Collaborative Research: A joint seismic and geodetic investigation into the structure and behavior of an intracontinental subduction zone, Nepal

俯冲带，一个构造板块在另一个板块下面滑动，产生世界上最大的地震，通常也是最具破坏性的地震。在典型的俯冲带中，地震发生的部分位于近海和水下，在几公里的海洋之下，那里的下沉板块是海洋。在许多这些地区，最严重的危险是海啸造成的，而不是地震本身造成的震动。由于位于海底，大多数俯冲带大逆冲断层的详细特征难以确定：大逆冲断层的形状如何；是一个尖锐的、离散的界面，还是一个较厚的、分布的剪切层；这些特征在大地震的形成、发生和传播过程中是如何表现的？然而，在喜马拉雅之下，印度大陆板块在西藏之下俯冲，形成了一种独特的情况，整个俯冲带都在陆地上，由地震台站测量，从太空成像，并通过连续的大地测量标记进行监测。由于大型人口中心位于大型逆冲断层的浅层部分之上，因此地震的风险比典型的海洋俯冲带要高得多。2015年尼泊尔廓尔喀大地震（里氏7.8级和7.3级）摧毁了加德满都、珠峰大本营和周边地区，为通过研究大陆俯冲带的数据更直接地研究大型逆冲断层的一般特性提供了难得的机会。廓尔喀地震破坏了喜马拉雅主逆冲断层——印度和欧亚大陆之间的巨型逆冲板块边界断层。地震传感器（记录地震期间的震动）和大地测量卫星（记录地震引起的地表变形）以一种前所未有的方式记录了地震及其余震。本项目旨在利用这些独特的数据集对喜马拉雅主逆冲构造的隐伏构造进行成像，以更好地了解俯冲巨型逆冲构造的详细结构和力学，提高我们对喜马拉雅锋面孕震带及其地震危险性的认识，并探索地震和大地测量观测联合分析地球构造成像的新方法。这项研究需要使用详细的余震重新定位、各向异性接收函数分析和有限断层滑动反演来解决以下问题：在发震深度，俯冲通道剪切带是否是喜马拉雅主逆冲构造的特征？2. 廓尔喀地震是破坏了这个俯冲水道的顶部、底部还是内部？3. 河道内是否存在剪切构造？如果存在，是否沿喜马拉雅主逆冲断层的垂盘或下盘形成？4. 如果俯冲通道模式是相关的，那么在喜马拉雅主逆冲构造中是否存在沿走向的构造变化影响破裂区域？利用来自事件前后区域宽带地震仪装置的现有数据，研究人员将对径向和横向分量接收函数进行联合分析，以绘制喜马拉雅主逆冲断层的宽度、深度、速度和剪切结构以及沿走向几何形状。此外，将使用详细的地震重新定位来阐明廓尔喀余震序列照亮的拟议俯冲通道的允许深度范围和厚度。干涉合成孔径雷达（InSAR）、Landsat-8图像和GPS偏移量将用于绘制Mw7.8和7.3级地震的同震源图，重点是定义与现有地震和大地测量观测结果一致的断层几何形状（倾角和深度范围）。最后，该团队将在地震和大地测量结果之间进行迭代，以确定喜马拉雅主逆冲的性质和结构的内部一致描述。

Subduction zones, where one tectonic plate slides beneath another, produce the world's largest and often most destructive earthquakes. The earthquake-generating portions of typical subduction zones, where the downgoing plate is oceanic, are located offshore and under water, beneath several kilometers of ocean. The most severe hazard in many of these regions is posed by tsunami rather than by the shaking during the earthquake itself. Because of the submarine location, it is difficult to address the detailed characteristics of most subduction zone megathrust faults: what is the shape of the megathrust; is it a sharp, discrete interface or a thicker, distributed shear layer; how do these characteristics behave during the build-up time, generation, and propagation of great earthquakes? Under the Himalaya, however, the continental Indian plate subducts beneath Tibet, creating a unique situation where the entire subduction zone is on land and is instrumented by seismic stations, imaged from space, and monitored by continuous geodetic markers. Because large population centers sit immediately atop the shallow portions of the megathrust, the risk from shaking is much higher than in typical oceanic subduction zones. The large 2015 Gorkha, Nepal earthquakes (magnitude Mw7.8 and 7.3) that devastated Kathmandu, Everest basecamp, and surrounding regions provide a rare opportunity to investigate the general properties of megathrust faults more directly by studying data from this continental subduction zone. The Gorkha earthquakes ruptured the Main Himalayan Thrust - the megathrust plate boundary fault between India and Eurasia. The earthquakes and their aftershocks were recorded by seismic sensors (which record shaking during earthquakes) and geodetic satellites (which record surface deformation caused by the earthquakes) in a way that is unprecedented for subduction zone settings. This project aims to exploit these unique data sets to image the buried structure of the Main Himalayan Thrust with the goal to better understand the detailed structure and mechanics of a subduction megathrust, improve our knowledge of the seismogenic zone and resulting earthquake hazard along the Himalayan front, and explore new methods for the joint analysis of seismic and geodetic observations in imaging Earth structure.This investigation entails the use of detailed aftershock relocations, anisotropic receiver function analysis, and finite fault slip inversions to address the following questions: 1. Does a subduction channel shear zone characterize the Main Himalayan Thrust at seismogenic depths?; 2. Did the Gorkha earthquakes rupture the top, bottom, or interior of this subduction channel?; 3. Is a shear fabric present within the channel, and if so, has it formed along the hanging or footwall of the Main Himalayan Thrust?; 4. If the subduction channel model is relevant, are there along-strike structural variations in the Main Himalayan Thrust that influence rupture area? Using existing data from pre- and post-event regional broadband seismometer installations, the investigators will conduct joint analysis of radial and transverse component receiver functions to map the width, depth, velocity and shear fabric structure, and along-strike geometry of the Main Himalayan Thrust. Additionally, the will use detailed earthquake relocations to elucidate permissible depth ranges and thicknesses of the proposed subduction channel that are illuminated by the Gorkha aftershock sequence. Interferometric synthetic aperture radar (InSAR), Landsat-8 imagery, and GPS offsets will be used to map the co-seismic source of the Mw7.8 and 7.3 events with an emphasis on defining a population of fault geometries (dip and depth range) that are consistent with the available seismic and geodetic observations. Lastly, the team will iterate between the seismic and geodetic results to define internally consistent descriptions of the nature and structure of the Main Himalayan Thrust.