ALBERT

Description: Shear-wave splitting parameters (fast polarization direction and delay time) are determined using data from LA RISTRA (Colorado pLAteau RIo Grande Rift/Great Plains Seismic TRAnsect), a deployment of broadband seismometers extending from the Great Plains, across the Rio Grande Rift and the Jemez Lineament, to the Colorado Plateau. Results show that the fast polarization directions are sub-parallel to North American absolute plate motion. The largest deviations from the plate motion are observed within the western edge of the Great Plains and in the interior of the Colorado Plateau where lithospheric anisotropy may be significant. Delay times range from 0.8 to 1.8 seconds with an average value of 1.4 seconds; the largest values are along the Jemez Lineament and the Rio Grande Rift which are underlain by an uppermost mantle low velocity zone extending to depths of ∼200 km. The anisotropy beneath the central part of LA RISTRA shows a remarkably consistent pattern with a mean fast direction of 40° ± 6°. Seismic anisotropy can be explained by differential horizontal motion between the North American lithosphere and westerly to southwesterly flow of the asthenospheric mantle. The approximately N-S fast direction found beneath western Texas is similar to that observed beneath the southern rift and may reflect a different dynamic regime.

Description: Group and phase velocities of fundamental mode Rayleigh waves, in the period range of 10 to 70 s, are obtained for southern and northern Tibet. Significant variations in crustal velocity structure are found. The group velocity minimum for Tibet occurs at ∼33 s and the minimum is ∼0.12 km/s lower for southern Tibet than for northern Tibet. At periods greater than 50 s, however, group velocities are up to 0.2 km/s faster in southern Tibet. The group and phase velocities are inverted for layered S wave models. The dispersion observations in southern Tibet can only be fit with a low‐velocity layer in the middle crust. In contrast, the velocity models for northern Tibet do not require any low‐velocity zone in the crust. The S wave velocity of the lower crust of southern Tibet is ∼0.2 km/s faster than the lower crust of northern Tibet. In southern Tibet the sub‐Moho velocity increases with a positive gradient that is similar to a shield, while there is no velocity gradient beneath northern Tibet. The high‐velocity lower crust of southern Tibet is consistent with the underthrusting of Indian continental lithosphere. The most plausible explanation of the mid‐crustal low velocity zone is the presence of crustal melt resulting from H2O‐saturated melting of the interplate shear zone between the underthrusting Indian crust and overflowing Asian crust. The lack of a pronounced crustal low‐velocity zone in northern Tibet is an indication of a relatively dry crust. The low S wave velocity in the lower crust of northern Tibet is interpreted to be due to a combination of compositional differences, high temperatures, presumably caused by a high mantle heat flux, and possibly small amounts of partial melt. Combined with all available observations in Tibet, the new surface wave results are consistent with a hot and weak upper mantle beneath northern Tibet.