Significant and complex seismic anisotropy beneath the Himalayas and the southern Tibetan Plateau

喜马拉雅山和青藏高原南部地区显着而复杂的地震各向异性

• 批准号: 0911346
• 负责人: Kelly Liu
• 金额: $ 12.43万
• 依托单位: Missouri University of Science and Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-07-15 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911346&HistoricalAwards=false
• 关键词: Significant; complex; seismic; anisotropy; beneath

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Physical and chemical processes inside the Earth's deep interior are the ultimate causes of features and phenomena observed on the surface of the Earth. The rising of mountain chains such as the Himalayas, the formation of deep valleys such as the Death Valley and the Rio Grande rift, and the occurrence of devastating earthquakes and volcanic eruptions are just a few examples of the consequence of the mighty internal forces of the planet Earth. Obviously, a better understanding of those internal processes will lead to a better understanding of natural hazards such as earthquakes and volcanoes. Unfortunately, the vast majority of the Earth's interior is inaccessible: while the center of the earth is about 6370 km deep, the deepest hole that the human being can drill so far is merely 15 km. If we assume that the Earth has a volume of a regular chicken egg, the hole that people can drill is about half way through the eggshell. As a result, indirect techniques are routinely used to image the Earth's deep interior. The most effectively techniques come from computer analysis of elastic waves produced by earthquakes. Many motion-detection devices called seismographs have been recording ground vibrations over the past 50 years. The technique is similar to CAT-Scan used by medical doctors in the hospital to image the internal structure of a patient. This project measures the direction and strength of fabrics formed in the Earth's mantle beneath the Tibetan Plateau using teleseismic P-to-S converted phases at the core-mantle boundary. Splitting of teleseismic shear-waves is mostly the consequence of lithospheric deformation and asthenospheric flow. Significant seismic anisotropy with an averaged splitting time of about 1 s has been observed in the vicinity of most present-day subduction zones and in ancient collisional mountain belts, as a result of asthenospheric flow and lithosphere shortening, respectively. Surprisingly, previous shear-wave splitting measurements in the Himalayas and southern Tibet, which are the locations of the prototype of active continental collision, suggested an isotropic or weakly anisotropic upper mantle (with the majority of splitting times of 0.5 s or less). A number of conflicting models regarding the geometry of the Indian lithosphere beneath southern Tibet have been proposed based on shear-wave splitting and other measurements. Reassessment of all the available data (Gao and Liu, 2009, G-cubed) from station LSA which is located in the southern part of the Lhasa block in southern Tibet revealed clear evidence of significant anisotropy, with a splitting time of up to 1.5 s. When the PKS and SKKS in addition to SKS phases are used, remarkable azimuthal variations of the splitting parameters have been identified. The majority of the splitting parameters can be interpreted as a combined effect of two layers of anisotropy. The top layer has a NE-SW fast direction which can be considered as the result of lower-crustal plastic flow, and the lower layer has a nearly E-W fast direction which can be interpreted as reflecting the asthenospheric flow associated with the motion of the Eurasian plate.The project expands the reassessment of mantle anisotropy in southern Tibet from one station to about 100 stations by applying a systematic shear-wave splitting measurement procedure. A uniform data processing method is used to all the data sets which were collected by a total of 6 portable seismic experiments since 1991.

该奖项是根据2009年的《美国回收与再投资法》（公法111-5）资助的。地球深层内部内部的物理和化学过程是地球表面上观察到的特征和现象的最终原因。喜马拉雅山等山脉的上升，诸如死亡谷和里奥格兰德裂谷之类的深山谷的形成以及发生毁灭性地震和火山喷发的发生只是地球地球强度内部力量的几个例子。显然，对这些内部过程的更好理解将使人们更好地了解天然危害，例如地震和火山。不幸的是，地球内部的绝大多数是无法访问的：虽然地球中心约为6370公里，但人类可以钻的最深孔仅是15公里。如果我们假设地球上有一个普通的鸡蛋，那么人们可以钻的孔大约是通过蛋壳的一半。结果，间接技术通常用于成像地球深内部。 最有效的技术来自对地震产生的弹性波的计算机分析。在过去的50年中，许多称为地震仪的运动检测设备一直在记录地面振动。 该技术类似于医院医院在医院使用的猫扫描，以对患者的内部结构进行想象。该项目通过使用核心 - 壳命中边界处的远距离p-to-s转换的相位，衡量在藏族高原下地球地幔中形成的织物的方向和强度。伸展性剪切波的分裂主要是岩石圈变形和动态圈流动的结果。在大多数当今俯冲带的附近和古代碰撞山带的附近，显着的地震各向异性已分别观察到大约1 s的平均分裂时间。令人惊讶的是，喜马拉雅山脉和西藏南部的先前剪切波分裂测量值是主动大陆碰撞原型的位置，这表明各向同性或弱向各向异性的上层地幔（大部分分布时间为0.5 s或更少））。基于剪切波分裂和其他测量值提出了许多有关印度岩石圈下岩石圈几何形状的矛盾模型。 重新评估位于西藏南部LHASA块南部的LSA站的所有可用数据（Gao and Liu，2009年，G-Cubed），揭示了明显的证据表明明显的各向异性证据，分裂时间为1.5 s。当使用SKS相位的PK和SKK时，已经确定了分裂参数的显着方位角变化。大多数分裂参数可以解释为两层各向异性的综合效应。顶层具有NE-SW的快速方向，可以将其视为下层塑料流动，而下层几乎具有E-W快速方向，可以解释为反映与欧亚板板运动相关的小圈流动。该项目扩大了从一个站点的STATIBERTION，将近南部的壁炉旁的陆上脉冲的重新安排扩展到100个系统的范围内，将其施加到一个系统的范围内。 自1991年以来，总共有6个便携式地震实验收集的所有数据集均匀数据处理方法。