HR: 10:35h
AN: T32A-02    [Abstracts]
TI: Scattered Wave Imaging of the Lithosphere and the Asthenosphere Beneath Eastern North America
AU: * Rychert, C A
EM: Catherine_Rychert@Brown.edu
AF: Brown University , Department of Geology Box 1846 , Providence, RI 02906 United States
AU: Rondenay, S
EM: Rondenay@mit.edu
AF: MIT, Department of Eath, Atmospheric, and Planetary Sciences 77 Massachusetts Ave. , Cambridge, MA 02139 United States
AU: Karen, F M
EM: Karen_Fischer@Brown.edu
AF: Brown University , Department of Geology Box 1846 , Providence, RI 02906 United States
AB: To better understand the properties that define the lithosphere and the asthenosphere beneath continents, we have analyzed P-to-S phases produced by scattering beneath permanent seismic stations HRV, PAL, LMN, LBNH, BINY, and SSPA in eastern North America. Our prior work has shown that a strong (3-11%) drop in shear-wave velocity occurs at depths (90-110 km), consistent with the westward dipping lithosphere-asthenosphere boundary imaged by surface-wave tomography. Inversions of data at LMN and HRV require that this velocity gradient be sharp and occur over less than 11 km in depth. This result indicates that the lithosphere-asthenosphere boundary is not defined by temperature alone and suggests the presence of water or partial melt in the asthenosphere. To investigate the properties of the asthenosphere at greater depths, we are now investigating later scattered arrivals at these stations. A strong phase in the ~~290 km depth range is observed at stations PAL and HRV. Although modeling discontinuities at such depths is often hindered by the interference of reverberations from shallower discontinuities, the phase at PAL appears to be uncontaminated, and we have been able to model the ~~290 km depth phase at HRV by carefully including shallower discontinuities and their reverberations. This ~~290 km depth phase appears on both SH and SV migrated waveforms, and analysis of these particle motions demonstrates that the phase has undergone shear-wave splitting. We plan to further investigate data at HRV and PAL as well as other stations in the region to determine the implications of this phase for the depth distribution of anisotropy and the velocity gradient structure at the base of the asthenosphere.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting