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产生了一些地球上最大的地震。这些地震的大小基本上与破裂带的长度有关。这个项目的目的是利用在智利北部阿塔卡马沙漠（地球上最干燥的地方）暴露和保存良好的开放式张力裂缝套件来确定长期平均破裂长度。为了完成这项任务，将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（Cornell University）和迈阿密大学（Miami University）的科学家领导的一个研究小组，在构造地质学、干旱土壤形成、地表暴露年代测定和地球物理学方面具有专业知识，他们正在访问智利北部的关键地区，制定客观标准，以区分不同来源的裂缝。通过测定不同但相邻表面不同裂缝密度的年代，研究小组正在确定裂缝产生的速率，并通过研究裂缝沿线湿润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时打开的裂缝图像。有了客观标准，在智利北部和秘鲁南部的2米分辨率图像上识别出的裂缝数量进行了评估，以确定哪些是由大板块边界地震形成或重新激活的。这些裂缝的方向可以通过模拟与假设和实际板块边界破裂相关的静态和动态应力来与地震破裂区有关。这项工作的主要科学成果将是对跨越数千到数万个事件的特征地震概念的第一次长期评估，而不是用古地震学技术可能解决的少数事件。这项工作将直接导致对影响智利北部和秘鲁南部的大地震的长期、时间平均性质的评估。通过反演裂缝的方向来获得平均滑动分布，这项工作可以帮助灾难应对规划者预测哪些地区可能受到大地震的影响最大。这一信息与伊基克市和阿利卡市（每个城市都有数十万人）息息相关，这两个城市自19世纪末以来就没有经历过大地震。每隔100到150年，这一段是目前整个南美边缘最危险的部分。以前在智利边缘发生的地震引发了海啸，对整个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-非洲板块的平均滑动分布将有助于在这些地区建立更准确的海啸产生模型。该项目由美国国家科学基金会地球科学部构造计划和美国国家科学基金会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.