Benefiting from the deployment of the USArray and La Ristra array which distribute closely-spaced broadband stations in the region of interest, I am able to resolve the 3-D tomographic images of the upper mantle structure beneath the southwestern United States with significantly improved resolutions. The dataset used to constrain the P- and S-wave velocity models comes from frequency-dependent travel time shifts measured by cross correlation of bandpass-filtered waveforms between stations. The main scientific results and their implications for the tectonic evolutions under such the complicated tectonic region are summarized as follows.
1. Different from previous regional tomographic studies conducted in the area which primarily rely on ray theory and regular-grid parameterization, the novel advanced approach that includes finite-frequency theory and multi-scale parameterization is employed in this tomographic study. As such, the intrinsic wavefront healing and 3-D finite-frequency sensitivity of seismic travel time data as well as the naturally unfavorable condition of uneven data sampling can be appropriately accounted for.
2. The multi-scale, finite-frequency models are compared with those obtained from classical linearized ray theory and grid parameterization invoked with a priori smoothness constraints. Built on the same model parameterization with similar data fits, the finite-frequency tomography tends to yield the models with large amplitudes of velocity perturbations. On the other hand, for the same data kernel or forward theory used in the inversion, the multi-resolution tomography leads to the much lower model variance. The optimal P- and S-wave velocity models based on the new multi-scale, finite-frequency approach are chosen by means of the tradeoff
variance reduction for the P and ~60% for the S-wave model.
3. The 3-D variations of the resulting P and S velocity structures reveal a prominent smile-shaped region of low wave speed anomalies encircling the Colorado Plateau and confined within the uppermost 250 km depth. Such features are abruptly terminated at the depth of ~250 km. The lowest velocity anomalies are highly correlated to the late Cenozoic volcanic fields but not beneath the center of the RGR. Combined with the evidence of rather flat topography of the 410- and 670-km seismic discontinuities underneath [Wilson et al., 2003], the tomographic image resolved from this study do not support the existence of a deep-rooted, active upwelling beneath the RGR.
4. At depths greater than 250 km, the variations of P and S velocity structures appear less coherent with the surface geologic features. A very slow-velocity core inside the Colorado Plateau extending from ~300 km to greater depths may provide a plausible explanation of the high elevation of the Plateau.
Reference
Baig, A. M., F. A. Dahlen and S.-H. Hung (2003). Traveltimes of waves in
three-dimensional random media. Geophysical Journal International, 153(2):
467-482.
Baldridge, W. S., F. V. Perry, D. T. Vaniman, L. D. Nealey, B. D. Leavy, A. W. Laughlin, P. Kyle, Y. Bartov, G. Steinitz and E. S. Gladney (1991). Middle to late cenozoic magmatism of the southeastern Colorado plateau and central Rio Grande rift (New Mexico and Arizona, U.S.A.) : a model for continental rifting.
Tectonophysics, 197(2-4): 327-354.
Bassin, C., G. Laske and G. Masters (2000). The Current Limits of Resolution for
Surface Wave Tomography in North America. AGU Fall Meeting, San Francisco, CA, USA.
Bijwaard, H., W. Spakman and E. Engdahl (1998). Closing the gap between regional and global travel time tomography. J. Geophys. Res., 103(B12): 30055-30078.
Chiao, L.-Y. and B.-Y. Kuo (2001). Multiscale seismic tomography. Geophysical Journal International, 145(2): 517-527.
Chiao, L.-Y. and W.-T. Liang (2003). Multiresolution parameterization for geophysical inverse problems. Geophysics, 68(1): 199-209.
Dahlen, F. A., S.-H. Hung and G. Nolet (2000). Fréchet kernels for finite-frequency traveltimes -I. Theory. Geophysical Journal International, 141(1): 157-174.
Dueker, K., H. Yuan and B. Zurek (2001). Thick-Structured Proterozoic Lithosphere of the Rocky Mountain Region. GSA Today, 11(12): 4-9.
Engdahl, E. R., R. v. d. Hilst and R. Buland (1998). Global teleseismic earthquake relocation with improved travel times and procedures for depth determination.
Bulletin of the Seismological Society of America, 88(3): 722-743.
Gök, R., J. F. Ni, M. West, E. Sandvol, D. Wilson, R. Aster, W. S. Baldridge, S. Grand, W. Gao, F. Tillmann and S. Semken (2003). Shear wave splitting and mantle flow beneath LA RISTRA. Geophys. Res. Lett., 30(12): 16-1 - 16-4.
Gao, W., S. P. Grand, W. S. Baldridge, D. Wilson, M. West, J. F. Ni and R. Aster (2004).
Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography. J. Geophys. Res., 109(B3): 1-16.
Grand, S. (1994). Mantle shear structure beneath the Americas and surrounding oceans.
J. Geophys. Res., 99(B6): 11591-11621.
Humphreys, E. D. (1995). Post-Laramide removal of the Farallon slab, western United
Humphreys, E. D. and K. G. Dueker (1994). Western U.S. upper mantle structure. J.
Geophys. Res., 99(B5): 9615-9634.
Hung, S.-H., F. A. Dahlen and G. Nolet (2000). Fréchet kernels for finite-frequency traveltimes -II. Examples. Geophysical Journal International, 141(1): 175-203.
Hung, S.-H., F. A. Dahlen and G. Nolet (2001). Wavefront healing: a banana-doughnut perspective. Geophysical Journal International, 146(2): 289-312.
Hung, S.-H., E. J. Garnero, L.-Y. Chiao, B.-Y. Kuo and T. Lay (2005). Finite frequency tomography of D" shear velocity heterogeneity beneath the Caribbean. J.
Geophys. Res., 110(B7): 1-20.
Hung, S.-H., Y. Shen and L.-Y. Chiao (2004). Imaging seismic velocity structure beneath the Iceland hot spot: A finite frequency approach. J. Geophys. Res., 109(B8): 1-16.
Keller, G. R. and W. S. Baldridge (1999). The Rio Grande rift: A geological and geophysical overview. Rocky Mountain Geology, 34(1): 121-130.
Kennett, B. L. N., E. R. Engdahl and R. Buland (1995). Constraints on seismic velocities in the Earth from traveltimes. Geophysical Journal International, 122(1): 108-124.
Lee, S. v. d. and G. Nolet (1997). Upper mantle S velocity structure of North America. J.
Geophys. Res., 102(B10): 22815-22838.
Marquering, H., F. A. Dahlen and G. Nolet (1999). Three-dimensional sensitivity kernels for finite-frequency traveltimes: the banana-doughnut paradox.
Geophysical Journal International, 137(3): 805-815.
Menke, W. (1989). Geophysical data analysis: discrete inverse theory. San Diego, Academic Press.
Meyerholtz, K. A., G. L. Pavlis and S. A. Szpakowski (1989). Convolutional quelling in seismic tomography. Geophysics, 54(5): 570-580.
Montelli, R., G. Nolet, F. A. Dahlen, G. Masters, E. R. Engdahl and S.-H. Hung (2004).
Finite-Frequency Tomography Reveals a Variety of Plumes in the Mantle.
Science, 303(5656): 338-343.
Olsen, K. H., W. S. Baldridge and J. F. Callender (1987). Rio Grande rift: An overview.
Tectonophysics, 143(1-3): 119-139.
Owens, T. J., H. P. Crotwell, C. Groves and P. Oliver-Paul (2004). SOD: Standing Order for Data. Seismological Research Letters, 75(1): 515-520.
Paige, C. C. and M. A. Saunders (1982). LSQR: An Algorithm for Sparse Linear Equations and Sparse Least Squares. ACM Trans. Math. Softw., 8(1): 43-71.
Parker, R. L. (1977). Understanding Inverse Theory. Ann. Rev. Earth Planet. Sci., 5(1):
35-64.
Perry, F. V., W. S. Baldridge and D. J. DePaolo (1988). Chemical and isotopic evidence
for lithospheric thinning beneath the Rio Grande rift. Nature, 332(6163):
432-434.
Sigloch, K., N. McQuarrie and G. Nolet (2008). Two-stage subduction history under North America inferred from multiple-frequency tomography. Nature
Geoscience, 1(7): 458-462.
Slack, P. D., P. M. Davis, W. S. Baldridge, K. H. Olsen, A. Glahn, U. Achauer and W.
Spence (1996). The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation. J. Geophys.
Res., 101(B7): 16003-16023.
Song, T.-R. A. and D. V. Helmberger (2007). Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States. Geophysical Journal International, 171(1):
244-258.
Spence, W. and R. S. Gross (1990). A Tomographic Glimpse of the Upper Mantle Source of Magmas of the Jemez Lineament, New Mexico. J. Geophys. Res., 95(B7): 10829-10849.
Trampert, J. and R. Snieder (1996). Model Estimations Biased by Truncated Expansions:
Possible Artifacts in Seismic Tomography. Science, 271(5253): 1257-1260.
VanDecar, J. C. and R. S. Crosson (1990). Determination of teleseismic relative phase arrival times using multi-channel cross-correlation and least squares. Bulletin of the Seismological Society of America, 80(1): 150-169.
West, M., J. Ni, W. S. Baldridge, D. Wilson, R. Aster, W. Gao and S. Grand (2004).
Crust and upper mantle shear wave structure of the southwest United States:
Implications for rifting and support for high elevation. J. Geophys. Res., 109(B3): 1-16.
Wilson, D., R. Aster, J. Ni, S. Grand, M. West, W. Gao, W. S. Baldridge and S. Semken (2005). Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau using receiver functions. J.
Geophys. Res., 110(B5): 1-14.
Wilson, D., R. Aster and T. R. Team (2003). Imaging crust and upper mantle seismic structure in the southwestern United States using teleseismic receiver functions.
The Leading Edge, 22(3): 232-237.
Yang, H.-Y. and S.-H. Hung (2005). Validation of ray and wave theoretical travel times in heterogeneous random media. Geophys. Res. Lett., 32.
Zhao, L., T. H. Jordan and C. H. Chapman (2000). Three-dimensional Fréchet
differential kernels for seismicdelay times. Geophysical Journal International, 141(3): 558-576.
McGraw-Hill. I: 648.