Mantle Anisotropic Structure and Dynamics Beneath the Western United States

Constraints from Shear-wave Splitting Analysis

Figure 1. About 16,000 pairs of shear wave splitting parameters in the study area. The orientation of the red bars represents the fast polarization direction, and the length is proportional to the splitting time (Liu et al., 2014).

Science Nugget - By Kelly Liu (Missouri University of Science and Technology), and Stephen S. Gao (Missouri University of Science and Technology) - NSF # EAR 0952064 | 2010-2013

We measured ~16,000 pairs of shear-wave splitting parameters beneath all USArray andmost other broadband stations in the western and central United States from teleseismic events occurred over the period of 1989-2012. Seismic waveforms are requested from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (Top Figure). The dataset can be accessed from the following IRIS website: http://ds.iris.edu/ds/products/sws-db- mst/.

Circular pattern of fast directions: The pattern in the Great Basin and in the SAF vicinity observed previously is enhanced and it extends further to the south and east (Top Figure). Outside the area with the circular pattern, the fast directions are dominantly parallel to the Absolute Plate Motion (APM) direction of the North American Plate, suggesting anasthenospheric origin of most of the observed anisotropy. However,inconsistency between the fast direction and the APM directions exists in a number of regions such as the southwestern US, the Black Hills region,the Snake River valley/Yellowstone area,Colorado Plateau, and the vicinity of the Rio Grande rift.

Fig 1 — Figure 2. Schematic diagram showing direction of flow lines in the asthenosphere relative to the lithosphere. The solid black arrows indicate shear-strain in the asthenosphere beneath the craton, and the thick dashed line represents flow deflected by the root of the North American craton. The red arrow shows the APM direction of the North American plate, and the thin red bars represent individual shear-wave splitting measurements. The background image show P-wave travel-time residuals, and the white lines indicate the approximate edge of the craton (Refayee et al., 2014).

Mantle flow and lithosphere-asthenosphere coupling beneath the southwestern edge of the North American Craton: The western and southern edges of the North American Craton show edge-parallel fast directions with larger-than normal splitting times, and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the APM direction of North America (Figure 2). The spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere.

Top Figure: About 16,000 pairs of shear wave splitting parameters in the study area. The orientation of the red bars represents the fast polarization direction, and the length is proportional to the splitting time (Liu et al., 2014).

Bibliography

Gao, S.S., and K.H. Liu (2012), AnisDep: A FORTRAN program for the estimation of the depth of anisotropy using spatial coherency of shear-wave splitting parameters, Computers & Geosciences, doi: 10.1016/j.cageo.2012.01.020.

Liu, K.H., A. Elsheikh, A. Lemnifi, U. Purevsuren, M. Ray, H. Refayee, B. Yang, Y. Yu, and S.S. Gao (2014), A uniform database of teleseismic shear wave splitting measurements for the western and central United States, Geochemistry Geophysics Geosystems , doi: 10.1002/2014GC005267.

Liu, K.H. and S.S. Gao (2011), Estimation of the depth of anisotropy using spatial coherency of shear-wave splitting parameters, Bulletin of the Seismological Society of America, 101, pp. 2153-2161, doi: 10.1785/0120100258.

Liu, K.H, and S.S. Gao (2013), Making reliable shear-wave splitting measurements, Bulletin of the Seismological Society of America, Volume 103, No.5, 14 pages, doi: 10.1785/0120120355.

Refayee, H.A., B.B. Yang, K.H. Liu, and S.S. Gao (2014), Mantle flow and lithosphere-asthenosphere coupling beneath the southwestern edge of the North American Craton: Constraints from shear-wave splitting measurements, Earth and Planetary Science Letters , 10.1016/j.epsl.2013.01.031.

Yang, B.B., S.S. Gao, K.H. Liu, A.A. Elsheikh, A.A. Lemnifi, H.A. Refayee, and Y. Yu (2014), Seismic anisotropy and mantle flow beneath the northern Great Plains of North America, Journal of Geophysical Research, , volume 119, doi: 10.1002/2013JB010561.

Yang, B.B., K.H. Liu, H.H. Dahm, and S.S. Gao (2016), A uniform database of teleseismic shear wave splitting measurements for the western and central United States: December 2014 update, Seismological Research Letters, 87, 295-300, doi: 10.1785/0220150213.