Collaborative Research: Deciphering upper plate deformation and faulting processes in Central America with integrated geodetic and seismic analyses

在俯冲带中，一个构造板块下降到另一个板块之下的地球，上板块的一部分称为前弧，有时会水平移动，方向几乎垂直于两个板块之间的会聚方向。 为了使这种弧前迁移发生，在上板块中必须存在一个变形带，在那里断层的运动适应了弧前的运动。 然而，变形带中断层的位置和方向，以及它们与上板块火山的关系，在俯冲带中各不相同。 这项研究将提高全球的理解，使弧前迁移的过程中，通过研究他们在尼加拉瓜的科科斯板块俯冲到加勒比板块之下的东北部，和加勒比板块弧前正在向西北移动。 特别是，这项工作将测试尼加拉瓜的火山是否在上板块中产生了薄弱区，这些薄弱区将断层和地震集中在火山之间板块的较强部分。 全球定位系统数据将用于测量与弧前运动有关的剪切的局部位置。 大地震及其余震的位置将得到改进，并用于确定活动断层的方向。 地震波将被用来探测上板块的结构，并确定部分融化上升到火山削弱了上板块。 除了增进对弧前输送过程的了解外，这项工作还将有助于评估地震断层对尼加拉瓜人口构成的危险。该项目将有助于宾夕法尼亚州立大学和布朗大学研究生的教育和职业发展。至少有一名本科生将通过领导联盟（布朗）和PA空间赠款（宾夕法尼亚州立大学）参与该项目，这些项目从STEM领域代表性不足的群体中招募学生。该项目还将通过研究小组会议和课程接触到宾夕法尼亚州立大学和布朗大学更广泛的学生和博士后群体，并将与RI的普罗维登斯的小学进行外展。在地球仪周围的许多俯冲带中，已经记录了由于斜向会聚引起的应变分配和迁移弧前岩块的形成，然而，主要的问题仍然是关于如何在上板中适应这些运动的运动。 关于弧岩浆活动对与弧前搬运有关的上板块断层作用的影响，以及变形和断层作用的表观几何形状在不同俯冲带之间是否以及为什么不同，以及在某些情况下沿着单个弧前长条的走向，对不同俯冲带的研究得出了不同的结论。 该项目将解决这些全球性的问题，通过测试的假设，即弧前运输在尼加拉瓜代表一个端元的情况下，地壳弱化，由于火山中心下的岩浆强烈介导的断层和变形的几何形状。沿弧上板块变形的变化，揭示了一个巨大的旋转方向的震间速度从边缘正常在中央哥斯达黎加边缘平行在尼加拉瓜的弧前银运输是最明显的。在尼加拉瓜，具有大地震的边缘正断层提供了书架断层和弧前块体绕垂直轴旋转的证据。然而，书架断层与弧平行走滑断层的分布，它们在容纳弧前运输中的作用，以及断层与流变学弱弧的关系，仍然不确定。这个项目将利用大地测量和地震数据来检验这样一种假设，即与岩浆中心有关的流变学薄弱区会大大改变上板块变形和断层的模式。 这一假说的四个预测将被检验：1）与弧前搬运有关的变形局限于弧内; 2）断层局限于火山中心之间的强地壳带; 3）变形由书架断层和边缘平行走滑断层提供，其中断层方向部分受火山中心间距的控制;弧前搬运部分受岩浆活动的调节。综合大地测量和地震分析将检验四个模型的预测。对新的和现有的GPS数据的分析将指出最大应变梯度的位置（1）以及剪切和剪切应变率的异常区域（2-4）。GPS导出的同震位移与地震重新定位和震源方向性分析相结合将指示断层几何形状（2-3）。面波和转换体波数据的联合分析和反演将改进上板块结构和地壳及地幔流变学的模型，包括对地壳中部分熔融分布的限制（即，与电弧相关联的弱区）。研究人员将与INETER（尼加拉瓜监测地震和火山的机构）的同事合作，改善与人口有关的上板块地震的危险评估。新的全球定位系统网络装置将扩大COCONet全球定位系统网络的效用。另一个成果是软件和工作流程，使INETER的地震学家能够独立地进行地震序列的双差重定位。该奖项反映了NSF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。

In subduction zones, where one tectonic plate descends into the Earth beneath another, a portion of the upper plate called the fore-arc sometimes moves horizontally in a direction that is nearly orthogonal to the direction of convergence between the two plates. For such fore-arc migration to occur, a zone of deformation must exist in the upper plate where movement on faults accomodates the motion of the fore-arc. However, the locations and orientations of faults in the zone of deformation, and their relationship to volcanoes in the upper plate, differ from one subduction zone to the next. This study will improve a global understanding of the processes that enable fore-arc migration by studying them in Nicaragua where the Cocos plate subducts to the northeast beneath the Caribbean plate, and the Caribbean plate fore-arc is moving to the northwest. In particular, this work will test whether the volcanoes in Nicaragua create weak zones in the upper plate that concentrate faulting and earthquakes in the stronger parts of the plate that lie between volcanoes. GPS data will be used to measure where shearing related to fore-arc motion is localized. The locations of large earthquakes and their aftershocks will be improved and used to determine the orientations of active faults. Seismic waves will be used to probe the structure of the upper plate and determine where partial melt rising up to the volcanoes has weakened the upper plate. In addition to improving understanding of fore-arc transport processes, this work will help in assessing where earthquake faults pose hazards to the population of Nicaragua. The project will contribute to the education and career development of graduate students at Penn State and Brown. At least one undergraduate will work on this project through the Leadership Alliance (Brown), and the PA Space Grant (Penn State), programs that recruit students from groups underrepresented in STEM fields. The project will also reach a broader group of students and postdocs at Penn State and Brown through research group meetings and courses, and will be featured in outreach with elementary schools in Providence, RI.Strain partitioning and the formation of migrating fore-arc terranes due to oblique convergence have been documented in numerous subduction zones around the globe, yet major questions remain regarding how the motion of these terranes is accommodated in the upper plate. Studies in different subduction zones have reached varying conclusions about the impact of arc magmatism on upper plate faulting associated with fore-arc transport, and whether and why the apparent geometry of deformation and faulting varies between subduction zones, and in some cases along the strike of a single fore-arc sliver. This project will address these global questions by testing the hypothesis that fore-arc transport in Nicaragua represents an end-member case where crustal weakening due to magma beneath volcanic centers strongly mediates the geometry of faulting and deformation. Along-arc changes in upper plate deformation are revealed by a dramatic rotation in the direction of interseismic velocities from margin-normal in central Costa Rica to margin-parallel in Nicaragua where fore-arc sliver transport is most pronounced. In Nicaragua, evidence for bookshelf faulting and the rotation of fore-arc blocks about vertical axes is provided by margin-normal faults with large earthquakes. However, the distribution of bookshelf faulting versus arc-parallel strike-slip faults, their roles in accommodating fore-arc transport, and the relationship of faulting to a rheologically weak arc, remain uncertain. This project will use geodetic and seismic data to test the hypothesis that zones of rheological weakness associated with magmatic centers significantly alter the mode of upper plate deformation and faulting. Four predictions of this hypothesis will be tested: 1) deformation associated with fore-arc transport is localized in the arc; 2) faulting is localized in zones of strong crust between volcanic centers; 3) deformation is accommodated by bookshelf faulting and margin-parallel strike-slip faulting, where fault orientation is partially controlled by the spacing of volcanic centers; and 4) fore-arc transport is partially accommodated by magmatism. Integrated geodetic and seismic analyses will test the four model predictions. The analysis of new and existing GPS data will indicate the location of the maximum strain gradient (1) and anomalous regions of shear and dilatational strain rates (2-4). GPS-derived coseismic displacements combined with earthquake relocations and analyses of source directivity will indicate fault geometry (2-3). Joint analyses and inversions of surface wave and converted body wave data will improve models of upper plate structure and crust and mantle rheology, including constraints on the distribution of partial melt in the crust (i.e., weak zones associated with the arc). The investigators will work with colleagues at INETER (the agency that monitors earthquakes and volcanoes in Nicaragua) to improve hazard assessment from upper plate earthquakes in relation to population. New GPS network installations will expand the utility of the COCONet GPS network. Another outcome will be software and a workflow to enable the seismologists at INETER to independently carry out double-difference relocation of earthquake sequences.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.