Collaborative Research: Deep imaging of the south-central Chile margin to understand plate boundary development and its control on megathrust slip behavior

作为“火环”的一部分，南美洲西海岸经常发生大地震。这是因为大洋板块正被推入(俯冲)南美洲大陆之下。智利边缘的高水平活动使其成为研究地质结构如何影响地震行为的绝佳场所，这些知识可以转移到世界各地的其他俯冲带。1960年智利边缘破裂的那部分地区似乎每隔100到150年就会发生一次类似的大地震，而在智利北部，历史模式并不那么规律，有几次较小的地震穿插着偶尔发生的较大地震。该项目的目标是利用朗塞斯号研究船上可用的地震成像工具，深入这一边缘(深度达~15公里)，以确定可能控制其破裂方式的海洋和大陆板块之间边界的物理性质。这一结果将对全球地震灾害评估工作产生广泛的社会影响。这一背景与美国西海岸外的卡斯卡迪亚俯冲带非常类似，更好地了解智利的巨型逆冲应该会为卡斯卡迪亚(以及全球各地的利润率)带来更有见地的危险预测和缓解计划。研究巡航的科学团队将包括来自美国和智利(可能还有其他国家)的教授和学生，并提供地震反射数据采集和处理方面的实践培训。研究巡航也将是一个机会，既可以通过学生主导的博客进行虚拟接触，也可以面对面(通过港口的船只之旅)与学童和公众进行接触。智利中南部俯冲带(S度34-47度)在1960年经历了有仪器记录的最大地震(MW9.5)，并在2010年在北部相邻的重叠部分经历了第六大地震(MW8.8)。1960年地震期间破裂的地段有整个1000公里长的地段有规律破裂的历史，而在2010年地震期间破裂的500公里长的地段通常以较小(100公里)的小块和不规则的间隔破裂。这两项赛事都有良好的滑移模型可供选择，2010年赛事之前还提供了板间耦合模型。由于地震历史悠久，再加上现代对板间滑动和地震间耦合的广泛制约，该地区是比较弧前地质结构沿走向差异并确定它们在控制大地震行为中沿走向差异的作用的绝佳地点。近年来，为了解释在全球俯冲带观察到的不同滑动行为，人们提出了几种观测结果和假说。其中最重要的是观察到，历史上最大的地震(如智利中南部、阿拉斯加、苏门答腊岛和卡斯卡迪亚)往往位于沉积较厚的俯冲带，可能是因为厚厚的沉积物覆盖导致了平滑、均匀的板块边界。该项目将获得地震反射数据，以成像板块边界深处的详细结构，并确定进入板块上的沉积物如何与基底地形相互作用，以及沉积物厚度是否控制导致大地震的上下板块之间的相互作用。

As part of the "ring of fire," the western coast of South America is subject to frequent large earthquakes. This is because the oceanic plate is being thrust under (subducted) beneath the continent of South America. The high level of activity along the Chile margin makes it an excellent place to study how geologic structure can influence earthquake behavior, knowledge that can be transferred to other subduction zones around the world. The portion of the Chile margin that ruptured in 1960 seems to produce similar great earthquakes every 100 to 150 years whereas to the north the historical pattern is less regular, with several smaller earthquakes interspersed with occasional larger earthquakes. The goal of this project is to use seismic imaging tools available on the research vessel Langseth to look deep into this margin (up to ~15 km in depth) to characterize the physical properties of the boundary between the oceanic and continental plates that may control how it ruptures. The results will have broad societal impact by informing efforts to evaluate seismic hazards globally. This setting is a good analog to the Cascadia subduction zone off the westcoast of the US, and a better understanding of the Chilean megathrust should lead to more informed hazard forecasts and mitigation plans for Cascadia (and margins around the globe). The science team for the research cruise will include professors and students from the US and Chile (and possibly other countries) and provide hands-on training in seismic reflection data acquisition and processing. The research cruise will also be an opportunity to reach out both virtually (through student-led blogs) and in person (through ship tours in port) with school children and the general public. The south-central Chile subduction zone (34-47 degrees S) experienced the largest instrumentally-recorded earthquake (Mw 9.5) in 1960 and the sixth largest (Mw 8.8) in 2010 on an adjacent, overlapping segment to the north. The segment that ruptured during the 1960 event has a history of regular rupture of this entire 1000-km long segment, while the 500-km long segment that ruptured during the 2010 event usually ruptures in smaller (100 km) patches and at irregular intervals. Good slip models are available for both events as well as a model for interplate coupling leading up to the 2010 event. Because of the long earthquake history coupled with extensive modern constraints on interplate slip and interseismic coupling, this region is an excellent place to compare the along-strike differences in forearc geologic structure and determine their role in controlling these major along-strike differences in megathrust earthquake behavior. Several observations and hypotheses have been presented in recent years to explain the different slip behaviors observed in subduction zones globally. Foremost among these is the observation that the largest historic earthquakes (e.g. south-central Chile, Alaska, Sumatra and Cascadia) tend to be in thickly-sedimented subduction zones, possibly because a thick sediment cover leads to a smooth, homogeneous plate-boundary. This project will obtain seismic reflection data to image the detailed structure of the plate boundary at depth and determine how sediment on the incoming plate interacts with basement topography and whether sediment thickness controls the interaction between the upper and lower plates that leads to great earthquakes.