Seismic evidence for present-day plume upwelling at the core-mantle boundary
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Transcript of Seismic evidence for present-day plume upwelling at the core-mantle boundary
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Seismic evidence for present-Seismic evidence for present-day plume upwelling at the day plume upwelling at the
core-mantle boundarycore-mantle boundary
Sebastian Rost Edward J. GarneroSebastian Rost Edward J. Garnero
Quentin WilliamsQuentin Williams
Michael MangaMichael Manga
University of CaliforniaUniversity of CaliforniaSanta CruzSanta Cruz
University of CaliforniaUniversity of CaliforniaBerkeleyBerkeley
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ULVZ structure and detection
0.5 to 10’s km thick0.5 to 10’s km thick 10 to 30 % velocity decrease10 to 30 % velocity decrease density ?density ?
Thorne and Garnero, 2004
CMB are probed < 50 %CMB are probed < 50 % ULVZ evidence < 10%ULVZ evidence < 10%
(of CMB area)(of CMB area)
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ScP waveform variationsScP waveform variationsScP waveform variations
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Topography from NOAA 2’ datasetTopography from NOAA 2’ dataset
Using two small-scale arrays
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- Tonga-Fiji seismicity- Tonga-Fiji seismicity
- deep earthquakesdeep earthquakes
- 97 earthquakes97 earthquakes
- Seismicity from:Seismicity from:10/1990 – 01/199810/1990 – 01/1998
WRA dataset
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- 51 earthquakes- 51 earthquakes
- deep seismicitydeep seismicity
- Seismicity from:Seismicity from:11/1996 – 12/200011/1996 – 12/2000
ASAR dataset
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WRA beam-trace profile
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WRA beam-trace profile
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All precursor eventsAll precursor events+ summation trace+ summation trace
Precursor summation tracePrecursor summation trace
Non-precursor summationNon-precursor summation
WRA double-beam
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All precursor eventsAll precursor events+ summation trace+ summation trace
Precursor summation tracePrecursor summation trace
Non-precursor summationNon-precursor summation
WRA double-beam
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ASAR beam-trace profile
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WRA : 0.5Hz – 1.4HzWRA : 0.5Hz – 1.4Hz
ASAR: 1Hz – 3 HzASAR: 1Hz – 3 Hz
Higher ASAR resolutionHigher ASAR resolutiongives evidence for SdP gives evidence for SdP and perhaps SPcPand perhaps SPcP
ScP/P waveform comparison
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ScP CMB sampling
Tomo from Ritsema and van Heijst, 2002
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CRZ evidence from CRZ evidence from Rost & Revenaugh,Rost & Revenaugh,Science, 2001Science, 2001
ScP ULVZ evidence
- ~50 by 50 km- ~50 by 50 km
- northern boundary –24.5northern boundary –24.5
- southern boundary –25.5southern boundary –25.5
- some boundaries not well some boundaries not well
resolvedresolved
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Forward modeling parameter space
1D Gaussian Beam Synthetics1D Gaussian Beam Synthetics
constant layer velocityconstant layer velocity
ScP, ScsP, SdP, SPcPScP, ScsP, SdP, SPcP
PREM backgroundPREM background
sharp upper boundarysharp upper boundary
4 parameter grid-search4 parameter grid-search
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Forward modeling waveforms
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Partial MeltPartial MeltChemical Chemical
HeterogeneityHeterogeneity
Best fit grid-search
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Best-fit model properties: Best-fit model properties:
ThicknessThickness : 8.5 (: 8.5 (1) km1) km VVPP : -10 (: -10 (2.5) %2.5) % VVSS : -25 (: -25 (4) %4) % : +10 (: +10 (5) %5) %
VVPP// VVS S indicates partially molten materialindicates partially molten material
~50 by 50 km lateral extension~50 by 50 km lateral extension
small lateral extent raises stability questionssmall lateral extent raises stability questions
High-frequency data indicate very sharp upper boundaryHigh-frequency data indicate very sharp upper boundary
sharpness < 400 msharpness < 400 m
Data and modeling results
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1D modeling restrictions
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Experiment probes very slow mantleExperiment probes very slow mantle
(Ritsema and van Heijst, 2002)(Ritsema and van Heijst, 2002)
Region of strong lateral gradient Region of strong lateral gradient chemical heterogeneity chemical heterogeneity
(Thorne et al., 2004)(Thorne et al., 2004)
Probably dense material at CMB Probably dense material at CMB (McNamara and Zhong, 2004)(McNamara and Zhong, 2004)
Thorne et al., 2004Thorne et al., 2004
red: lowest velocities for S20RTSred: lowest velocities for S20RTS
green: strongest Vgreen: strongest VSS gradients gradients
Data and modeling results
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5 to 30 vol.% melt5 to 30 vol.% melt
no spreading along CMBno spreading along CMB
trapped intercumulus trapped intercumulus
liquidliquid
incompatible-element incompatible-element
enriched liquidenriched liquid
crystals are initially over-crystals are initially over-
grown and trap residualgrown and trap residual
requires large overlying thermal anomalyrequires large overlying thermal anomaly
downward percolation of meltdownward percolation of melt
correlation to dynamic instabilities/upwellingscorrelation to dynamic instabilities/upwellings
probably a fixed base for mantle upwellingsprobably a fixed base for mantle upwellings
Preferred physical model
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(from Jellinek and Manga, RoG, 2004)(from Jellinek and Manga, RoG, 2004)
Similar Tank experiment
D” aspect ratio of tank experiment !!
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5 to 30 vol.% melt5 to 30 vol.% melt
no spreading along CMBno spreading along CMB
trapped intercumulus trapped intercumulus
liquidliquid
requires large overlying thermal anomalyrequires large overlying thermal anomaly
downward percolation of meltdownward percolation of melt
incompatible-element enriched liquidincompatible-element enriched liquid
correlation to dynamic instabilities/upwellingscorrelation to dynamic instabilities/upwellings
probably a fixed base for mantle upwellingsprobably a fixed base for mantle upwellings
Preferred physical model