HR: 1330h
AN: S22B-0446 INVITED     [PDF]
TI: Compressional Wave Q in the Uppermost Mantle Beneath the Tibetan Plateau Measured Using Pn Wave Spectra
AU: * Xie, J
EM: xie@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: Pn waves from three near-colocated seismic events in the eastern Tarim Basin are well-recorded by the INDEPTH III and II arrays, which are deployed from northern to southern Tibet with a small east-west spread (between $\sim$88 and $91\deg$E). The paths run southward and sample the Tibetan mantle with epicentral distances increasing from 870 to 1540 km. These waves have spectral contents that are distinctly different from those collected from the Kyrghistan network (KNET), to which the paths traverse westward through the eastern Tienshan. Pn Q beneath Tibet and Tienshan must therefore be different. Xie and Patton (1999,JGR, 104, 941-954) have simultaneously estimated source spectra of the co-located events, and path-averaged Pn Q to the KNET stations. Under a simplified geometrical spreading of $\Delta ^{-1.3}$, they have estimated $Q_0$ and $\eta$ (Pn Q at 1 Hz and its frequency dependence) to KNET to be about 360 and 0.5, respectively. Using those estimates as a priori knowledge, we estimate that $Q_0$ and $\eta$ are ~180 and 0.3 along paths to northern Tibet, and $\sim$260 and 0.0 along paths to southern Tibet. The southward increase of $Q_0$ correlates well with a similar increase in Pn velocity contained in previous tomographic images. Additionally, we measured Pn Q using a two-station method along two profiles (from station SANG to TUNL, and GANZ to MAQI) deployed during the 1991-1992 Sino-US Tibetan Plateau experiment. Both profiles are located to the east of $92\deg$E. Along profile SANG-TUNL, we estimate $Q_0$ and $\eta$ to be $\sim$270 and 0.0, respectively. The $Q_0$ value is rather high, but correlates well with the high Pn velocities of $>$ 8.1 km/s re-measured in this study. Our results suggest that the zone of low Pn $Q_0$ and velocity in northern Tibet, which is likely caused by high mantle temperature and partial melting, is confined to the west of $92\deg$E. This is so despite that the zone of high Sn attenuation extends to further east.
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting