Collaborative Research: Relationship between plate boundary obliquity, strain accommodation, and fault zone geometry at oceanic-continental transforms: The Queen Charlotte Fault

夏洛特女王断层（QCF）系统通常被称为“北方的圣安德烈亚斯”，是一个走滑板块边界，将太平洋和北美构造板块分开，加拿大西部近海和阿拉斯加东南部。QCF可以说是世界上这种类型中最活跃的断层：在上个世纪，整个约900公里的离岸长度在7次7级地震中破裂，并且它保持着已知的最高变形率（50毫米/年）。该断层系统代表了阿拉斯加东南部和加拿大卡斯卡迪亚以外最大的地震危险，并导致加拿大？美国有记录以来最大的地震（8.1级）发生在1949年。尽管在2012年和2013年的7级地震之后做出了快速反应，但由于缺乏现代地球物理成像技术，关于地震期间断层系统如何变形以及控制断层破坏的过程的一级问题仍然没有得到解答。这次实验将是第一次在区域尺度上对该板块边界进行深度表征的综合尝试。该项目将利用来自海洋声源和地震源的地震能量，测量地震活动的深度和程度，对断裂带进行深度成像，并确定断层的速度和热结构。所有这些数据将使人们更好地了解这个断层和其他主要的走滑断层系统，以便更好地进行灾害评估和地震预报。该科学团队是一个由美国和加拿大研究人员组成的国际合作小组，由三名早期职业女性领导。与当地社区的接触将通过在阿拉斯加锡特卡科学中心的驻留和在当地高中和社区中心的讲座来进行。与收敛的大陆-海洋板块边界相比，大陆-海洋转换边缘的时空演化研究尚不充分，尽管它们在地球上扮演着重要的角色。S板块构造体系。大陆-海洋转换断层可能是俯冲起始最有利的构造环境之一，因为密度和热结构不同的岩石圈并置在一起——小程度的汇聚可能导致失败。QCF系统提供了一个理想的位置来研究大陆-海洋转换断层如何在岩石圈尺度上系统地响应不断增加的收敛程度。研究区包括两个潜在的断层段边界，它们标志着海底形态和浅层地震反射结构的变化表明了逆压变形机制的突变：南部的应变划分和逆冲作用过渡到北部高度局部化的走滑变形。由于缺乏有关微地震深度和位置、深部断层的变形历史和几何形状以及岩石圈速度结构的信息，许多基本问题没有得到解答：为什么QCF会在这里形成，它的变形历史是怎样的？PAC沿边缘逆冲的历史是怎样的？最大辐合区以北逆冲物质的命运又是怎样的？控制海陆转换地震发生的主要物理条件是什么？走滑和压应变在地质和发震时间尺度上是如何调节和划分的？利用主动源和被动源相结合的海洋地震成像策略，本研究将对地壳和上地幔速度结构、断裂带结构和流变学以及地震活动性进行表征。数据将使用R/V Marcus G. Langseth的长偏移二维地震反射和广角反射折射能力，以及由64台仪器组成的美加宽带海底地震仪阵列，部署约1年。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Often called the "San Andreas of the North", the Queen Charlotte fault (QCF) system is a strike-slip plate boundary that separates the Pacific and North American tectonic plates offshore western Canada and Southeast Alaska. The QCF is arguably the most active fault of its type in the world: the entire ~900 km offshore length has ruptured in seven M7 earthquakes during the last century and it sustains the highest known deformation rates (50 mm/yr). The fault system represents the largest seismic hazard to southeastern Alaska and Canada outside of Cascadia, and caused Canada?s largest recorded earthquake (M8.1) in 1949. Despite rapid response efforts following M7 earthquakes in 2012 and 2013, first-order questions regarding how the fault system deforms and the processes controlling fault failure during earthquakes remain unanswered due to the lack of modern geophysical imaging. This experiment will be the first comprehensive attempt to characterize this plate boundary at depth on a regional scale. Using seismic energy from marine acoustic and earthquake sources, the project will measure the depth and extent of seismicity, image the fault zone at depth, and determine velocity and thermal structure across the fault. All these data will lead to an improved understanding of this, and other major strike-slip fault systems, for better hazard assessment and earthquake forecasting. The science team is a collaborative, international group of US and Canadian researchers, led by three early-career women. Outreach to local communities will be conducted through a residency at the Sitka Science Center in Alaska and lectures at local high schools and community centers. Compared to convergent continental-oceanic plate boundaries, the time-space evolution of continental-oceanic transform margins is understudied, despite their important role in the planet?s plate tectonic system. Continental-oceanic transform faults are potentially one of the most favorable tectonic settings for subduction initiation due to the juxtaposition of lithospheres of contrasting density and thermal structure -- small degrees of convergence can lead to failure. The QCF system provides an ideal location to investigate how a continental-oceanic transform fault responds to systematically increasing degrees of convergence at the lithospheric scale. The study area includes two potential fault segment boundaries that mark abrupt changes in transpressive deformation mechanisms as suggested by changes in seafloor morphology and shallow seismic reflection structure: strain partitioning and underthrusting in the south transition to highly localized strike-slip deformation in the north. Lack of information on microseismic depths and locations, the deformation history and geometries of faults at depth, and lithospheric velocity structure leave multiple fundamental questions unanswered: Why has the QCF formed where it is, and what is its deformation history? What is the history of PAC underthrusting along the margin and the fate of underthrust material north of the area of maximum convergence? What are the primary physical conditions controlling seismogenesis along oceanic-continental transforms? How are strike-slip and compressive strain accommodated and partitioned over geologic and seismogenic timescales? Using a combined active- and passive-source marine seismic imaging strategy, this research will characterize crustal and uppermost mantle velocity structure, fault zone architecture and rheology, and seismicity. Data will be acquired using long-offset 2D seismic reflection and wide-angle reflection-refraction capabilities of the R/V Marcus G. Langseth and a combined US-Canadian broadband ocean bottom seismometer array of 64 instruments deployed for ~1 year.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.