Virginia Tech Global Seismological Lab

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. read more

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.
read more

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.
read more

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.
read more

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.
read more

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.
read more

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.
read more

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.
read more
Ying Zhou June 2012