Guided Seismic Waves: Possible Mantle-Plume

Diagnostics

Bruce R. Julian

John R. Evans

U. S. Geological SurveyMenlo 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. (“Banana”).

• Sensitivity = 0 on ray! (“Doughnut”).

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Lλ

Lower-Mantle Anomalies (Princeton)

• Based on high-frequency (using ray theory) and low-frequency (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 “Banana-Doughnut” theory of Dahlen et al. (2000)

ScS2 Sensitivity Kernel

• Similar to ScS kernel in upper mantle.

ScS2-ScS Sensitivity Kernel

• Almost zero small-scale 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.