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

2015年的廓尔喀7.8级地震造成9,000人死亡，是由于沿着一个大型逆冲断层--喜马拉雅主逆冲断层--发生断裂，印度次大陆俯冲到变形的喜马拉雅带和亚洲大陆之下。大多数巨型逆冲断层系统，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，通常由一个主断层和额外的展布断层组成，这些断层共同作用以适应构造板块之间的碰撞。最近发现的复杂的断层系统西部的廓尔喀震中地区，尼泊尔西部断层系统（WNFS），似乎发挥了重要作用，容纳喜马拉雅碰撞，并构成了重大的地震危险，尼泊尔的一个大区域。该项目将记录长期的运动速率，确定史前地震的时间和位置，并绘制出WNFS的完整范围。这些数据将用于制定对区域人口地震灾害风险的限制，并为低成本、可持续和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，如妇女充分参与STEM，通过参与当地公共宣传活动提高公众科学素养和公众对STEM的参与，通过本科生和研究生培训发展多元化的具有全球竞争力的STEM劳动力，以及促进国际合作。地质和地球物理观测长期以来表明，斜向板块会聚的区域往往形成滑动分隔系统，其中基底巨型逆冲断层适应边缘垂直会聚，而走滑展布或弧后断层适应边缘平行运动。考虑到大多数会聚边缘的曲率，会聚倾角通常沿着走向变化，在某个点变为零。然而，由于这些地区通常是无法进入近海俯冲带，运动学和地震周期的行为，这些展布断层知之甚少。该项目的目标是新发现的西尼泊尔断层系统作为一个主要的，陆上的，暴露良好的扇断层系统在喜马拉雅逆冲楔，似乎转移右向应变从喀喇昆仑断层在倾斜收敛的西北喜马拉雅后弧中央喜马拉雅前弧收敛主要是边缘正常。本项目的目的是测试多个工作假设，这些假设使用WNFS的变形模式来约束断层分割模型和区域滑动分割和展布断层模型的连接：（1）强应变分配假说，其中WNFS是一个连接良好的活动断层系统，该系统从西北部的Karakoram- Gurla Mandhata/Humla断层系统转移滑动，穿过逆冲楔，（2）弱应变分配假说，其中WNFS是一组不连续的断层，广泛地适应弧平行走滑断层和喜马拉雅弧条的西北平移;（3）逆冲断层和斜坡道假说，其中WNFS是一组断层，适应逆冲楔内的缩短。为了解决这些假设并利用区域地震危险的新限制，研究小组将进行为期三年的多学科调查，包括三个主要组成部分：（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）活动断层系统的几何学和运动学，（B）位移量，（c）断层滑动速率，（d）利用古地震学的应变释放的历史模式;（2）结合地质研究结果的最新概率地震危险和风险分析;以及（3）结构性和非结构性脆弱性的效益-成本分析-从家庭到社区的减少战略，为通过减缓和恢复减少风险和提高复原力提供建议。活动断层图、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。通过与尼泊尔合作者的合作，研究结果将被用于提供最新的概率地震危险性分析，并为一个被广泛认为地震风险较高的地区制定实用的减灾策略。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。

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.