
3056A Derring Hall (Mail Code 0420)
Department of Geosciences,
Virginia Tech,
Blacksburg, VA 24061
To improve global tomographic resolution
in the upper mantle, we developed finitefrequency 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 smallscale wavespeed anomalies are
better resolved in
finitefrequency tomographic
models.
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FiniteFrequency Theory for Seismic Anisotropy
Observations of seismic anisotropy provide valuable constraints on the
strain (stress) orientation in the
mantle. We have developed finitefrequency theory for multimode
surface wave phasedelay and amplitude measurements to image
radial anisotropy in the upper mantle and transition zone.
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FiniteFrequency Sensitivity for Body Waves
In bodywave tomography, asymptotic traveltime and amplitude
sensitivity kernels may be computed very
efficiently for known bodywave phases based on kinematic and dynamic
ray tracing. This approach encounters difficulties for diffracted waves and triplicated phases.
We developed finitefrequency sensitivity kernels for dispersion measurements of
body waves which are valid for waves traveling along the coremantle
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 3D Q models are developed based upon the assumption that
amplitude anomalies are mainly
caused by 3D 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 "counterintuitive": waves
propagating through more anelastic
(lowQ) regions experience amplification.
We have developed finitefrequency theory for imaging mantle
anelasticity,fully accounting for the dual dependence of surfacewave
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 finitefrequency
sensitivity kernels for Moho depth variations based on Born scattering
approximation and
investigated finitefrequency effects of surfacewave phase delays
upon variations in crustal thickness
as well nonlinear 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 smallscale heterogeneities. Our model FFSW1 revealed distinctly
different ridge anomalies beneath
fast and slow spreading centers, this observation provides important
constraints on the dynamics of seafloor 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 Pwave coda. Those PtoS 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 timedomain deconvolution based on
singular value decomposition works better than frequencydomain
deconvolution as the problem is often illposed and requires
regularization.
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Ying Zhou
June 2012
 