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年美国复苏和再投资法案（公法111-5）资助的。地球内部深处的物理和化学过程是地球表面观测到的特征和现象的最终原因。喜马拉雅山等山脉的上升，死亡谷和格兰德河裂谷等深谷的形成，以及毁灭性地震和火山爆发的发生，只是地球强大的内部力量造成的后果的几个例子。显然，更好地了解这些内部过程将导致更好地了解地震和火山等自然灾害。不幸的是，地球内部的绝大多数是无法进入的：虽然地球的中心大约有6370公里深，但人类迄今为止可以钻的最深的洞只有15公里。如果我们假设地球有一个普通鸡蛋的体积，人们可以钻的洞大约是蛋壳的一半。因此，间接技术通常用于地球深部的成像。 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年里，许多被称为地震仪的运动探测设备一直在记录地面振动。 该技术类似于医院医生用于对患者内部结构进行成像的CAT扫描。该项目利用核幔边界的P-S转换相测量了青藏高原下地幔中形成的组构的方向和强度。地壳剪切波的分裂主要是岩石圈变形和软流圈流动的结果。在现今俯冲带附近和古老的碰撞造山带中，由于软流圈流动和岩石圈缩短，分别观察到了平均分裂时间约为1 s的显著地震各向异性。令人惊讶的是，以前在喜马拉雅山和西藏南部，这是活跃的大陆碰撞的原型位置的剪切波分裂测量，建议各向同性或弱各向异性的上地幔（与大多数分裂时间为0.5秒或更少）。根据剪切波分裂和其他测量结果，人们提出了一些关于藏南印度岩石圈几何形状的相互矛盾的模型。 重新评估位于西藏南部拉萨地块南部的LSA站的所有可用数据（Gao和Liu，2009，G-cubed），发现明显的各向异性证据，分裂时间高达1.5 s。当PKS和SKKS除了SKS相位使用，显着的方位角变化的分裂参数已被确定。大多数分裂参数可以解释为两层各向异性的组合效应。上层为NE-SW向快速方向，可认为是下地壳塑性流动的结果，该项目将藏南地区地幔各向异性的再评价从一个测站扩展到100个测站，利用系统剪切-剪切分波测量程序。 对1991年以来6次便携式地震实验所采集的全部数据集采用统一的数据处理方法。