Himalayan Seismotectonics at Deep Structure

喜马拉雅尼泊尔西藏地震实验(HIMNT)是第一个同时覆盖尼泊尔南部平原、大喜马拉雅和小喜马拉雅以及青藏高原南部的宽带地震实验。HIMNT项目包括在2001-2003年在尼泊尔东部和西藏南部部署了29个宽带地震台。首次利用HIMNT地震资料研究了尼泊尔和西藏下方的上地幔地震、尼泊尔下方的印度-喜马拉雅滑脱运动、碰撞带中的地壳-地幔边界(MOHO)结构以及西藏下部地壳解耦的震源机制。目前的项目继续对HIMNT数据进行分析，以解释与印度/欧亚碰撞有关的结构和过程。到目前为止所分析的远震接收功能现在得到了当地深震的地壳转换的补充。滑脱上的斜坡-平坦几何学被用来解释喜马拉雅前缘的地震聚集和隆升模式。虽然来自远程地震接收器功能的初始叠加图像在斜坡区域缺乏分辨率，但对接收器函数和来自深部地方地震的转换震相的模拟现在正在进行，以显示滑脱斜坡的存在或不存在。从上青藏地壳正断层到下地壳走滑机制的震源机制变化表明，下青藏地壳存在一条解偶带，这可能与根据面波分析和大地测量学在别处提出的下地壳流通道有关。结合速度层析成像的衰减研究将有助于确定地壳的物理状态，以及在喜马拉雅高原下发现的明显的地壳中段脱钩带的性质。原始地震记录的频率成分表明，西藏(高衰减，低Q)和尼泊尔(低衰减，高Q)之间的衰减差异很大。由局部和区域层析成像得到的应力状态、由局部和区域层析成像得到的材料速度、由接收函数得到的地壳几何形状、新的衰减测量结果以及基于层析和接收函数结果的新的重力模拟结果被结合起来解释西藏地壳在碰撞带中的行为。区域面波分析揭示了研究区东部与西部岩石圈厚度的根本变化。获得的岩石圈厚度被用作挠曲模拟的输入，以研究印度次大陆俯冲的动力学。岩石圈厚度和俯冲角度之间的相关性表明，对于较厚的岩石圈来说，要么是高刚性，要么是浮力增加，这一点现在可以测试。通过地震和其他方法的联合应用，调查人员正在组装喜马拉雅碰撞带的完整运动学和动力学图景。与该项目相关的更广泛的影响包括为科罗拉多大学科学探险者计划开发自然灾害课程。该计划包括为科罗拉多州的中学教师和学生团队举办的一系列为期一天的研讨会。该项目覆盖了全州20个不同地点的300多名教师和1500名学生，为期一天的研讨会。其他更广泛的影响是研究生教育和代表不足的群体参与项目。通过IRIS向地震界提供这些数据，并将向国际地震学中心提供包括精选数据在内的事件信息，以供进一步传播。

The Himalayan Nepal Tibet Seismic Experiment (HIMNT) was the first broadband seismic experiment to simultaneously cover the plains of southern Nepal, the Lesser and Greater Himalaya, and the Southern Tibetan Plateau. The HIMNT project included the deployment of 29 broadband seismc stations in eastern Nepal and southern Tibet in 2001-2003. The first studies with the HIMNT earthquake seismic data revealed upper mantle earthquakes under Nepal and Tibet, a first view of the Indian-Himalayan decollement under Nepal, crust mantle boundary (Moho) structure in the collision zone, and earthquake focal mechanisms suggesting decoupling in the lower Tibetan crust. The current project continues this analysis of HIMNT data to interpret structure and processes associated with the India/Eurasia collision. Teleseismic receiver functions analyzed to date are now supplemented with crustal conversions from local deep earthquakes. A ramp-flat geometry on the decollement has been proposed to explain clustering of earthquakes and uplift patterns at the Himalayan front. While an initial stacked image from teleseismic receiver functions lacked in resolution in the ramp area, modeling of receiver functions and converted phases from deep local earthquakes to show the presence or nonexistence of a decollement ramp is now underway. Changes in focal mechanisms from normal faulting in the upper Tibetan crust to strike-slip mechanisms at subcrustal depths indicate a decoupling zone in the lower Tibetan crust, which may be identified with a lower crustal flow channel suggested elsewhere based on surface wave analysis and geodesy. Attenuation studies combined with velocity tomography will help ascertain the physical state of the crust, and the nature of an apparent midcrustal decoupling zone discovered beneath the Himalayan Plateau. The frequency content of raw seismograms suggests a strong variation in attenuation between Tibet (high attenuation, low Q) and Nepal (low attenuation, high Q). The resulting stress regimes, material velocities derived from local and regional tomography, crustal geometry from receiver functions, new attenuation measurements, and new gravity modeling based on tomographic and receiver function results are combined to explain the behavior of the Tibetan crust in the collision zone. Regional surface wave analysis reveals radical changes in lithospheric thickness in the eastern versus western part of the study area. The obtained lithospheric thicknesses are used as input into flexural modeling to investigate the dynamics of subduction of the Indian subcontinent. The correlation between lithospheric thickness and subduction angle suggests either high rigidity or increased buoyancy for the thicker lithosphere, which can now be tested. Through the joint application of seismic as well as other methods, the investigators are assembling a complete kinematic and dynamic picture of the Himalayan collision zone. Broader impacts associated with the project involve the development of a natural hazards curriculum for the University of Colorado Science Explorers program. This program includes a series of one-day workshops for teams of middle school teachers and students throughout the state of Colorado. The project reaches over 300 teachers and 1500 students in one-day workshops at twenty different locations throughout the state. Other broader impacts are graduate student education and project participation by underrepresented groups. The data are available to the seismological community through IRIS and event information including picks will be made available to the International Seismological Centre for further dissemination.