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Guided Seismic Waves: Possible Mantle-Plume Diagnostics

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,

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Guided Seismic Waves: Possible Mantle-Plume Diagnostics

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  1. Guided Seismic Waves: Possible Mantle-Plume Diagnostics Bruce R. Julian John R. Evans U. S. Geological Survey Menlo Park, California

  2. 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.

  3. Teleseismic Tomography (of Iceland) • Resolution is limited to depth < aperture of seismometer array. Looking South Looking West

  4. 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

  5. Finite-Frequency Tomography • Travel time “feels” Fresnel zone, of width . (“Banana”). • Sensitivity = 0 on ray! (“Doughnut”).

  6. Lower-Mantle Anomalies (Princeton) • Based on high-frequency (using ray theory) and low-frequency (using finite-frequency theory) data.

  7. 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.

  8. 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.

  9. ScS Sensitivity Kernel • Using “Banana-Doughnut” theory of Dahlen et al. (2000)

  10. ScS2 Sensitivity Kernel • Similar to ScS kernel in upper mantle.

  11. ScS2-ScS Sensitivity Kernel • Almost zero small-scale sensitivity in upper mantle

  12. 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.

  13. Analog: Fault-Zone Waveguide • Theoretical computations from Li & Leary (1990)

  14. Cylindrical Rod in Homogeneous Medium • Torsional modes

  15. 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. • ???

  16. 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.

  17. Another Possible Experiment:Earthquake at a Hot Spot • Exchange source and observer: Same result (reciprocity principle).

  18. 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.

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