HR: 09:15h
AN: S41E-06    [Abstracts]
TI: Strong Evidence for 380 and 580 km Negative Velocity Gradients Beneath the Lodore Array in NW Colorado
AU: * Jasbinsek, J J
EM: johnjj@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: Dueker, K
EM: dueker@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AB: Teleseismic data from the Lodore Array, a one year deployment of 31 broadband stations with a 100 km aperture in NW Colorado, were analyzed with receiver functions for mantle transition zone discontinuity structure. The array provides a dense data set with which to isolate converted S-wave (Pds) arrivals. Events from NW and SE back-azimuths are selected from 38°-93° in distance allowing for excellent phasing analysis. To test for lateral discontinuity homogeneity, the dataset was divided into four sub-arrays of seven to eight stations. Statistical comparison of the sub-array radial RF stacks show that no significant variations exist, permitting all stations recording each event to be stacked together. This has the advantage of minimizing the strong signal generated noise in teleseismic P-coda and permits good error estimation crucial to robust velocity modeling. Both NW and SE radial receiver functions stacks show 4-5% (amplitude with respect to vertical P-wave) Pds arrivals from the 410 and 660 km discontinuities. More interesting, however, is the observation of two negative amplitude arrivals. The NW stack has a large (-5%) negative amplitude Pds arrival at 380 km depth that phases very well. This discontinuity is broadly consistent with the velocity predictions of the "410 water filter" hypothesis of Karato and Bercovici (2003). The SE stack shows a negative amplitude Pds arrival at 580 km depth that also phases very well. We infer that this discontinuity must be related to chemical layering because no solid-state phase transition is predicted at this depth. A plausible origin for this chemical velocity discontinuity would be subducted oceanic crust. The observations that the NW and SE stacks find different structure suggests that the 380 and 580 km discontinuities are not continuous over the approximate 250-300 km lateral sampling of our Pds dataset. Frequency dependence of the radial stacks is observed that will be used to constrain the sharpness of the velocity gradients. Tangential receiver function stacks show comparatively weak energy in the transition zone. The most noteworthy tangential signal is from the 660 km discontinuity. Reflectivity modeling of our observed phases will be performed to constrain the amplitude and sharpness of these shear-wave velocity discontinuities.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7203 Body waves
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005