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

俯冲带占世界面积的90%以上？是美国最大的地震，也是几乎所有海啸产生的原因。虽然地球科学家了解此类地震的驱动力，但他们对地震发生的原因知之甚少，特别是俯冲断层（称为大逆冲断层）如何锁定大地震的发展。这些环境仍然难以理解，因为它们通常发生在海上，并且很难用主要的陆地地球物理仪器进行观测。然而，哥斯达黎加的尼科亚半岛具有独特的几何形状，可以在陆地上观测地震活跃界面。此外，大地震大约每50年发生一次，最近一次发生在2012年9月。现在，在这次大地震之后的几年里，有一个独特的机会可以观察到巨型逆冲断层的重新结合以及与此过程相关的余震的消失。该项目将支持该地区在最近一次大地震后的三年时间内收集和模拟地面变形和地震活动，以评估环境如何恢复到未来地震的建筑应力状态。在这项研究中进行的研究将提高我们对地震后行为与过去以及潜在的未来俯冲带地震之间关系的理解。这项工作具有重要的社会意义，因为它确定了先前地面变形数据的效用，以在这些地区开发更准确的地震潜力评估。该项目将支持一个团队执行两个时间关键的现场GPS活动，并运行一个本地地震网络，以捕捉尼科亚地下的瞬态变形和地震活动，同时它将转变为地震间行为。该项目将支持一名研究生将活动数据与正在进行的连续GPS数据、地震后立即收集的现有活动GPS数据以及先前记录的微地震活动相结合，以成像并了解沿活动板块界面重新耦合的状态。该团队将结合GPS和地震数据集，模拟由界面后滑、粘弹性地幔松弛和空间可变界面重耦合控制的变形。该团队将使用一个新构建的几何精确的板块界面模型来开发数值模型，该模型可以识别与海底山和下行板块缝合线相对应的大型结构特征。受局地台网记录的持续微震活动的约束，模型将区分余震区和地幔松弛区，阐明界面和地幔控制对变形的相对贡献。通过对活动和连续GPS的评估，以及2012年9月尼科亚地震后几年微震活动的演变，该团队将阐明巨型逆冲耦合的发展。地幔弛豫、界面复杂性和邻近慢滑活动的贡献将有助于理解俯冲带环境的应力发育和传递。这项工作将提供：1)余震和早期震间微震活动的高分辨率图像；2) 2012年尼科亚地震后5年地面形变的空间密集影像；3)界面滑移与时间耦合的详细模型；4)哥斯达黎加北部地幔楔的应力依赖粘弹性特征；5)对从晚震间锁定到同震滑动，再到早震间锁定模型的转变进行有意义的比较；6)评价俯冲地形对新的震间锁闭的影响。该项目虽然只关注一个俯冲环境，但跨越了晚震间期和同震期。这项工作的结果不仅将用于哥斯达黎加Nicoya地震周期的未来评估，而且还将应用于全球俯冲带，因为该社区使用GPS、海底大地测量学和其他技术开发了更多更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.