origins can be distinguished. By dating different, but adjacent surfaces with different crack densities, the team is determining the rate of crack production, and by studying the isotopic composition of both moist soil along the cracks and gypsum veins formed within the cracks, the role of ground water in crack formation is being determined. Finally, radar interferometry provide images of cracks that open during actual earthquakes affecting the forearc. With objective criteria in hand, crack populations identified on 2 meter resolution imagery throughout northern Chile and southern Peru are evaluated to determine which ones were formed or have been reactivated by large plate boundary earthquakes. The orientations of those cracks can be related to earthquake rupture areas by modeling the static and dynamic stresses associated with both hypothetical and real plate boundary ruptures. The major scientific outcome of this work will be the first long term evaluation of the concept of characteristic earthquakes that spans thousands to tens of thousands of events, rather than the few events that are possible to resolve with paleoseismological techniques.This work will lead directly to an assessment of the long term, time average nature of great earthquakes that affect northern Chile and southern Peru. By inverting the crack orientation for the average slip distribution, the work can help disaster response planners to predict what areas are likely to be most affected by major earthquakes. This information is immediately pertinent to the cities of Iquique and Arica (each involving hundreds of thousands of people) which have not experienced a major earthquake since the late 1800's. With a repeat time of 100 to 150 years, this segment is currently the most dangerous part of the entire South American margin. Previous earthquakes along the Chile margin have produce tsunamis that have caused damage all around the Pacific margin. For example, the tsunami from the 1960 Chile earthquake destroyed the town of Hilo on the big island of Hawaii. Knowing the average slip distribution for the Iquique Arica segment will help build more accurate models of tsunami generation in those areas. The project is supported by the NSF Earth Sciences Division Tectonics Program and the NSF Office of International Science and Engineering.