Date of Award
Doctor of Philosophy
Christine A Powell
Charles A Langston
Mitchell M Withers
Robert Smalley Jr
Part 1: We have determined detailed crust and upper mantle 3-D P wave and S wave velocity models to a depth of 400 km for the Mississippi Embayment (ME) and the New Madrid Seismic Zone (NMSZ). This study incorporates data from three networks; the New Madrid Seismic Network (CNMSN) operated by the Center for Earthquake Research and Information (CERI), the Earthscope Transportable Array (TA), and the Flex Array (FA) Northern Embayment Lithospheric Experiment (NELE) project stations to aid in constructing the most detailed velocity images of the NMSZ to date. For the crust we observe a separation of velocity highs from velocity lows along the axis of the Mississippi Valley Graben (MVG). For the upper mantle, we image a significant low velocity anomaly of ~ -3% to -5% at depths of 100 - 250 km. A high velocity anomaly of ~ +3% to +4% is observed at depths of 80 - 160 km and it occurs along the sides and top of the low velocity anomaly. The Vp and Vs solutions in the upper mantle show a remarkable similarity both in shape and anomaly magnitude. We propose that the observed low velocity features in the upper mantle are as a result of various tectonic activities in the area, which could result in: 1) Rejuvenated/primitive mantle, 2) Elevated temperatures, and 3) Increased fluid content. A combination of elevated temperatures and increased fluid content reduce P wave velocity (Vp) whereas the three effects combined significantly reduce S wave velocity (Vs). The high velocity anomalies observed are associated with mafic rocks emplaced in the lithosphere beneath the ME during initial rifting in the early Paleozoic and/or remnants of the depleted, lower portion of the lithosphere. Part 2: Using teleseismic SKS phases recorded with the Northern Embayment Lithospheric Experiment (NELE), and the USArray Transportable Array (TA), we apply the SplitLab processing environment to measure shear wave splitting within and outside the Mississippi Embayment (ME) for the period 2011 - 2015. Our dataset consists of ~4570 individual splitting measurements from 112 earthquakes. Stations outside of the ME exhibit significant shear wave splitting, with average delay times at individual stations between ~ 0.4 sec and ~ 1.5 sec. To the northeast and east of the ME, nearly all observed fast directions are approximately oriented NE-SW, in agreement with the absolute plate motion (APM) predicted by HS3-Nuvel-1A. The homogeneity of the fast directions in this region could be indicative of active flow in the asthenosphere beneath these stations. A counter-clockwise rotation in the splitting pattern is observed moving through the study area from northeast to northwest. Inside the ME, there is a complex pattern of anisotropy with large but systematic deviations ranging from ~ 0.7 sec to ~ 2.5 sec. This region of large deviations also coincides with imaged low P- and S-wave velocity anomalies in the upper mantle. The splitting complexity could be due to a contribution from 'frozen in' lithospheric fabrics formed during past orogenic events and additional fabric due to a northeast to southwest (NE-SW) asthenospheric flow as a result of the sinking of the Farallon slab in the lower mantle beneath the Central and Eastern United States (CEUS).
dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Nyamwandha, Cecilia Anyango, "A Joint Local, Regional and Teleseismic Tomography Study and Shear Wave Splitting Beneath the Mississippi Embayment and New Madrid Seismic Zone" (2015). Electronic Theses and Dissertations. 1295.