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3056A Derring Hall (Mail Code 0420)
Department of Geosciences,
Virginia Tech,
Blacksburg, VA 24061
Surface-wave Finite-Frequency Theory
To improve global tomographic resolution
in the upper mantle, we developed finite-frequency theory for surface
waves based on a single scattering (Born) approximation.
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Diffractional Effects in Tomography
When do diffractional effects become important?
Based on wave propagation simulations, we show that it depends on the length
scale of heterogeneities.
Our tomographic inversions show that small-scale wavespeed anomalies are
better resolved in
finite-frequency tomographic
models.
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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. We have developed finite-frequency theory for multi-mode
surface wave phase-delay and amplitude measurements to image
radial anisotropy in the upper mantle and transition zone.
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Finite-Frequency Sensitivity for Body Waves
In body-wave tomography, asymptotic traveltime and amplitude
sensitivity kernels may be computed very
efficiently for known body-wave phases based on kinematic and dynamic
ray tracing. This approach encounters difficulties for diffracted waves and triplicated phases.
We developed finite-frequency sensitivity kernels for dispersion measurements of
body waves which are valid for waves traveling along the core-mantle
boundary.
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Tomographic Theory for Mantle Anelasticity (Q)
It has been long recognized that the earth is not purely elastic, and
seismic energy loss caused by
the Earth's internal friction can be characterized by the seismic
quality factor, Q.Current
global 3-D Q models are developed based upon the assumption that
amplitude anomalies are mainly
caused by 3-D anelastic (Q) structure through wave attenuation.
We show that amplitudes of surface waves are
dominated by elastic focusing at periods longer than 50 seconds. More
interestingly, surface wave
amplitudes in 3D Q models are often "counter-intuitive": waves
propagating through more anelastic
(low-Q) regions experience amplification.
We have developed finite-frequency theory for imaging mantle
anelasticity,fully accounting for the dual dependence of surface-wave
amplitudes and traveltimes upon variations in wavespeed and Q.
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Tomographic Theory for for Moho Boundary Topography
Surface waves can be potentially used to constrain crustal structure
at a global scale as they propagate
in the outer shell of the earth and therefore are highly sensitive to
crustal structure, and, they
provide very good spatial coverage compared to other seismic data
sets. We developed finite-frequency
sensitivity kernels for Moho depth variations based on Born scattering
approximation and
investigated finite-frequency effects of surface-wave phase delays
upon variations in crustal thickness
as well non-linear dependence of phase delays upon Moho depth
variations.
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Global and Regional Seismic Tomography
In global diffractional tomography using surface wave dispersion data,
we improved the resolutions
of small-scale heterogeneities. Our model FFSW1 revealed distinctly
different ridge anomalies beneath
fast and slow spreading centers, this observation provides important
constraints on the dynamics of sea-floor spreading.
In joint diffractional tomography of global surface wave data and
regional USArray body wave
data, we investigated slab and plume interactions in the mantle
transition zone beneath the North
America.
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Receiver Function Studies of Mantle Discontinuities
Teleseismic P wave gives rise to converted S waves at significant
velocity discontinuities in the Earth, which travel slower than the P
wave and arrive later in the P-wave coda. Those P-to-S converted phases
provide constraints upon seismic interfaces and heterogeneities in the
lithosphere. We have investigated limitations of receiver functions in
imaging transition zone topography. We showed that time-domain deconvolution based on
singular value decomposition works better than frequency-domain
deconvolution as the problem is often ill-posed and requires
regularization.
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Ying Zhou
June 2012
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