Seismic evidence for presentday 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
 10 to 30 % velocity decrease
 density ?
 CMB are probed < 50 %
 ULVZ evidence < 10%
(of CMB area)
Thorne and Garnero, 2004
waveform variations
ScP waveformScP
variations
Using two small-scale arrays
Topography from NOAA 2’ dataset
WRA dataset
- Tonga-Fiji seismicity
- deep earthquakes
- 97 earthquakes
- Seismicity from:
10/1990 – 01/1998
ASAR dataset
- 51 earthquakes
- deep seismicity
- Seismicity from:
11/1996 – 12/2000
WRA beam-trace profile
WRA beam-trace profile
WRA double-beam
All precursor events
+ summation trace
Precursor summation trace
Non-precursor summation
WRA double-beam
All precursor events
+ summation trace
Precursor summation trace
Non-precursor summation
ASAR beam-trace profile
ScP/P waveform comparison
WRA : 0.5Hz – 1.4Hz
ASAR:
1Hz – 3 Hz
Higher ASAR resolution
gives evidence for SdP
and perhaps SPcP
ScP CMB sampling
Tomo from Ritsema and van Heijst, 2002
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
Forward modeling parameter space
 1D Gaussian Beam Synthetics
 constant layer velocity
 ScP, ScsP, SdP, SPcP
 PREM background
 sharp upper boundary
 4 parameter grid-search
Forward modeling waveforms
Best fit grid-search
Partial Melt
Chemical
Heterogeneity
Data and modeling results
 Best-fit model properties:
 Thickness
 DVP
 DVS
 Dr
: 8.5 (1) km
: -10 (2.5) %
: -25 (4) %
: +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
1D modeling restrictions
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)
Preferred physical model
 5 to 30 vol.% melt
 no spreading along CMB
 trapped intercumulus
liquid
 incompatible-element
enriched liquid
 crystals are initially overgrown and trap residual
 requires large overlying thermal anomaly
 downward percolation of melt
 correlation to dynamic instabilities/upwellings
 probably a fixed base for mantle upwellings
Similar Tank experiment
D” aspect ratio
of tank
experiment !!
(from Jellinek and Manga, RoG, 2004)
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