Imaging Seismic Velocity Structure of Crust and Upper Mantle

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					           Imaging Seismic Velocity Structure of Crust and Upper
              Mantle At Southern Tibet by Receiver Function
                                                 G. Jin 1 , Y. J. Chen 1 , E. Sandvol 2, D.C. Wilson 3
        Institute of Theoretical    and A pplied Geophysics (ITAG), School of Earth and Space Sciences, Peking University, B eijing
                                                                    100871, China
                             Department of G eological Sciences, U. Missouri , Columbia , MO 65211, United States
                             3 G eological Sciences Dept ., U of Texas at Austin, Austin, TX 78712, United States
E xploring the velocity structure is a key to understand the procession of the uplift of Tibet plateau . Several seismical
experiments have been programmed to explore the crust and mantle structure under Tibet , including Indepth I(1992), Indepth
II(1994-1995), Indepth III(1999) et al. From June , 2004 to September 2005, as a part of HiClimb project , a 2-D survey
including 32 broadband stations are installed to record signal from nature earthquakes . (Fig 1) In this paper, we focus on the
position of discontinuities and velocity structure under southern Tibet using receiver function method .

                                                                      3 P restack and P oststack MIGRATION
                                                                      Since the crustal lateral heterogeneities are significant in
                                                                      this area , we adopt different kinds of methods , including
                                                                      both pre -stack and post -stack migrations , to explore the
                                                                      crustal structure. We adopt 2.5D Kirchhoff style migration
                                                                      [David Wilson and Richard Aster, 2005] as pre -stack
                                                                      migration , while wave equation migration method [Ling
                                                                      Chen et al. 2005] as post-stack migration , which
                                                                      provides a further correction on the result of CCP.
                                                                                                          S                  IY             N


   Fig1: Study area and distribution of stations, Moho                                                                                 0
   and mantle discontinuities pierce points .
                                                                                                          W            XSR              E

From June 2004 to September 2005, all the
earthquakes of which magnitude over 5 and epicenter                                                           4 10

distance between 30° to 90° are involved and finally
                                                                      Figure 3: Result of west-east
nearly 1000 high qualified receiver function are selected
to process in the following methods . All the receiver                crust profiles : W1 and W3.               660

functions are firstly selected automatically due to their             Similar structure is moving from
signal to noise ratio , waveform and error in the iterative           south -west to north -east.
                                                                      Strong upper crust discontinuity  Figure 4: Result of mantle
deconvolution . After that, a carefully manual selection is                                             profiles shows 410km and
performed to insure the quality of the data .                         is observed .
                                                                                                        660km discontinuities are not
                                                                                                        affected by the collision and
                                                                                                        subsequence subduction .
                                                                       4 S lantstack and f-k filter for Moho depth

                                                                      The "slant stacking" method transforms the receiver
                                                                      function series from the time domain to the H-Vp/Vs
                                                                      domain is developed by Zhu and K anamori [2000]. In
                                                                      the stations that the data is reluctant enough, a f-k filter
                                                                      is processed to improve data quality .

                                                                      5 Conclusion:

                                                                      •Two independent middle crust
                                                                      discontinuities can be observed
                                                                      south and north of the Yalu-
                                                                      Zangbu suture zone and there
                                                                      is also a low velocity layer
                                                                      north of the Yalu -Zangbu suture
                                                                      zone .
                                                                      •We observe the lower crust               Figure 5: Moho depth
                                                                      undertrusting south of the Yalu-
                                                                      Zangbu suture zone , which may
  Figure 2. N1 profile : ①: middle crust discontinuity in
  the south dipping and disappeared. ②: lower crust
                                                                      also cause the heterogeneity in
                                                                      •There is strong Moho doublet in the direction of west to
                                                                      the in .
                                                                      east norththe southern Tibet crust, a strong discontinuity in
  underthrusting south of the Yalu-Zangbu suture zone .
  ③: middle crust discontinuity north of the Yalu-                    the upper crust is observed spreading from SSW to
  Zangbu suture zone and the presence of a low
  velocity layer . ④: Mohodoublet .                                   •Mantle discontinuities (410km and 660km) are quite flat,
                                                                      which indicates that the collision of Indian and Eurasia
                                                                      continent doesn ’t affect deep structure.
                                                                      •Moho near the north -south rifts seems to be thinner .