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”，它是一个走滑的板块边界，将加拿大西部和阿拉斯加东南部的太平洋和北美构造板块隔开。QCF可以说是世界上最活跃的这类断层：在上个世纪的七次M7地震中，整个近海长约900公里的断层破裂，它保持着已知的最高形变速率(50毫米/年)。该断裂系统是除卡斯卡迪亚以外对阿拉斯加东南部和加拿大最大的地震危害，并在1949年引发了加拿大S有记录以来最大的地震(8.1级)。尽管在2012年和2013年M7级地震后做出了快速反应，但由于缺乏现代地球物理成像，有关地震期间断层系统如何变形和控制断层失效的过程的一级问题仍然没有得到回答。这项实验将是第一次在区域尺度上对这一板块边界进行深度表征的全面尝试。利用来自海洋声波和地震源的地震能量，该项目将测量地震活动的深度和程度，对深部的断层带进行成像，并确定断层上的速度和热结构。所有这些数据将有助于更好地理解这一点，以及其他主要的走滑断层系统，以便更好地进行危险评估和地震预报。这个科学团队是一个由美国和加拿大研究人员组成的国际协作小组，由三名职业生涯早期的女性领导。将通过阿拉斯加锡特卡科学中心的驻留以及在当地高中和社区中心的讲座来开展对当地社区的外联活动。尽管陆洋转换缘在地球-S板块构造体系中具有重要作用，但与陆洋板块交汇边界相比，陆洋转换缘的时空演化研究较少。由于密度和热结构不同的岩石圈并置，大陆-海洋转换断层可能是俯冲开始的最有利的构造环境之一--小程度的会聚可能导致失败。QCF系统提供了一个理想的位置来研究大陆-海洋转换断层如何响应在岩石圈尺度上系统增加的会聚程度。研究区包括两个潜在的断层段边界，这两个边界标志着海侵变形机制的突变，海底形态和浅层地震反射结构的变化表明：南部的应变分配和俯冲作用，北部的高度局部化走滑变形。由于缺乏关于微震深度和位置、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，许多基本问题都没有得到解答：为什么QCF在它所在的地方形成，它的形变历史是什么？PAC沿边缘俯冲的历史和最大汇聚区以北的俯冲物质的命运是什么？控制大洋-大陆转换地震成因的主要物理条件是什么？走滑和挤压应变是如何在地质和孕震时间尺度上调节和分配的？采用主动和被动震源相结合的海洋地震成像策略，这项研究将表征地壳和上地幔的速度结构、断裂带的结构和流变性以及地震活动。数据将使用R/V Marcus G.Langseth的长炮检距2D地震反射和广角反射-折射能力以及美国-加拿大组合的宽带海底地震仪阵列来获取。该奖项反映了美国国家科学基金会的法定使命，并已通过使用基金会的智力优势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.