Recoupling the Megathrust: Evaluation of the Transition from Postseismic to Interseismic Behavior in Nicoya Costa Rica

俯冲带是世界上90%以上的地震的罪魁祸首--S，几乎所有的海啸都是由俯冲带引起的。虽然地球科学家了解这种地震的驱动力，但人们对它们为什么会发生在地震发生的地方，特别是俯冲断层(被称为巨型逆冲)如何锁定大地震的发展知之甚少。这些环境仍然很难理解，因为它们通常发生在近海，主要用陆基地球物理仪器很难观察到。然而，哥斯达黎加的尼科亚半岛有一个独特的几何结构，可以对地震活动界面进行陆上观测。此外，那里大约每50年发生一次大地震，最近一次发生在2012年9月。现在，在紧接这次大地震之后的几年里，有一个独特的机会来观察与这一过程相关的巨型逆冲断层的重新耦合和余震的死亡。该项目将支持在最近一次大地震之后的三年期间收集和模拟该区域的地面变形和地震活动，以评估环境如何在未来的地震中恢复到建筑物应力状态。在这项研究中进行的研究将提高我们对地震后刚刚发生的行为与俯冲带中潜在的未来地震之间的关系的理解。这项工作具有重要的社会意义，因为它确定了以前的地面形变数据的效用，以制定更准确的评估在这些地区的地震潜力。该项目将支持一个小组执行两个时间关键的现场GPS运动，并运行当地地震网络，以捕获尼科亚以下的瞬时变形和地震活动，当它过渡到地震之间的行为。该项目将支持一名研究生将活动数据与持续不断的全球定位系统、地震后立即收集的现有活动全球定位系统数据以及先前记录的微震活动同化，以成像和了解活动板块界面上的重新耦合状态。该团队将结合GPS和地震数据来模拟受界面后滑、粘弹性地幔松弛和空间可变界面重新耦合控制的变形。该团队将使用新构建的几何精确的板块界面模型来开发数值模型，该模型识别与下行板块中的海山和缝合线相对应的大型结构特征。受当地台网记录的持续微震活动的约束，模型将区分后滑区和地幔松弛，阐明界面和地幔控制对变形的相对贡献。通过对2012年9月尼科亚地震后几年内随着微震活动的演变而进行的运动和连续GPS的评估，该小组将阐明巨型逆冲耦合的发展。地幔松弛、界面复杂性和相邻的慢滑活动的贡献将有助于理解俯冲带环境中应力的发展和传递。这项工作将提供：1)余震和早期两次地震间微震活动的高分辨率图像；2)2012年尼科亚地震后5年内地面变形变化的空间密集图像；3)界面滑动和随时间重新耦合的详细模型；4)哥斯达黎加北部地幔楔体依赖应力的粘弹性行为的特征；5)建立从晚期两次地震间锁定到同震滑移再到早期两次地震间锁定模型的有意义的比较；以及6)评估俯冲地形在影响新的两次地震间锁定方面的作用。该项目虽然只专注于一种俯冲环境，但跨越了地震间隔期和同震期的晚期。这项工作的结果不仅将用于未来对哥斯达黎加尼科亚沿线的地震周期的评估，而且还将应用于全球俯冲带，因为该社区利用GPS、海底大地测量和其他技术开发了更多和先进的地震间锁定和同震行为图像。

Subduction zones are responsible for over 90% of the world?s largest earthquakes, and are responsible for virtually all tsunami generation. While geoscientists understand the driving force for such earthquakes, little is known about why they occur where they do, and particularly how the subduction fault (termed the megathrust) locks up for the development of major earthquakes. These environments remain difficult to understand because they generally occur offshore, and are difficult to observe with primarily land-based geophysical instrumentation. However, the Nicoya Peninsula of Costa Rica has a unique geometry that allows for land-based observations of the seismically active interface. Additionally large earthquakes occur there approximately every 50 years, with the most recent in September 2012. Now, in the years that immediately follow this major earthquake, a unique opportunity exists to observe the recoupling of the megathrust fault and die-off of aftershocks associated with this process. The project will support the collection and modeling of ground deformation and earthquake activity in the region over a three-year period following this latest large earthquake to evaluate how the environment returns to a state of building stress for future earthquakes. The research performed in this study will improve our understanding of the relationship between behavior just after an earthquake to past, and potentially future earthquakes in a subduction zone. This work is of significant societal relevance as it identifies the utility of prior ground deformation data to develop more accurate assessments of earthquake potential in such zones.This project will support a team to perform two time-critical field GPS campaigns and operate a local seismic network to capture the transient deformation and earthquake activity beneath Nicoya while it transitions back to interseismic behavior. The project will support a graduate student to assimilate the campaign data with ongoing continuous GPS, existing campaign GPS data collected immediately after the earthquake, and prior recorded microseismicity to image and understand the state of recoupling along the active plate interface. The team will combine GPS and seismic datasets to model the deformation as controlled by interface afterslip, viscoelastic mantle relaxation, and spatially variable interface recoupling. The team will develop numerical models using a newly constructed geometrically accurate plate interface model that identifies large structural features that correspond to seamounts and sutures in the downgoing plate. Constrained by ongoing microseismicity recorded by the local network, models will differentiate afterslip regions from mantle relaxation, clarifying the relative contribution of interface and mantle controls on deformation. Through the evaluation of campaign and continuous GPS along with the evolution of microseismicity in the years immediately following the September 2012 Nicoya Earthquake, the team will illuminate the development of megathrust coupling. The contributions from mantle relaxation, interface complexity, and adjacent slow-slip activity will be useful for understanding the development and transfer of stress along the subduction zone environment. This work will provide: 1) high-resolution images of aftershock and early interseismic microseismicity; 2) spatially-dense images of changes in ground deformation in the 5 years following the 2012 Nicoya Earthquake; 3) detailed models of interface slip and recoupling with time; 4) characterization of stress-dependent viscoelastic behavior of the mantle wedge beneath northern Costa Rica; 5) develop meaningful comparisons between the transitions from late-interseismic locking, to coseismic slip, then early-interseismic locking models; and 6) evaluate the role of subducted topography in affecting the new interseismic locking. This project, while focused on only one subduction environment, spans the late-interseismic and coseismic periods. The results from this work will be used for future evaluation of the seismic cycle not just along Nicoya, Costa Rica, but will also be applied to subduction zones globally as the community develops more and advanced images of both interseismic locking and coseismic behavior using GPS, seafloor geodesy, and other techniques.