Virginia Tech Global Seismological Lab


3056A Derring Hall (Mail Code 0420)
Department of Geosciences, Virginia Tech, Blacksburg, VA 24061

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Finite-Frequency Theory for Seismic Anisotropy

Observations of seismic anisotropy provide valuable constraints on the strain (stress) orientation in the mantle. Large discrepancies in radial anisotropy in the depth range of 400--1000 km exist among global models. Higher-mode surface waves (overtones) play an important role in imaging the Earths interior in that they provide complementary depth coverage to deep-turning body waves and fundamental mode surface waves. The main difficulty associated with overtone measurements is that different overtone modes can arrive simultaneously in the seismogram within a narrow frequency band, making it difficult to extract a single overtone mode. In addition, finite-frequency effects are more important for higher-mode surface waves (overtones) than for fundamental-mode surface waves as the associated Fresnel zones are wider for overtones due to their faster propagating speed.
We developed finite-frequency theory for multi-mode surface wave phase-delay and amplitude measurements made between data and synthetics without isolating any single overtone modes. The Born sensitivity kernels are a generalization of the theory in Zhou et al (2004) to allow for perturbations in radial anisotropy. In the presence of radial anisotropy, seismic waves of different polarizations can be strongly coupled. Multi-mode sensitivity kernels often exhibit 3-D structures of multiple-reflected body waves as a result of coupling among overtone modes.

Major-arc multi-mode kernel calcuted for 20-mhz major-arc transverse-component phase-delay measurement made with Slepian multi-tapers of 300 seconds, centered at the group arrival of the first higher-mode major-arc Love wave (G2) at 10 mhz. The geometry of the sensitivity kernels changes upon crossing the antipodes (AS and AR), including polarity changes This is due to a pi/2 phase shift upon waves crossing the antipodes, leading to different mode interaction patterns at different sides of the antipodes.

References

  • Ying Zhou (2009). "Multi-mode 3-D Surface-wave Sensitivity Kernels in Radially Anisotropic Media", Geophysical Journal International, 176, 865-888. pdf

  • Ying Zhou, Guust Nolet, F. A. Dahlen and Gabi Laske (2006). "Global upper-mantle structure from finite-frequency surface-wave tomography", Journal of Geophysical Research, 111, B04304, doi:10.1029/2005JB003677. pdf

  • Ying Zhou, F.A.Dahlen and Guust Nolet (2004). "3-D sensitivity kernels for surface-wave observables", Geophysical Journal International, 158, 142-168. pdf


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    Ying Zhou June 2012