HR: 1340h
AN: T33A-1342    [Abstracts]
TI: S-splitting in Eastern Nepal and Southern Tibet Across High Himalaya
AU: * Wu, F T
EM: wu@binghamton.edu
AF: Department of Geological Sciences, State University of New York, Binghamton, Vestal Parkway East, Binghamton, NY 13902-6000 United States
AU: Sheehan, A F
EM: afs@cires.colorado.edu
AF: CIRES, University of Colorado, CB399, Boulder, CO 80309 United States
AB: The 28 station HIMNT broadband seismic deployment in eastern Nepal and Southern Tibet in September, 2001-November, 2002 recorded teleseisms that allow the determination of S-splitting using multiple events from different back azimuths. We use mainly SKS waves for this study and the waves selected for analaysis are visually highly coherent in the band between 0.02 to 0.2 Hz. A code by Menke (see reference) is used for the determination of split parameters. The resulting fast directions for both eastern Nepal and southern Tibet are in the NNE direction (N $10\deg$-$30\deg$ E) with delay times varying in the range of less than 0.1 second to 0.9 seconds with most of the values in the 0.2 to 0.4 second range. The variations from station to station do not appear to have a clear pattern. These measurements are distinctly different from splitting parameters found from stations to the north of our study area. In northern part of the Tibetan plateau some of the largest splitting delays, up to 2.7 seconds (McNamara et al., 1993), in the world are found and the fast directions are generally trending ENE. Toward southern Tibet, the delays decrease and the direction changes to more northeasterly. Earlier sparse measurements of splitting in the high Himalaya shows NW or NE fast directions but with small delay times (Hirn, 1995). In many of the young mountain ranges of the world the fast directions areparallel to the trends of the ranges, e.g., New Zealand (Klosko et al., 1999), Taiwan (Rau et al., 2000), etc., some of which are undergoing significant shear. It is not yet clear as to the source of the small magnitude splitting that we observe across the High Himalaya. Such relatively small splitting delays could conceivably be generated mainly in the crust, with possible mantle contributions. The lack of large splitting delays with fast direction parallel to the mountain range reflect differences of the orogenic processes in the upper mantle beneath the Himalaya relative to other young mountain ranges. Selected References Hirn, A. and 11 others, 1995. Seismic anistropy as an indicator of mantle flow beneath the Himalayas and Tibet, Nature, 375,571-574. McNamara, D.E.,Owens, T.J.,Silver, P.G.,Wu, F.T.,1994. Shear wave anisotropy beneath the Tibetan Plateau,JGRB, 99, 13,655-13,665. Menke, W. (http://www.ldeo.columbia.edu/menke/software.html) Klosko, E. R.; Wu, F. T.; Anderson, H. J., and others, 1999, Upper mantle anisotropy in the New Zealand region,Geophysical Research Letters,26, 1497-1500 . Rau, R.J.Liang, W.T,, Kao,H., and others, 2000. Shear wave anisotropy beneath the Taiwan Orogen,EPSL,177, 177-192 .
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting