HR: 1340h
AN: T33B-0537    [Abstracts]
TI: Kirchhoff Migration of Synthetic Receiver Functions: Subducting Slabs and Mantle Discontinuities
AU: * Ham, S
EM: telnet2u@rice.edu
AF: Center for Computational Geophysics, Rice University, 6100 Main St. MS-126, Houston, TX 77005 United States
AU: Levander, A
EM: alan@rice.edu
AF: Center for Computational Geophysics, Rice University, 6100 Main St. MS-126, Houston, TX 77005 United States
AU: Niu, F
EM: niu@rice.edu
AF: Center for Computational Geophysics, Rice University, 6100 Main St. MS-126, Houston, TX 77005 United States
AB: Receiver function imaging provides locations of mantle discontinuities from large P-to-S conversions, providing higher resolution of interface structures than can be obtained from traveltime tomography, which are relatively insensitive to interface structure, or refracted waves, which tend to average properties of large regions in earthquake experiments. Properly stacked, or migrated, receiver functions can also be used to image dipping structures such as subducting slabs. From the prestack depth migration of receiver functions from HiNet data, we found a narrow, 150-200km wide, topographic high at the 410-km discontinuity and a broad, >400km wide, moderate topographic low at the 660-km discontinuity corresponding to slab structure in southwest Japan. The latter implies a flat lying slab along the lower boundary of the transition zone. The migrated image shows the subducting slab changing slope at the 410 discontinuity and flattening onto the 660, which implies that Pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone. These results are consistent with receiver function analyses and traveltime tomography done by several other research groups using a variety of data. To test the fidelity of Kirchhoff migration of receiver functions, we generated synthetic data using a two-dimensional elastic wave finite-difference code and to understand the relationship between the physical parameters of the subsurface and the migrated image. The slab geometry from high-resolution tomography in Japan (Zhao et al., 1994) was used to construct a synthetic model. We simulated the effect of various parameters like reference velocity model, epicentral distance (i.e. incidence angle of the incoming P-wave), and reference velocity, as well as processing parameters such as post-migration dip filters on the migrated image. We simulated the geometry of earthquakes with different epicentral distances, 45°, 55°, 65°, and 75°. We calculated the amplitude along the slab boundary, and along the 410-km and 660-km discontinuities in the images from different epicentral distances, and found that the amplitudes in the migrated images are strongly dependent on incidence angle, as expected from scattering calculations. Multiples generated from within the slab boundary obscure structure in the mantle beneath the dipping slab, but can be effectively removed by summing migrated images from different distances. To investigate the effect of the reference velocity distribution used in migration, we set up several different 1D velocity models and smoothed 2D velocity distributions for use as the migration velocity model. Migration with a smoothed 2D version of the velocity distribution gives an improved image of the slab geometry and discontinuity structure than that with a 1D velocity model only, underscoring the importance of taking into account upper mantle velocity fluctuations in receiver function imaging.
DE: 7200 SEISMOLOGY
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005