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

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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.