HR: 17:45h
AN: S14A-08    [Abstracts]
TI: Large-scale Shear Velocities Beneath Hotspot Locations: New Observations and Travel Time Synthesis
AU: An, Y
EM: yan@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AU: * Gu, Y J
EM: jgu@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AU: Sacchi, M D
EM: sacchi@phys.ualberta.ca
AF: Department of Physics, University of Alberta, CEB Building, Edmonton, AB T6G2G7, Canada
AB: The existence, lateral dimension, and depth of mantle plumes beneath hotspots have been issues of contentious debate in the past two decades. To a large extent, the difficulty lies in the insufficient data resolution in the transition region (400-1000 km) between the upper and lower mantle beneath major hotspot locations. In this study, we report a new type of observation, the ray parameter variation of SS precursors, to image large-scale mantle shear velocities from the surface down to 1200 km beneath 17 major hotspots. We significantly improve the resolution by a High-resolution Radon transform method that utilizes time-domain inversions to simultaneously constrain differential times and ray parameters. Perturbations of S410S-SS ray parameters present the most revealing observations of hot thermal anomalies in this study. We identify both positive and negative jumps in travel time curves (hence ray parameters) for rays bottoming beneath the majority of the hotspot locations; these anomalous jumps are not observed at non-hotspot locations. Through careful modeling of the observed ray parameter jumps using 2-D finite-difference ray tracing, and accounting for depth, width, sign and strength of the velocity columns, we are able to unequivocally categorize the mantle beneath the aforementioned 17 hotspots to have low seismic velocities at depth 1) comparable to transition zone (400-670 km) depths, or 2) down to 900-1200 km or more. While our data and modeling strategies do not enable us to probe the source of hotspots beyond 1200-km depth, they do have major implications for the discussion of shallow vs. deep-rooted hotspots around the world. For example, the Hawaii and Macdonald hotspots exhibit strong type 1 characteristics, while the Azores and Canary hotspots are describable by type 2 mantle velocity variations.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: 2007 Fall Meeting