HR: 0800h
AN: T31B-0463    [Abstracts]
TI: Evidence for Vertical Coherent Deformation in Eastern Tibet from Splitting of Crustal S Phases
AU: * Karalliyadda, S C
EM: sapi1982@yahoo.com
AF: Carnegie Institution of Washington, Broad Branch Rd, Washington, Dc 20015, United States
AU: Weeraratne, D S
EM: dsw@dtm.ciw.edu
AF: Carnegie Institution of Washington, Broad Branch Rd, Washington, Dc 20015, United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, Broad Branch Rd, Washington, Dc 20015, United States
AB: Regional shear wave phases recorded by stations in Yunnan Province and Eastern Tibet surrounding the Eastern Himalayan syntaxis are used to constrain seismic anisotropy at crustal depths which range from 45 km to 65 km, respectively. Shear wave splitting measurements were obtained using regional direct S phases recorded at stations operated by Carnegie Institution of Washington, Lehigh University, and MIT, with source depths that originate from 20 km to 55 km. S to P converted energy is avoided considering events with free surface incidence angles less than 37{°} and using records with low P energy. We apply a low pass filter with a corner frequency at 2.5 Hz to all records and correct for surface reflections using a free surface transform from previous methods. Anisotropy is small but well resolved for 14 records at 8 stations and indicate that delay times obtained for all stations are less than 0.4 {±} 0.05s. In Yunnan province the average delay time is 0.16 s but is twice this value in eastern Tibet at 0.31 s. The fast directions for all stations in Yunnan Province including KMI and CHTO (GSN) are consistent with fast directions for previous SKS studies, but with smaller delay times of 0.4s or less (where SKS are 1.0 – 1.5s). Stations in eastern Tibet display a {~}N-S azimuth roughly orthogonal to SKS studies in this area and may indicate sensitivity to cracks which form roughly parallel to the compressive stress direction. A N-S azimuth of anisotropy is also consistent with surface waves results for periods below 40s. By comparison with SKS splitting studies, we find that anisotropy in the crust is very small and contributes only about 15% to the total SKS splitting measurements, requiring about 1.0 – 1.2s of splitting to come from subcrustal depths. Such strong anisotropic fabric possibly present in the lithospheric mantle is inconsistent with a lower crustal flow model which predicts decoupling above the Moho. Consistently low splitting times for events with crustal depths which vary from 20 to 55 km suggest a shallow source possibly from cracks or other forms of SPO in the upper 20 km. The consistency between surface deformation studies of topography, structural geology, GPS observation and SKS fast direction suggests that crust and mantle flow around the Himalayan syntaxis is vertically coherent with clear evidence for lithospheric or sublithospheric LPO but indicating deformation of crustal material may not align in a preferred orientation as suggested in previous laboratory and seismic studies.
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting