Velocity and azimuthal anisotropy structure underneath the Reelfoot Rift region from Rayleigh wave phase velocity dispersion curves

Abstract

Summary Phase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope Transportable Array, and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20 – 100s). We obtain more than 12,000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the Reelfoot Rift, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the Reelfoot Rift is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the center of the Illinois Basin in the period range 75-100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the Reelfoot Rift from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.

Shear Velocity Structure Beneath the Central United States From the Inversion of Rayleigh Wave Phase Velocities

“…The tradeoff between azimuthal anisotropy and heterogeneity was minimal. A bleeding test was conducted using the method by Buehler and Shearer (2010) (Basu and Powell, 2021) Supporting Information S1), and thus azimuthal anisotropy was not considered in this study. Anisotropy results are discussed in detail in Basu and Powell (2021).…”

“…Boundaries of the Illinois Basin, the Mississippi Embayment, and the Reelfoot Rift are marked with dashed lines in blue, black, and yellow, respectively. Grid nodes used to present the phase velocity maps by Basu and Powell (2021) are denoted with empty dots.…”

“…The slow velocities form a northwest dipping zone and indicate the presence of a slab fragment below the central US region. Basu and Powell (2021) to obtain Rayleigh wave phase velocities. SS, the Sparta Shelf; OD, the Ozark Dome; ND, the Nashville Dome; OAT, the Oklahoma-Alabama Transform; OR, the Ouachita Rift; NMSZ, the New Madrid Seismic Zone; SGSZ, the Ste.…”

“…Figure 1. 348 Northern Embayment Lithosphere Experiment (NELE), CERI, OIINK, and transportable array (TA) stations utilized inBasu and Powell (2021) to obtain Rayleigh wave phase velocities. SS, the Sparta Shelf; OD, the Ozark Dome; ND, the Nashville Dome; OAT, the Oklahoma-Alabama Transform; OR, the Ouachita Rift; NMSZ, the New Madrid Seismic Zone; SGSZ, the Ste.…”

“…Figure2. The azimuthal distribution and ray paths of 104 regional and teleseismic events with magnitudes ≥5.0 and epicentral distances 5°-120° (measured from the center of the northern Mississippi Embayment), fromBasu and Powell (2021). They were selected over a period of 5 years (from 2011 to 2015).…”