Guided Seismic Waves:
Possible Mantle-Plume
Diagnostics
Bruce R. Julian
John R. Evans
U. S. Geological Survey
Menlo Park, California
Problem: Seismological methods offer
the highest resolution mantle images, but
current techniques are
• Most sensitive to large structures,
• Most effective in the upper mantle, and
• Limited by uneven data distribution.
Teleseismic Tomography (of Iceland)
• Resolution is limited to
depth < aperture of
seismometer array.
Looking South
Looking West
Whole-Mantle Tomography
• Model S20RTS
(Ritsema et al.,
1999, 2004)
• Section along MAR
through Iceland
• Resolution best in
upper mantle
(surface waves).
• Limited by ray
distribution, esp. in
lower mantle.
• Artifacts
Finite-Frequency Tomography
• Travel time “feels”
Fresnel zone, of width
L
. (“Banana”).
• Sensitivity = 0 on ray!
(“Doughnut”).

Lower-Mantle Anomalies (Princeton)
• Based on high-frequency (using ray theory) and lowfrequency (using finite-frequency theory) data.
Ray Distribution (Bolton & Masters, 2001)
• Plume-like anomalies in SW Pacific correspond closely to
clumps in data distribution (turning points).
• Tomography is limited by uneven data coverage much
more than by finite-frequency effects.
Multiple ScS Core Reflections
• Hawaii earthquake of
1973 April 26, recorded
on Oahu (Best et al.,
1974)
• Relative times indicate
high wave speeds and
low attenuation.
ScS Sensitivity Kernel
• Using “BananaDoughnut” theory of
Dahlen et al. (2000)
ScS2 Sensitivity Kernel
• Similar to ScS kernel
in upper mantle.
ScS2-ScS Sensitivity Kernel
• Almost zero smallscale sensitivity in
upper mantle
And Now for Something Completely Different!
• A channel of low seismic-wave speed will act as a
waveguide: Waves cannot escape from such a
structure, and will follow the channel even around
(not too sharp) corners.
• Same principal as fiber-optic cable, SOFAR
acoustic channel in the ocean, etc.
• Observation and identification of such guided
waves would be virtually conclusive evidence for
a continuous low-wave-speed channel.
Analog: Fault-Zone Waveguide
• Theoretical
computations
from Li &
Leary (1990)
Cylindrical Rod in Homogeneous Medium
• Torsional
modes
Excitation of Plume-Guided Waves
• Earthquakes in the deep mantle: These would be
great, but they don’t occur (we think…).
• Teleseisms recorded by seismometers at hot spots:
Adequate?
• Earthquakes at hot-spots recorded teleseismically:
These would produce signals comparable to the
case above.
• ???
A Possible Experiment:
Teleseism Recorded by Seismometer at a Hot Spot
• Caustic (large wave
amplitude) for PKP
near 120º.
• Various other core
phases have similar
caustics.
Another Possible Experiment:
Earthquake at a Hot Spot
• Exchange source
and observer:
Same result
(reciprocity
principle).
Conclusions
• Guided seismic waves are promising tools
for detecting mantle plumes.
• Positive result would be nearly conclusive.
• Negative result would be ambiguous:
Absence of plume or inadequate excitation
of guided wave? Interpretation would
require detailed theoretical computation of
excitation by various processes.