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Seismic evidence for present-day plume upwelling at the core-mantle boundary

Seismic evidence for present-day plume upwelling at the core-mantle boundary. Sebastian Rost Edward J. Garnero. Quentin Williams. University of California Santa Cruz. Michael Manga. University of California Berkeley. ULVZ structure and detection. 0.5 to 10’s km thick

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Seismic evidence for present-day plume upwelling at the core-mantle boundary

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  1. Seismic evidence for present-day plume upwelling at the core-mantle boundary Sebastian Rost Edward J. Garnero Quentin Williams University of California Santa Cruz Michael Manga University of California Berkeley

  2. ULVZ structure and detection • 0.5 to 10’s km thick • 10 to 30 % velocity decrease • density ? • CMB are probed < 50 % • ULVZ evidence < 10% (of CMB area) Thorne and Garnero, 2004

  3. ScP waveform variations ScP waveform variations

  4. Using two small-scale arrays Topography from NOAA 2’ dataset

  5. WRA dataset • - Tonga-Fiji seismicity • deep earthquakes • 97 earthquakes • Seismicity from: • 10/1990 – 01/1998

  6. ASAR dataset • - 51 earthquakes • deep seismicity • Seismicity from: • 11/1996 – 12/2000

  7. WRA beam-trace profile

  8. WRA beam-trace profile

  9. WRA double-beam All precursor events + summation trace Precursor summation trace Non-precursor summation

  10. WRA double-beam All precursor events + summation trace Precursor summation trace Non-precursor summation

  11. ASAR beam-trace profile

  12. ScP/P waveform comparison WRA : 0.5Hz – 1.4Hz ASAR: 1Hz – 3 Hz Higher ASAR resolution gives evidence for SdP and perhaps SPcP

  13. ScP CMB sampling Tomo from Ritsema and van Heijst, 2002

  14. ScP ULVZ evidence • - ~50 by 50 km • northern boundary –24.5 • southern boundary –25.5 • some boundaries not well • resolved CRZ evidence from Rost & Revenaugh, Science, 2001

  15. Forward modeling parameter space • 1D Gaussian Beam Synthetics • constant layer velocity • ScP, ScsP, SdP, SPcP • PREM background • sharp upper boundary • 4 parameter grid-search

  16. Forward modeling waveforms

  17. Best fit grid-search Chemical Heterogeneity Partial Melt

  18. Data and modeling results • Best-fit model properties: • Thickness : 8.5 (1) km • DVP : -10 (2.5) % • DVS : -25 (4) % • Dr : +10 (5) % • DVP/DVS indicates partially molten material • ~50 by 50 km lateral extension • small lateral extent raises stability questions • High-frequency data indicate very sharp upper boundary • sharpness < 400 m

  19. 1D modeling restrictions

  20. Data and modeling results red: lowest velocities for S20RTS green: strongest VS gradients Thorne et al., 2004 • Experiment probes very slow mantle (Ritsema and van Heijst, 2002) • Region of strong lateral gradient  chemical heterogeneity (Thorne et al., 2004) • Probably dense material at CMB (McNamara and Zhong, 2004)

  21. Preferred physical model • 5 to 30 vol.% melt • no spreading along CMB • trapped intercumulus liquid • incompatible-element enriched liquid • crystals are initially over- grown and trap residual • requires large overlying thermal anomaly • downward percolation of melt • correlation to dynamic instabilities/upwellings • probably a fixed base for mantle upwellings

  22. Similar Tank experiment D” aspect ratio of tank experiment !! (from Jellinek and Manga, RoG, 2004)

  23. Preferred physical model • 5 to 30 vol.% melt • no spreading along CMB • trapped intercumulus liquid • requires large overlying thermal anomaly • downward percolation of melt • incompatible-element enriched liquid • correlation to dynamic instabilities/upwellings • probably a fixed base for mantle upwellings

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