One of the major discoveries in the geological science made over the past 50 years is that the crustal and the upper-most mantle of the Earth are divided into tens of 'pieces', or tectonic plates. Movement of the plates against one another is responsible for the formation of ocean basins, mountain ranges, rifted valleys, volcanoes, and earthquakes. In spite of numerous studies, however, the forces that are driving the movement of the plates remain unresolved. The ongoing project is designed to provide critical information on the driving mechanism of plate motion, by measuring the strength and direction of the fabrics formed by the movement of the plates. A database of thousands of shear-wave splitting parameters, which are measures of deformation in the lithosphere and flow in the underlain asthenosphere, is being established as an updated version of an existing shear-wave splitting database (NA-SWS-1.1 which can be accessed at www.mst.edu/~liukh/SWS) that the PI established for North America. The new database, like its first version, will be made public as soon as possible over the course of the project. The researchers involved in the current project are collecting geological and geophysical information to interpret the shear-wave splitting observations. Major scientific issues to be addressed by the proposed study include whether the western US is underlain pervasively by two-layer anisotropy, why the Colorado Plateau and adjacent areas show complex anisotropy, and what are the characteristics of mantle structure and dynamics beneath the transitional zone between the tectonically active western US and apparently inactive Great Plains.
Intensive geological and geophysical research over the past several decades has demonstrated that mechanical anisotropy is a ubiquitous property of the Earth's crust and upper mantle. Measurements of the strength and direction of seismic anisotropy from splitting of teleseismic P-to-S converted phases at the core-mantle boundary on the receiver side have provided the geoscience community with important information on the structure and deformation of the Earth's deep interior. The SWS parameters, fast polarization direction (φ) and splitting time between the fast and slow split shear-waves (δt) are measures of the orientation and strength of the anisotropy, respectively. This project aimed at studying seismic anisotropy in the western US using shear wave splitting measurements. Here are the outcomes. Data products We measured a total of about 9598 pairs of shear-wave splitting parameters beneath all the USArray and most other broadband stations in the western and central United States (west of 900W), from teleseismic events occurred between 1/1/1980 and 12/31/2011. Seismic waveforms from events with magnitude >=5.6, which is reduced to 5.5 if the focal depth is greater than 100 km, to take the advantage of sharper waveforms from deeper earthquakes, and in the epicentral distance range of 84-1800 are requested from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC). Figure 1 shows all the 9598 SWS measurements plotted above the ray piercing points at the depth of 200 km. We are in the process of working with the DMC to transfer the data sets from the PI’s homepage to the DMC to merge with the existing (mostly station-averaged) shear-wave splitting data base developed by the University of Montpellier (France). Currently, the dataset can be accessed from the following website: http://web.mst.edu/~liukh/WUSSWS/. Notes, in addition to the measurements proposed in the proposal, some of the measurements from the events recorded in 2012 were also included in the data products. 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 (Figure 1). 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 an asthenospheric 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. Mantle flow and lithosphere-asthenosphere coupling beneath the southwestern edge of the North American Craton Systematic spatial variations of anisotropic characteristics were revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations in the southwestern United States (Figure 2). 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. At the majority of the stations, no significant systematic azimuthal variations of the splitting parameters were observed, suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations. The spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere (Figures 3 and 4). Mantle anisotropy beneath the northern Great Plains A total of 4138 pair of high quality splitting parameters were obtained at 492 USArray and other broadband stations on the northern Great Plains (Figure 5). The mean value of the splitting time is 0.98±0.32 s which is almost the same as the global average of 1.0 s for continents. A clear rotation pattern can be seen along the boundary between Yavapai and Mazatzal provinces. The Superior Craton shows large splitting times (1.15±0.13s) and NE fast directions which are parallel to the APM direction of the North American plate. Fast directions in the Trans-Hudson Orogeny show three general patterns: NE in the North, W-E in the Center and SE in the South. One of the most interesting features of the measurements is that along a zone of thinned lithosphere, the splitting parameters show systematic azimuthal variations with a 90o periodicity, suggesting the existence of multiple layer anisotropy. Estimation of the depth of anisotropy using spatial coherency of shear-wave splitting parameters We tested an approach by using both synthetic and real data to search for the optimal depth by computing a spatial variation factor, and applied the procedure for estimating the depth of anisotropy in the Afar Depression of NE Africa and North America (Figure 6). The source codes have been published as part of a Computers and Geosciences paper for open access.
