MISSOURI SYSTEM, UNIVERSITY OF
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.
Our recent 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. In addition, 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.
Intellectual Merit: The proposed project will expand our reassessment of mantle anisotropy in the Himalayas and southern Tibet from one station to about 150 stations by applying a uniform data processing and measurement procedure to all the data sets, which were collected by 6 portable seismic experiments between 1991 and 2006. The project is meritorious because:
Most previous results only used the SKS phase (except for a few SKKS measurements), while our results obtained at LSA suggest that the use of PKS and SKKS in addition to SKS can provide critical information regarding the existence and characteristic of mantle anisotropy beneath southern Tibet.
All the previous studies presented their results in the form of averaged splitting parameters over all the events from a station. Obviously, when strong azimuthal dependence presents at a station (such as LSA), the averaged splitting parameters are biased toward the most populous azimuthal group of events and thus cannot reflect the actual anisotropy structure.
Various data processing and analysis procedures or techniques and different criteria for accepting/rejecting and ranking the splitting parameters have been used by previous studies, and thus the results are intrinsically inhomogeneous. The coherent database of splitting parameters to be produced as a result of the proposed study will be used not only to infer mantle anisotropy in the study area, but can also be used as critical constraints for geodynamic modeling and for the understanding of active continental collision in general.
Broader Impacts: Given the interdisciplinary nature of the study for the mechanisms and consequences of continental collision, results from the proposed study will be used by non-seismology researchers from a variety of fields such as geodynamics, geodesy, mineralogy, and climate change etc.. Another impact of the proposed study is that the PIs have recently moved to the Missouri University of Science and Technology (known as Univ. of Missouri-Rolla prior to 1/1/08) to establish a brand-new program in seismology. The project, if funded, will lay a much-needed foundation to reach such a goal.
A footnote ' this is a resubmission of proposal 0810124 (submitted in Dec. 2007). We believe that we have addressed all the concerns of the reviewers. See Section 9 of the text for d