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，小喜马拉雅和大喜马拉雅以及西藏高原南部的宽带地震实验。HIMNT项目包括2001-2003年在尼泊尔东部和西藏南部部署29个宽带地震台站。 HIMNT地震数据的首次研究揭示了尼泊尔和西藏的上地幔地震，尼泊尔下的印度-喜马拉雅滑脱，碰撞带的壳幔边界（Moho）结构和地震震源机制表明西藏下地壳的解耦。 目前的项目继续HIMNT数据的分析，解释与印度/欧亚大陆碰撞的结构和过程。迄今为止分析的遥测接收器功能现在补充了来自当地深地震的地壳转换。一个斜坡平坦的几何形状的滑脱已被提出来解释集群的地震和隆升模式在喜马拉雅前线。虽然最初的叠加图像从滑脱斜坡接收器功能缺乏在斜坡区的分辨率，接收器功能的建模和转换阶段从深本地地震，以显示存在或不存在的滑脱斜坡现在正在进行中。震源机制的变化，从正常的断层在西藏地壳的上地壳的走滑机制在地壳下的深度表明，在西藏地壳的下地壳，这可能是确定与下地壳流动通道建议基于表面波分析和大地测量的其他地方去耦带。衰减研究与速度层析成像相结合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中地壳解耦带的性质。原始地震记录的频率内容表明，西藏（高衰减，低Q）和尼泊尔（低衰减，高Q）之间的衰减变化很大。由此产生的应力制度，材料的速度来自本地和区域层析成像，地壳几何形状的接收器功能，新的衰减测量，以及新的重力建模的基础上层析成像和接收器功能的结果相结合，以解释西藏地壳在碰撞区的行为。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。所获得的岩石圈厚度被用来作为输入到弯曲建模，以调查印度次大陆的俯冲动力学。 岩石圈厚度和俯冲角之间的相关性表明，对于较厚的岩石圈，要么是高刚性，要么是浮力增加，现在可以进行测试。通过地震和其他方法的联合应用，调查人员正在收集喜马拉雅碰撞带的完整运动学和动力学图像。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索者项目制定自然灾害课程。该计划包括一系列为期一天的研讨会，为整个科罗拉多州的中学教师和学生团队。该项目在全州20个不同地点的为期一天的讲习班上接触了300多名教师和1500名学生。其他更广泛的影响是研究生教育和代表性不足的群体参与项目。地震学界可通过综合资源信息系统获得这些数据，而包括选点在内的事件信息将提供给国际地震中心，以便进一步传播。

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.