Collaborative Research: Investigating fault geometries and rupture processes in the Nepal Himalaya following the April 25, 2015 Gorkha earthquake

继2015年4月25日戈尔卡7.8级地震和2015年5月12日7.3级地震之后，来自俄勒冈州立大学、加州大学河滨分校、斯坦福大学和德克萨斯大学埃尔帕索分校的合作小组在喜马拉雅山中段发生了80年来最大的地震，在大地震发生的喜马拉雅主断层上部署了由NSF快速资助的由45个地震台(41个宽带和短周期地震仪，14个强震传感器)组成的余震阵列。该阵列被称为尼泊尔阵列，测量余震跟踪地震(NAMASTE)的地震活动，保持了11个月，使一个独特的数据集恰好在活动的喜马拉雅逆冲系统之上。在这项提议中，这个合作小组将专门分析新收集的数据集，以研究断层的几何形状，包括MHT断层、该地区地壳和上地幔的组成、该地震序列的破裂过程，以及喜马拉雅和西藏南部更广泛的大地构造背景。美国和尼泊尔地震学家之间的协作和合作对这一项目至关重要，从而有助于尼泊尔科学基础设施的改善。新收集的高质量的纳玛斯特阵列数据将有助于更好地了解这些大地震的构造背景和破裂过程，并将阐明引发喜马拉雅大地震和大俯冲带地震的过程。具体地说，我们将分析NAMASTE阵列数据，以解决：1)MHT断裂带的几何形状，2)MHT地区的地壳组成，3)余震特征的断层动力学，以及4)更广泛的大地构造背景。为了解决MHT的几何问题，我们将执行精确的余震位置，根据Namaste和其他区域阵列开发强大的地震活动目录，并执行余震的震源特征，这将提供对余震序列中断层几何形状、滑动动力学、震源属性和应力演变状态的有意义的见解。为了确定地壳成分，我们将使用当地地震层析成像和环境噪声层析成像来得出地壳速度，利用速度和地壳成分之间的已知关系，并评估地壳中流体的存在和作用，特别是在活动断裂区域。对于断层动力学，我们将研究余震的时空分布以及与通过反投影获得的破裂斑块的关系，以便更好地了解主震和余震期间发生的破裂过程和应力变化。最后，我们将通过将NAMASTE阵列数据与其他区域台站相结合来探索该地区更广泛的大地构造背景，以从接收函数推导出层析速度模型、地壳和上地幔各向异性以及莫霍面和可能的岩石圈深度。我们将进一步将我们的地震学数据集与其他地球物理和地质数据集(例如大地电磁、重力、地表测绘、形变)进行比较，以得出迄今为止对尼泊尔和周边国家喜马拉雅和西藏南部构造的最好理解。

Following the April 25, 2015 Gorkha magnitude 7.8 and the May 12, 2015 magnitude 7.3 earthquakes, the largest earthquakes to occur in the central segment of the Himalaya in 80 years, a collaborative team from Oregon State University, U.C. Riverside, Stanford University, and the University of Texas at El Paso deployed an NSF RAPID-funded aftershock array of 45 seismic stations (41 broadband and short-period seismometers, 14 strong motion sensors) across the Main Himalayan Thrust (MHT) fault where the large earthquakes occurred. The array, known as the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake (NAMASTE) remained in place for 11 months, allowing for a unique data set right over the active Himalayan thrust system. In this proposal, this same collaborative team will analyze the newly collected data set specifically to study the geometry of faults, including the MHT fault, the composition of the crust and upper mantle in the region, the rupture process of this earthquake sequence, and the broader geotectonic setting in the Himalaya and southern Tibet. Collaboration and cooperation between U.S. and Nepalese seismologists is essential to this project, thereby contributing to improvements of Nepalese scientific infrastructure.The newly collected, high quality NAMASTE array data will allow for a better understanding of the tectonic setting and rupture process of these large earthquakes and will both illuminate the processes causing large Himalayan earthquakes as well as large subduction zone earthquakes. Specifically, we will analyze the NAMASTE array data to address: 1) The geometry of the MHT fault zone, 2) the crustal composition in the MHT region, 3) the fault dynamics from aftershock characterization, and 4) the broader geotectonic setting. To address the geometry of the MHT, we will perform precise aftershock locations, develop a robust seismicity catalog from the NAMASTE and other regional arrays, and perform source characterization of aftershocks that will provide meaningful insights into the fault geometry, slip dynamics, source properties, and evolving state of stress during the aftershock sequence. To determine crustal composition, we will use local earthquake tomography and ambient noise tomography to derive crustal velocities and use known relationships between velocity and crustal composition and assess the presence and role of fluids in the crust, especially in the regions of active faulting. For fault dynamics, we will examine the spatiotemporal distribution of aftershocks and relation with rupture patches obtained by backprojection to allow for a better understanding of the rupture process and stress changes that occurred during the main shock and aftershocks. Finally, we will explore the broader geotectonic setting of the region by combining NAMASTE array data with other regional stations to derive tomographic velocity models, crustal and upper mantle anisotropy, and Moho and possibly lithospheric depths from receiver functions. We will further compare our seismology datasets with other geophysical and geological datasets (e.g., magnetotellurics, gravity, surface mapping, deformation) to derive the best possible understanding to date of the tectonics of the Himalaya and southern Tibet in Nepal and the surrounding countries.