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.

“Receiver Functions” Across Snake River Plain near Yellowstone (Ken Dueker, pers. comm.) Sensitive to discontinuities (upper mantle only)

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”).

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)

ScS 2 Sensitivity Kernel Similar to ScS kernel in upper mantle.

ScS 2 -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.