Geodetic Characterization of the Easternmost Alaska Subduction Zone

俯冲带，即一个构造板块下降到另一个构造板块之下的地方，会产生强烈的地震，从而造成重大的地震危险。了解板块的特征，如下降板块的几何形状，以及它们相互滑动的过程，是评估地震风险程度的关键。两个板块的部分或全部可能会粘在一起，积累的张力可能会在大地震中释放，或者它们可能会更安静地相互滑动，包括在“慢滑事件”中，能量在几天或几年内释放，而不是几秒钟。阿拉斯加中南部是一个极好的实验室，可以用来研究俯冲带中板块和上板块的特征和滑动行为之间的联系。该地区发生了有记录以来的第二大地震（1964年的9.2级），但关于这一边缘的过程仍存在主要问题。地震活动表明阿拉斯加-阿留申板块在阿拉斯加南部部分地区俯冲。这次地震活动在发生9.2级地震的地方的东部突然停止，导致人们猜测这个板块在整个地区可能不是连续的。最近的GPS研究表明，在大部分地区，该板块存在并与上部板块锁定，地震调查也发现了瞬态行为可能发生的证据，但稀疏的数据限制了进一步的工作。来自普渡大学的项目团队正在安装四个新的连续GPS站点和五个临时GPS站点。该团队正在将这些地点的数据与现有地点的数据相结合，以调查该地区的俯冲过程，包括了解板块和上部板块的几何形状，并确定板块锁定的位置和程度。该团队还在寻找瞬变事件的微妙证据；如果发现，这些事件可能与板块几何形状和板块锁定有关。该项目还为当地K-12教师提供了参与实地考察的机会，并了解如何将GPS数据用于构造研究。与这些教师一起制定的课程计划将该项目的研究纳入该地区构造的更大范围视图，并为学生提供使用科学数据的经验。该团队正在通过与国家公园管理局工作人员的互动和公开讲座，将其研究成果传播给更多的受众。总的来说，该项目正在产生并传达对阿拉斯加最东部俯冲带过程的更完整理解，这将提高地震危险性估计。本提案旨在提高对复杂的板块几何形状（如平板俯冲、板块的边缘或弯曲）与俯冲相关过程（如上部板块变形、沿俯冲界面的耦合和瞬态事件（如慢滑））之间关系的理解。来自该地区4个新的连续GPS站点、5个新的活动站点以及现有的连续和活动站点的数据被用于检查应变模式、俯冲板的几何形状、沿俯冲界面的耦合、上板块运动以及沿阿拉斯加最东部俯冲带的瞬变过程。该项目正在解决以下问题：1)俯冲界面的几何形状是什么？活动俯冲界面的东缘在哪里？这与弗兰格尔火山场和观测到的地震活动有什么关系？2)在阿拉斯加最东部的俯冲带内，耦合是如何变化的？从锁定到爬行的转变发生在哪里？这与观测到的震颤、上板块变形和地震活动性有何关系？3)在俯冲界面上是否存在类似慢滑事件的瞬变事件？这些瞬态事件如何与界面几何形状和耦合分布相关？新的数据使得对板块边界特征的评估在空间和时间上的密度比目前可能的要大得多，从而导致对雅库塔板块界面行为及其对该地区构造的影响的更详细和更现实的模型。

Non-Technical SummarySubduction zones, where one tectonic plate descends beneath another, produce great earthquakes and thus pose significant seismic hazards. Understanding the characteristics of the plates, such as the geometry of the descending plate (or slab), and the processes by which they slip past each other is key to assessing the extent of the seismic risk. Parts or all of the two plates may be stuck together, accumulating strain that may be released in a large earthquake, or they may slide past each other more quietly, including in "slow slip events", where energy is released over days or years instead of seconds.South central Alaska is an excellent laboratory for investigating links between slab and upper plate characteristics and slip behavior in subduction zones. The region generated the second-largest earthquake ever recorded (a magnitude 9.2 in 1964), but major questions remain about processes along this margin. Earthquake activity shows that the Alaska-Aleutian slab is subducting beneath part of southern Alaska. This earthquake activity stops abruptly to the east of where the magnitude 9.2 earthquake occurred, leading to speculation that the slab may not be continuous throughout the area. Recent GPS studies have suggested that the slab is present and locked with the upper plate across much of the area and seismic investigations have found evidence that transient behavior may also occur, but sparse data has limited further work.The project team, from Purdue University, is installing four new continuous GPS sites and five temporary GPS sites. The team is combining data from these sites with data from existing sites to investigate subduction processes in the area, including understanding the geometries of the slab and upper plate and determining where, and to what degree, the plates are locked. The team is also searching for subtle evidence of transient events; if found, those events can be related to plate geometries and plate locking. The project is also providing local K-12 teachers the opportunity to participate in fieldwork and learn about how GPS data is used in tectonic studies. Lesson plans developed with these teachers incorporate this project's research into a larger scale view of the region's tectonics and give students experience working with scientific data. The team is spreading its research results to a larger audience through interactions with National Park Service staff and public lectures. Overall, this project is generating and communicating a more complete understanding of processes in the easternmost Alaska subduction zone, which will improve seismic hazard estimates.Technical DescriptionThis proposal seeks to improve understanding of the relationship between complicated slab geometries (e.g. flat slab subduction, edges or kinks in slabs) and subduction-related processes such as upper plate deformation, coupling along the subduction interface, and transient events (e.g. slow-slip). Data from 4 new continuous GPS sites, 5 new campaign sites, and existing continuous and campaign sites in the region are being used to examine strain patterns, the geometry of the subducting slab, coupling along the subduction interface, upper plate motion, and transient processes along the easternmost Alaska subduction zone. The project is addressing the following questions: 1) What is the geometry of the subduction interface? Where is the eastern edge of the active subduction interface? How does this relate to the Wrangell Volcanic field and observed seismicity? 2) How does coupling vary within the easternmost Alaska subduction zone? Where does the transition from locked to creeping occur? How does this relate to the observed tremor, upper plate deformation, and seismicity? 3) Are there transient events resembling slow-slip events on the subduction interface? How do these transient events relate to the interface geometry and coupling distribution? The new data are allowing an assessment of plate boundary characteristics on a much more spatially and temporally dense scale than currently possible, leading to more detailed and realistic models of the behavior of the Yakutat slab interface and its influence of the region's tectonics.