GROUP VELOCITY DISPERSION MAPS

Page content: Method description Acknowledgments References Interactive visualization: Description Orthographic projection Mercator projection Contact information

The group velocity measurements at periods between 16 and 200s are performed at the Center for Imaging the Earth's Interior in the University of Colorado at Boulder. The group velocities are measured with the frequency-time analysis ( Levshin et al., 1989) in which for every waveform a human analyst defines the frequency range of measurements and separate the signal form a variety of noise sources (e.g., overtones, fundamental modes of different types, other earthquakes, multipaths, scattered arrivals). We used broadband waveforms following earthquakes occurred from 1997 to present and recorded at stations from both global networks (GDSN, GSN, GEOSCOPE) as well as temporary regional arrays. At present, the group velocity data-set is composed of about 200000 paths.

Data coverage is generally better for Rayleigh waves than for Love waves, is better at intermediate periods than at very short or very long periods, and is better in the northern than in the southern hemisphere. This heterogeneous data coverage is imposed by the distribution of seismic stations and earthquakes. Data coverage optimizes in Eurasia and is currently worst across Africa, the central Pacific, parts of the Indian Ocean, and Antarctica.

Traditionally, the surface wave tomography has been based on the ray theory. Description of the ray-based tomographic method is done by Barmin et al. (2001). However, the ray theory models adequately represents the wave propagation only when the scale of the heterogeneity is larger than the size of the Fresnel zones. Therefore, the ray-based tomography becomes inaccurate to infer a relatively small-scale structure. Recently, we have developed a tomographic method based on a physical model of the surface-wave Fresnel zones and accounts fro associated diffraction effects like the wave-front healing. This diffraction tomography is described by Ritzwoller et al. (2002). Here, we show group velocity dispersion maps created with the diffraction tomography.

We provide interactive visualization of group velocity dispersion maps with following projections:

Center of the projection is selected by the user. Colors show the percent perturbation relative to the average global group velocity velocity at the selected period. This reference velocity is indicated in the figure.

Geographical region is selected by the user. Colors show the percent perturbation relative to the average global velocity at the selected period. This reference velocity is indicated in the figure.

CUB surface-wave research has been partially supported by NSF Office of Polar Programs and Defense Threat Reduction Agency. All plotting tools utilize the GMT software.

Barmin, M.P., Ritzwoller, M.H., and Levshin, A.L., A fast an reliable method for surface wave tomography, PAGEOPH, 158, 1351-1375, 2001. (PDF file)

Ritzwoller, M.H., Shapiro, N.M., Levshin, A.L., and Leahy, G.M., Crustal and upper mantle structure beneath Antarctica, J. Geophys. Res., 106, 30645-30670, 2001. (PDF file, Science editors choice)

Ritzwoller, M.H., N.M. Shapiro, M.P. Barmin, and A.L. Levshin, Global surface wave diffraction tomography, J. Geophys. Res., 107(B12), 2335, 2002. (PDF file)

Shapiro, N.M. and M.H. Ritzwoller, Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle, Geophys. J. Int., 151, 88-105, 2002. (PDF file)

Wessel, P., and W.H.F. Smith, Free software helps map and display data, EOS, 72, 441, 1991. (GMT web-page)

Wessel, P., and W.H.F. Smith, New version of the Generic Mapping Tools released, EOS, 76, 329, 1995. (GMT web-page)