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

俯冲带是指一个构造板块在另一个板块之下滑动的地方，会产生世界上最大的地震，通常也是最具破坏性的地震。典型俯冲带的地震孕育部分位于近海和水下，位于几公里深的海洋之下，向下的板块是海洋。其中许多地区最严重的危险是由海啸造成的，而不是地震本身造成的震动。由于海底的位置，很难解决大多数俯冲带巨型逆冲断裂的详细特征：巨型逆冲断裂的形状是什么；它是一个尖锐的离散界面还是更厚的分布剪切层；这些特征在大地震的形成时间、发生和传播过程中表现如何？然而，在喜马拉雅山脉之下，印度大陆板块俯冲到西藏之下，创造了一个独特的情况，整个俯冲带都在陆地上，由地震台测量，从太空成像，并由连续的大地标志进行监测。由于大型人口聚集地紧挨着巨型逆冲带的浅部，因此发生地震的风险比典型的海洋俯冲带要高得多。2015年尼泊尔高尔卡大地震(里氏7.8级和7.3级)摧毁了加德满都、珠穆朗玛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调查巨型逆冲断裂的一般性质提供了难得的机会。高尔加地震破坏了喜马拉雅山脉的主要逆冲断层--印度和欧亚大陆之间的巨型逆冲板块边界断层。地震及其余震是由地震传感器(记录地震期间的震动)和大地测量卫星(记录地震造成的地表变形)以一种在俯冲带环境中前所未有的方式记录下来的。这个项目的目的是利用这些独特的数据集来成像喜马拉雅主冲断带的埋藏结构，目的是更好地了解俯冲巨型逆冲带的详细结构和力学，提高我们对喜马拉雅前缘孕震带和由此产生的地震灾害的认识，并探索在成像地球结构中联合分析地震和大地测量观测的新方法。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：1.俯冲通道剪切带是否表征了孕震深度的主喜马拉雅冲断带？2.高尔加地震是否破坏了俯冲通道的顶部、底部或内部？3.通道内是否存在剪切组构？如果是，它是沿着喜马拉雅主冲断层的悬垂或下盘形成的？4.如果俯冲通道模型是相关的，那么主喜马拉雅逆冲是否存在影响断裂区的沿走向的构造变化？利用震前和震后区域宽带地震仪装置的现有数据，调查人员将对径向和横向分量接收函数进行联合分析，以绘制喜马拉雅主冲断带的宽度、深度、速度和剪切组构结构以及沿走向的几何图形。此外，将使用详细的地震重新定位来阐明被戈尔卡余震序列照亮的拟议俯冲通道的允许深度范围和厚度。将使用干涉合成孔径雷达(InSAR)、Landsat-8图像和GPS偏移量绘制Mw7.8和7.3地震的同震源图，重点是定义与现有地震和大地测量观测相一致的断层几何图形(倾角和深度范围)的总体。最后，该小组将在地震和大地测量结果之间进行迭代，以确定对主要喜马拉雅冲断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.