Collaborative Research: Filling in the Central Himalayan Seismic Gap: A Structural, Neotectonic, and Paleoseismic Investigation of the Western Nepal Fault System

2015年发生的导致9000人死亡的Gorkha Mw 7.8地震，是沿着一条巨型逆冲--主要的喜马拉雅逆冲--的破裂造成的，印度次大陆俯冲到变形的喜马拉雅带和亚洲大陆之下。大多数巨型逆冲断裂系统，其中一个构造板块被压在另一个构造板块之下，发生在世界海洋深处，由于难以进入海底，观测受到限制。这些系统产生了世界上有记录以来最大的地震，通常由一个主要断层和额外的伸展断层组成，它们共同作用以适应构造板块之间的碰撞。最近在Gorkha震中以西发现的一个复杂的断裂系统，即尼泊尔西部断裂系统，似乎在喜马拉雅碰撞中发挥了重要作用，并对尼泊尔的一大片地区构成了重大地震危险。这个项目将记录长期的运动速度，确定史前地震的时间和位置，并绘制出世界地震网络的全部范围。这些数据将被用来制定对区域人口地震风险暴露的限制，并为低成本、可持续和对文化敏感的地震风险缓解战略提供建议。该项目将推动其他预期的社会成果，例如妇女充分参与STEM，提高公众的科学素养，通过参加当地的公共外联活动提高公众对STEM的参与，通过本科生和研究生培训培养多样化的、具有全球竞争力的STEM劳动力队伍，以及促进国际合作。地质和地球物理学观察长期以来表明，斜向板块会聚的地区往往形成滑动分隔系统，其中基底的巨型逆冲容纳了边缘垂直会聚，走滑展布或弧后断裂容纳了边缘平行运动。考虑到大多数收敛边缘的曲率，收敛倾角通常沿走向变化，在某个点变为零。然而，由于这些带在近海俯冲带中通常是不可到达的，这些伸展断层的运动学和地震周期行为鲜为人知。该项目的目标是新发现的尼泊尔西部断裂系统，它是喜马拉雅逆冲楔内的一个主要的、地下的、暴露良好的张开断裂系统，它似乎将右旋应变从喜马拉雅西北弧后斜向会聚的喀喇昆仑断层转移到喜马拉雅弧前中部，那里的会聚主要是边缘正常的。该项目的目的是检验多个工作假说，这些假说利用WNFS的变形模式来约束区域滑动分割和张开断裂模型的断层分段和连接模型：(1)强应变分割假说，其中WNFS是一个连接良好的活动断裂系统，它从西北部的喀喇昆仑-古拉曼达塔/胡姆拉断裂系统传递滑动，穿过逆冲楔体，分支的主锋推力在楔形前部；(2)弱应变分割假说，WNFS是一组断裂带的集合，它广泛地容纳了弧形平行走滑断裂和喜马拉雅弧条的西北平移；(3)逆冲断层和斜坡道假说，认为逆冲断层是一组在逆冲楔体内适应缩短的断层。为了解决这些假设并利用对区域地震灾害的新限制，研究小组将开展一项为期三年的多学科调查，包括三个主要部分：(1)构造地质学、新构造学、地质力学和地质年代学与古地震学，以约束(A)活动断裂系统的几何和运动学，(B)位移震级，(C)断层滑移率，以及(D)利用古地震学的应变释放的历史模式；(2)综合地质研究结果的最新概率地震灾害和风险分析；以及(3)在家庭到社区范围内对结构性和非结构性脆弱性减少战略进行效益-成本分析，为通过缓解和恢复减少风险和提高复原力提供建议。活动断层填图、滑动速率和古地震数据也是地震危险性分析的基本组成部分。与尼泊尔的合作者合作，结果将被用于提供最新的概率地震风险分析，并为被广泛认为处于高地震风险的地区制定实用的缓解策略。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The 2015 Gorkha Mw 7.8 earthquake, which caused 9,000 fatalities, resulted from rupture along a megathrust - the Main Himalayan thrust - where the Indian sub-continent subducts beneath deformed Himalayan belt and the Asian continent. Most megathrust fault systems, where one tectonic plate is being forced underneath another tectonic plate, occur deep beneath the world's oceans where observations are limited due to difficult submarine access. These systems produce the largest recorded earthquakes in the world and typically consist of a primary fault and additional splay faults that work together to accommodate collision between the tectonic plates. A recently discovered complex fault system west of the Gorkha epicentral region, the Western Nepal Fault System (WNFS), appears to play a major role in accommodating the Himalayan collision and poses a major seismic hazard for a large region of Nepal. This project will document the long-term rate of motion, determine the timing and location of prehistoric earthquakes, and map out the full extent of the WNFS. The data will be used to develop constraints on the seismic hazard exposure of regional populations and provide recommendations for low-cost, sustainable, and culturally sensitive earthquake risk mitigation strategies. The project would advance other desired societal outcomes such as full participation of women in STEM, increased public scientific literacy and public engagement with STEM through participation in local public outreach activities, development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training, and fostering international collaboration.Geologic and geophysical observations have long shown that areas of oblique plate convergence tend to form slip partitioned systems, where the basal megathrust accommodates margin-perpendicular convergence, and a strike-slip splay or backarc fault accommodates margin-parallel motion. Given the curvature of most convergent margins, convergence obliquity often varies along strike, becoming zero at some point. However, because these zones are usually inaccessible in offshore subduction zones, the kinematics and earthquake cycle behavior of these splay faults are poorly known. This project targets the newly discovered Western Nepal Fault System as a major, subaerial, well-exposed splay fault system within the Himalayan thrust wedge that appears to transfer dextral strain from the Karakoram Fault in the obliquely convergent northwest Himalayan backarc to the central Himalayan forearc where convergence is primarily margin-normal. The objective of this project is to test multiple working hypotheses that use the deformation patterns of the WNFS to constrain models of fault segmentation and linkage for regional slip-partitioning and splay faulting models: (1) strong strain partitioning hypothesis in which the WNFS is a well-connected system of active faults that transfer slip from the Karakoram- Gurla Mandhata/Humla fault system in the northwest, across the thrust wedge, and branches with the Main Frontal Thrust at the front of the wedge; (2) weak strain partitioning hypothesis in which the WNFS is a collection of disconnected faults that broadly accommodate arc-parallel strike-slip faulting and northwest translation of the Himalayan arc sliver; and (3) thrust faulting and oblique ramp hypothesis in which the WNFS is a collection of faults that accommodate shortening within the thrust wedge. To address these hypotheses and leverage new constraints on regional seismic hazard, the research team will carry out a three year multidisciplinary investigation consisting of three primary components: (1) structural geology, neotectonics, geomechanics, and geochronology with paleoseismology to constrain (a) the geometries and kinematics of active fault systems, (b) displacement magnitude, (c) fault slip rates, and (d) historical patterns of strain release using paleoseismology; (2) an updated probabilistic seismic hazard and risk analysis integrating the results of the geologic studies; and (3) a benefit-cost analysis of structural and non-structural vulnerability-reducing strategies at household to community scales to provide recommendations for reducing risk and promoting resilience through mitigation and recovery. The active fault mapping, slip rate, and paleoearthquake data are also fundamental components of seismic hazard analyses. Working with Nepali collaborators, the results will be used to provide updated probabilistic seismic hazard analysis and develop practical mitigation strategies for a region widely thought to be at heightened seismic risk.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.