Dissertation Defense Announcement

College of Arts and Sciences announces the Final Dissertation Defense of

Urbi Basu

for the Degree of Doctor of Philosophy

April 1, 2019 at 10:15 AM in Center for Earthquake Research and Information, Long Building Seminar Room

Advisor: Christine Powell

Seismic velocity and anisotropy structure underneath the central United States

ABSTRACT: We present detailed seismic velocity and anisotropy structure of the central United States using two independent seismic tomography methods. The presence of the US- Transportable array in the region and the installation of the northern Embayment Lithosphere Experiment (NELE) flex array have provided an excellent dataset for the research. The first method is a tomographic inversion of Pn traveltimes. The technique simultaneously inverts for P-wave velocity and seismic anisotropy of the uppermost mantle. Anomalously fast Pn velocities are identified at the intersection of the Reelfoot Rift and Rough Creek graben near the northern edge of the Mississippi Embayment. A circular pattern of fast anisotropy directions is centered on the New Madrid seismic zone. The fast axis directions display complex patterns and differ from absolute plate motion directions and SKS splitting directions. Several other prominent P-wave velocity anomalies are also identified which correlate with major geologic features in the region. In some regions, the Pn anisotropy can be related to past orogenic events and is interpreted as frozen-in fabric from past deformation. In the second study, we investigate the presence of multi-layer seismic anisotropy underneath the Reelfoot Rift region by inverting fundamental mode Rayleigh wave phase velocity dispersion curves from 20-100s periods. Excellent raypath coverage is obtained for the Reelfoot Rift region in 30-75s periods with greater than 10,000 unique two-station paths. Tomographic inversion of the dispersion curves provide isotropic phase velocity maps and azimuthal anisotropy maps for each period. The phase velocity maps demonstrate consistently low velocities underneath the Reelfoot Rift in the upper mantle. Anisotropy fast axis directions tend to parallel the Reelfoot Rift at periods greater than 50s suggesting that the low velocities are indicative of flow directions in the mantle.