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

合作研究：对尼泊尔大陆内俯冲带的结构和行为进行联合地震和大地测量调查

• 批准号: 1645009
• 负责人: Vera Schulte-Pelkum
• 金额: $ 7.73万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-04-01 至 2023-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1645009&HistoricalAwards=false
• 关键词: Collaborative; research; joint; seismic; geodetic

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.

俯冲带是一个构造板块滑动到另一个构造板块之下的地方，会产生世界上最大且往往最具破坏性的地震。典型的俯冲带的地震发生部分，其中下行板块是海洋，位于离岸和水下，在几公里的海洋之下。在这些地区，最严重的灾害是海啸，而不是地震本身的震动。由于海底的位置，这是很难解决的大部分俯冲带巨型逆冲断层的详细特征：是什么形状的巨型逆冲断层;它是一个尖锐的，离散的界面或较厚的，分布剪切层;这些特征如何表现在建立时间，生成和传播的大地震？然而，在喜马拉雅山下，印度大陆板块俯冲到西藏之下，形成了一种独特的情况，即整个俯冲带都在陆地上，并由地震站进行测量，从太空成像，并由连续的大地测量标记进行监测。由于大型人口中心就坐落在巨型逆冲断层的浅水部分，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（震级Mw 7.8和7.3）摧毁了加德满都、珠峰大本营及周边地区，这提供了一个难得的机会，可以通过研究该大陆俯冲带的数据来更直接地研究大型逆冲断层的一般特性。廓尔喀地震破坏了喜马拉雅主冲断层-印度和欧亚大陆之间的巨型冲断层板块边界断层。地震传感器（记录地震期间的震动）和大地测量卫星（记录地震造成的地表变形）记录了地震及其余震，这在俯冲带环境中是前所未有的。该项目旨在利用这些独特的数据集对喜马拉雅主冲断层的埋藏结构进行成像，目的是更好地了解俯冲巨型冲断层的详细结构和力学，提高我们对喜马拉雅前缘沿着孕震区和由此产生的地震危险的认识，并探索在地球结构成像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位移，各向异性接收函数分析，和有限断层滑动反演，以解决以下问题：1。俯冲通道剪切带是否是喜马拉雅主冲断层孕震深度的特征？2.廓尔喀地震是在俯冲通道的顶部、底部还是内部破裂的？3.河道内是否存在剪切组构，如果存在，它是否沿着主喜马拉雅冲断层的上盘或下盘形成？4.如果俯冲通道模型是相关的，在主喜马拉雅逆冲断层中是否存在影响破裂区域的沿走向的构造变化？ 利用事件前和事件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器功能进行联合分析，以绘制喜马拉雅主冲断层的宽度，深度，速度和剪切组构结构以及沿走向的几何形状。此外，将使用详细的地震重新定位来阐明Gorkha余震序列所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（干涉合成孔径雷达）、陆地卫星8号图像和全球定位系统偏移量来绘制Mw7.8和7.3事件的同震震源，重点是确定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，该小组将在地震和大地测量结果之间进行比较，以确定对喜马拉雅主冲断层的性质和结构的内部一致的描述。

项目成果

Mantle earthquakes in the Himalayan collision zone

• DOI: 10.1130/g46378.1
• 发表时间: 2019-09
• 期刊: Geology
• 影响因子: 5.8
• 作者: V. Schulte‐Pelkum;G. Monsalve;A. Sheehan;P. Shearer;Francis T. Wu;S. Rajaure