HR: 14:20h
AN: T23G-03    [Abstracts]
TI: Imaging the Crust and Upper Mantle beneath the Hi-CLIMB Seismic Array in Tibet Using Gaussian-Beam Migration of Radial Receiver Functions
AU: * Nowack, R L
EM: nowack@purdue.edu
AF: Purdue University, Dept. of Earth and Atmos. Sci., West Lafayette, IN 47906, United States
AU: Chen, W
EM: wpchen@uiuc.edu
AF: University of Illinois, Dept. of Geology, Urbana, IL 61801, United States
AU: Tseng, T
EM: tseng1@uiuc.edu
AF: University of Illinois, Dept. of Geology, Urbana, IL 61801, United States
AU: Seismic Team, H
EM: wpchen@uiuc.edu
AF: University of Illinois, Dept. of Geology, Urbana, IL 61801, United States
AB: A broadband, linear seismic array of over 800 km in length was deployed across the Himalayan-Tibetan collision zone as part of Project Hi-CLIMB. Overall, more than 200 seismic station locations were occupied during 2002- 2005, with an average station spacing of 3-8 km along the linear array. In this study, we use Gaussian-Beam (GB) migration of scattered P-wavefield to obtain images of crust and upper mantle beneath Tibet. The sources of illumination are teleseismic P-waves and we form seismic profiles of radial receiver-functions from groups of earthquakes, with each group restricted to a small range of azimuths and distances. The best data come from a group of 12 located to the southeast of the array in the southwestern Pacific, and another group of 5 events located to the northeast of the array in the northwestern Pacific. Prior to imaging, we first analyze azimuthal variations in both radial and transverse components of receiver- functions from individual events at selected stations. We then stack the receiver-functions for each group of earthquakes and interpolate the data to a uniform spacing of 8 km before applying the GB migration algorithm to obtain images of variations in seismic wave speeds in the crust and the upper mantle. In a previous study, we already tested the GB algorithm with full synthetic seismograms to ensure that it is robust in imaging geologic features beneath a thickened crust. Initial results show a number of interesting features throughout the thickened crust of Tibet. Moreover, broad bandwidth of the data produces images at different frequency bands up to more than 1 Hz, constraining scattering properties at multiple scales. Overall, there is stronger lateral heterogeneity or anisotropy beneath the Qiangtang terrane in the heartland of Tibet than the Lhasa terrane in southern Tibet. Upper crustal anisotropy seems particularly strong at stations within 50 km south of the surface trace of the Bangong Nujiang Suture (BNS) – a feature corroborated by azimuthal variability and/or polarity switches in the transverse component of receiver-functions. P-to-S conversions across the Moho are generally strong and continuous under much of the Lhasa terrane, corresponding to a well-defined Moho. A noticeable exception occurs about 80 km north of the Indus-Yarlong suture where an apparently gradual Moho correlates well with a curious anomaly of low P-wave speeds detected by travel-time tomography. Disturbances to the Moho are also evident in the vicinity of the BNS, apparently a lithospheric structure; and increased scattering near the Moho generally persists under much of the Qiangtang terrane to the north where the most recent episode of regional magmatism was concentrated.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 8108 Continental tectonics: compressional
DE: 8110 Continental tectonics: general (0905)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting