Velocity structure

Helffrich G. (1996) Subducted lithospheric slab velocity structure observations and mineralogical inferences. In Subduction Top to Bottom, Geophys. Monogr. 96 (eds. G. Bebout, D. Scholl, S. Kirby, and J. Platt). AGU, Washington, DC, pp. 215-222. 

Ritsema J., van Heijst H. J., and Woodhouse J. H. (1999) Complex shear wave velocity structure imaged beneath Africa and Iceland. Science 286, 1925-1928. 

Saltzer R. L. and Humphreys E. D. (1997) Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region. 

Christensen N. I. and Mooney W. D. (1995) Seismic velocity structure and composition of the continental crust a global view. J. Geophys. Res. 100(B7), 9761-9788. 

Meltzer A. and Christensen N. (2001) Nanga Parbat crustal anisotrophy implication for interpretation of crustal velocity structure and shear-wave splitting. Geophys. Res. Lett. 28(10), 2129 -2132. 

Dawson P. B., Chouet B. A., Okubo P. G., Villasenor A., and Benz H. M. (1999) Three-dimensional velocity structure of the Kilauea caldera. Hawaii. Geophys. Res. Lett. 26, 2805-2808. 

Haslinger F., Thurber C., Mandernach M., and Okubo P. (2001) Tomographic image of P-velocity structure beneath Kilauea s east rift zone and South Flank seismic evidence for a deep magma body. Geophys. Res. Lett. 28, 375-378. 

Miller D. S. and Smith R. B. (1999) P and S velocity structure of the Yellowstone volcanic field from local earthquake and controlled-source tomography. J. Geophys. Res. Solid Earth 104, 15105-15121. 

Toomey D. R., Purdy G.M., Solomon S.,and Wilcox W. (1990) The three dimensional seismic velocity structure of the East Pacific Rise near latitude 9°30 N. Nature 347, 639-644. 

Zhao D., Horiuchi S., and Hasegawa A. (1992) Seismic velocity structure of the crust beneath the Japan Islands. Tectonophysics 212(3-4), 289-301. 

Tomographic results. Upper-mantle velocity structure (James et al. 20016) was determined by tomographic techniques based on the analysis of delay times from teleseismic broadband waveform data. Relative arrival times of phases P, PKPdf, S and SKS were retrieved via a multichannel cross-correlation procedure using all possible pairs of waveforms (VanDecar Crosson 1990). This procedure produces highly accurate delay times, with typical standard errors for the... 

The inversion method for obtaining velocity structure has been fully described by (VanDecar 1991). By this method, P- and S-wave delay times are inverted independently for structure beneath the array. The model is parameterized identically for the P- and the S-wave inversion with splines under tension constrained by a series of regular knots (Fig. 4). Within the interior portion of the model, the knots are spaced 50 km apart in depth and 5 degree apart in latitude and longitude. Corrections for station elevation and crustal thickness from Nguuri (2002) are applied to the data before inversion. The data are inverted simultaneously for the slowness perturbation field, earth-... 

Priestley, K. 1999. Velocity structure of the continental upper mantle evidence from southern Africa. Lithos, 48, 45-56. 

Ritsema, J., Nyblade, A. A., Owens, T. J., Langston, C. A. VanDecar, J. C. 1998. Upper mantle seismic velocity structure beneath Tanzania, East Africa implications for the stability of... 

Fig. 2. Large-scale velocity structure beneath the Canadian Shield as determined by Grand et al. (1997). The scale indicates the shear-wave velocity anomaly as a percentage relative to IASP91 (Kennett Engdahl 1991). O, locations of the TWiST seismic stations A, from south to north, the permanent seismic stations ULM (CNSN), FFC ffRIS) and FCC (CNSN).

Seismic tomography has been used to provide a more detailed picture of the velocity structure beneath the Western Superior. Here we review the results and interpretations presented by Sol et al. (2002). 

We see evidence for velocity structure and anisotropy within the tectosphere. Surface waves reveal evidence for a thin high-velocity layer, 5-20 km thick, which lies beneath a 37-43 km thick crust. Wide-angle refractions also show these features (Kay et al. 1999a Musacchio et al. 

Snell, C. S. 1999. Surface wave analysis of upper mantle velocity structure. Western Superior Province, Canada. MGeophy thesis. University of Leeds. 

Van der Lee, S. Nolet G. 1977. Upper mantle S velocity structure of North America, Journal of Geophysical Research, 102, 22815-22838. 

To a geologist who has not worked with seismic data, this situation is confusing. Even among seismologists, there is disagreement as to what features of the different models are genuine characteristics of the velocity structures beneath shields. For this reason, we thought it would be helpful to discuss what features can and cannot be well resolved by the various seismic methods as well as the seismic data that have been used. However, it was unexpectedly difficult in some cases to make accurate comparisons of the various models because they use different radial... 

However, the details of the velocity structure beneath the lid are not well constrained. Figure 3d shows that the minimum velocity at these depths can range from 4.35 to 4.45 km s and has only a small effect on the synthetic fits to the observed waveforms at 2106 km distance, but increasing the minimum velocity to 4.5 km s or greater results in an unacceptably early highermode arrival for the synthetic seismograms compared with the observed seismograms at this distance. 

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