Significant and complex seismic anisotropy beneath the Himalayas and the southern Tibetan Plateau
PIs: Kelly Liu and Stephen Gao
07/2009 - 7/2011 by National Science Foundation; EAR #: 0911346
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.
