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

作为“火山带”的一部分，南美洲西海岸经常发生大地震。这是因为大洋板块被推入（俯冲）到南美洲大陆之下。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地点，这些知识可以转移到世界各地的其他俯冲带。1960年智利边缘破裂的部分似乎每100到150年就会发生一次类似的大地震，而北部的历史模式则不那么规律，偶尔会有几次较小的地震穿插在偶尔发生的大地震中。该项目的目标是利用Langseth研究船上可用的地震成像工具深入研究这一边缘（深度约15公里），以表征海洋和大陆板块之间边界的物理特性，这些物理特性可能会控制它的破裂方式。研究结果将通过为评估全球地震灾害的努力提供信息，产生广泛的社会影响。这种情况很好地模拟了美国西海岸外的卡斯卡迪亚俯冲带，对智利巨型逆冲断层的更好了解应该会为卡斯卡迪亚（以及全球各地的边缘）带来更明智的灾害预测和减灾计划。这次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，通过虚拟（通过学生主导的博客）和亲自（通过在港口乘船游览）与学生和公众接触。智利中南部俯冲带（南纬34-47度）在1960年经历了仪器记录的最大地震（里氏9.5级），2010年在其北部相邻的重叠部分经历了第六大地震（里氏8.8级）。在1960年事件中破裂的部分在整个1000公里长的部分中有规律的破裂历史，而在2010年事件中破裂的500公里长的部分通常以较小的（100公里）斑块和不规则间隔破裂。对于这两个事件都有很好的滑动模型，以及导致2010年事件的板块间耦合模型。由于该区地震历史悠久，再加上现代对板间滑动和地震间耦合的广泛约束，是比较弧前地质构造沿走向差异并确定其在控制大逆冲地震行为中主要沿走向差异中的作用的绝佳场所。近年来提出了一些观测和假设来解释在全球俯冲带观测到的不同滑动行为。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、苏门答腊和卡斯卡迪亚）往往发生在厚沉积的俯冲带，这可能是因为厚沉积物覆盖导致了光滑、均匀的板块边界。该项目将获取地震反射数据，对板块边界的深层详细结构进行成像，并确定进入板块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了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.