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青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。首次利用HIMNT地震资料揭示了尼泊尔和西藏的上地幔地震，首次揭示了尼泊尔下的印度-喜马拉雅滑脱，碰撞带的壳幔边界（Moho）结构，以及表明西藏下地壳解耦的地震震源机制。目前的项目继续对HIMNT数据进行分析，以解释与印度/欧亚大陆碰撞相关的结构和过程。迄今为止分析的远震接收函数现在补充了来自局部深地震的地壳转换。人们提出了滑脱上的斜平几何来解释喜马拉雅锋面的地震聚集和隆起模式。虽然从远震接收函数获得的初始叠加图像在斜坡区域缺乏分辨率，但目前正在对接收函数和深层局部地震转换相进行建模，以显示滑脱斜坡的存在或不存在。从上藏地壳正断层到下藏地壳深处走滑机制的震源机制变化表明下藏地壳存在解耦带，这可能与其他地方根据面波分析和大地测量所提出的下地壳流动通道相一致。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地壳中解耦带的性质。原始地震记录的频率含量表明，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。由此产生的应力状态、来自局部和区域断层扫描的物质速度、来自接收函数的地壳几何形状、新的衰减测量以及基于断层扫描和接收函数结果的新的重力模型相结合，解释了西藏地壳在碰撞区的行为。区域面波分析表明，研究区东、西部岩石圈厚度变化剧烈。获得的岩石圈厚度被用作弯曲建模的输入，以研究印度次大陆的俯冲动力学。岩石圈厚度与俯冲角之间的相关性表明，较厚的岩石圈要么刚性高，要么浮力增加，这一点现在可以得到验证。通过地震和其他方法的联合应用，研究人员正在组装一个完整的喜马拉雅碰撞带的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开发自然灾害课程。该项目包括一系列为期一天的研讨会，由科罗拉多州的中学教师和学生组成。该项目涉及300多名教师和1500名学生，在全州20个不同地点举办为期一天的讲习班。其他更广泛的影响是研究生教育和未被充分代表的群体参与项目。地震学界可以通过“综合信息系统”获得这些数据，而包括精选在内的事件信息将提供给国际地震中心进一步传播。

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.