Collaborative research: Quantifying incoming plate hydration and role of fluids on megathrust properties in and around the Guerrero Gap, offshore Mexico

板块边界在俯冲带的设置是众所周知的，表现出广泛的行为，两个构造板块如何滑动过去对方。许多问题仍然是关于什么控制了世界各地不同俯冲带系统内部和之间的广泛的滑动行为。海水进入俯冲带的传入海洋板块通常被调用来解释板块界面滑动行为的变化。然而，并入下行大洋板块的海水的体积和分布及其在深度的命运因俯冲带而异，由于现有观测数量有限，人们对这些情况知之甚少。墨西哥近海中美洲俯冲带的格雷罗缺口和邻近部分是世界上板块边界滑动行为沿走向变化的最著名的例子之一。格雷罗缺口在100多年来没有在7级地震中破裂，但已知最大的相对较浅的慢滑事件，释放能量缓慢，不产生强烈的地震波发生约每4年。相比之下，相邻的段在7级地震中破裂，似乎被强烈锁定。该项目将涉及采集和分析格雷罗地震空区和邻近区段内的海洋主动源地震数据，以获得对俯冲带断层的性质和几何形状的关键制约因素，包括进入的大洋板块和俯冲带内的流体丰度和分布情况。这个项目的结果将是有价值的理解流体（如海水）的作用，在影响对比滑动行为都在这个边缘和类似的俯冲带。该项目将支持两名早期职业科学家和一名准终身科学家，并将涉及美国，墨西哥和日本科学家之间的强有力合作。拟议的研究还将有力地补充由墨西哥和日本合作者领导的正在进行的两栖大地测量和地震部署。将在Marcus G. R/V上进行为期47.5天的二维广角地震反射/折射和超长偏移距多道地震（MCS）联合勘探。朗塞斯该项目包括六个主要剖面沿着采集MCS和海底地震仪数据，以及将地震成像扩展到更大范围的边缘的附加MCS剖面。这项研究的格雷罗差距和邻近的部分将产生独特的见解到两个核心问题：（1）是什么样的水化状态的传入，年轻的沉积物饥饿的科科斯海洋板块在中美洲俯冲带，以及它如何变化沿走向和有关的俯冲带行为的变化？MCS成像以及来自OBS和拖缆数据的P和S波速度模型，包括全波形反演以及炮点和地震的联合反演，将提供在从脊轴向沟轴俯冲之前以及在格雷罗间隙内外的传入板块中水的数量和分布的第一次估计，使我们能够研究流体在俯冲带过程中的作用，并将结果与年轻但沉积良好的卡斯卡迪亚和南开俯冲板块进行比较，（2）俯冲和覆盖板块的几何和材料性质如何影响沿着巨型逆冲断层的滑动行为？可控震源地震数据将对俯冲带上部20-30 km的结构和性质带来新的高分辨率约束，这些俯冲带位于具有不同程度耦合和滑动行为的巨型逆冲断层的部分。这将使测试竞争的想法控制在其他俯冲带的滑动行为，如孔隙流体压力或断层带的异质性的变化。解决这个问题将需要一个综合的限制板块边界属性从这个项目的结果，从一个正在进行的多年部署的大地测量和地震站的滑动行为内的格雷罗Gap.This奖项反映了NSF的法定使命，并已被认为是值得通过使用该基金会的智力价值和更广泛的影响审查标准进行评估的支持。

The plate boundary in subduction zone settings is known to exhibit a wide spectrum of behavior for how the two tectonic plates slide past each other. Many questions remain about what controls the wide spectrum of slip behavior within and between different subduction zone systems worldwide. Seawater delivered into the subduction zone by the incoming oceanic plate is commonly invoked to explain variability of plate interface slip behavior. However, the volume and distribution of seawater incorporated into the downgoing oceanic plate and its fate at depth vary between subduction zones and is poorly known owing to the limited amount of existing observations. The Guerrero Gap and neighboring segments at the Middle America subduction zone, offshore Mexico, is one of the best-known examples in the world of along-strike variations in slip behavior of the plate boundary. The Guerrero Gap has not ruptured in a M7 earthquake in more than 100 years but the largest known relatively shallow slow-slip events that release energy slowly without generating strong seismic waves occur approximately every 4 yrs. In contrast, the neighboring segments have ruptured in M7 earthquakes and appear to be strongly locked. This project will involve the acquisition and analysis of marine, active-source seismic data within the Guerrero Seismic Gap and neighboring segments in order to obtain critical constraints on the properties and geometry of the subduction zone faults, including the abundance and distribution of fluids in both the incoming oceanic plate and within the subduction zone. Results from this project will be valuable for understanding the role of fluids (e.g. seawater) in influencing the contrasting slip behavior both at this margin and similar subduction zones. This project will support two early career scientists and one pre-tenured scientist and will involve a strong collaboration between US, Mexican and Japanese scientists. The proposed study will also strongly complement an ongoing amphibious geodetic and seismic deployment led by Mexican and Japanese collaborators. A 47.5 day, combined 2D wide-angle seismic reflection/refraction and ultra-long offset multi-channel seismic (MCS) survey will be conducted onboard the R/V Marcus G. Langseth. This project includes six primary profiles along which both MCS and Ocean Bottom Seismometer (OBS) data will be acquired, and additional MCS profiles that extend seismic imaging over a larger extent of the margin. This study of the Guerrero Gap and neighboring segments will yield unique insights into two core questions: (1) What is the hydration state of the incoming, young sediment-starved Cocos oceanic plate at the Middle America subduction zone, and how does it vary along-strike and relate to changes in subduction zone behavior? MCS imaging and P and S- waves velocity models from OBS and streamer data, including full-waveform inversion as well as joint inversion of shots and earthquakes, will provide the first estimates of the amount and distribution of water in the incoming plate prior to subduction from the ridge axis to the trench axis and both in and outside of the Guerrero Gap, allowing us to examine the role of fluids in subduction zone processes and results will be compared the young but well-sedimented Cascadia and Nankai subducting plates, (2) How do geometrical and material properties of the subducting and overriding plates influence slip behavior along the megathrust fault? Controlled- source seismic data will bring new, high-resolution constraints on the architecture and properties of the upper 20-30 km of this subduction zone on parts of the megathrust that have different degrees of coupling and slip behavior. This will enable the testing of competing ideas for controls on slip behavior developed at other subduction zones, such as variations in pore-fluid pressure or fault zone heterogeneity. Addressing this question will require a synthesis of constraints on plate boundary properties from this project with results on slip behavior from an ongoing multi-year deployment of geodetic and seismic stations within the Guerrero Gap.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.