Seismic anisotropy beneath the Mississippi Embayment and the New Madrid Seismic Zone: A study of shear wave splitting

Abstract

Shear wave splitting associated with the Mississippi Embayment (ME) is determined using teleseismic SKS phases recorded by the Northern Embayment Lithosphere Experiment (NELE), the USArray Transportable Array (TA), and the New Madrid seismic network for the period 2005–2016. Our data set consists of ~5900 individual splitting measurements from 257 earthquakes recorded at 151 stations within and outside the ME. Stations outside of the ME exhibit significant shear wave splitting, with average delay times between ~ 0.4 s and ~1.8 s. To the northeast and east of the ME, nearly all observed fast orientations are approximately oriented northeast-southwest, in agreement with absolute plate motion (APM) predicted by HS3-Nuvel-1A. The homogeneity of the fast orientations in this region suggests that the splitting is due to active flow in the asthenosphere. A counterclockwise rotation in the splitting orientation is observed moving northeast to northwest across the study area. Inside the ME, some stations show large and systematic deviations of the measured fast orientations from the APM. The delay times within the entire ME range from ~0.9 s to ~ 2.1 s. Splitting complexity is attributed to relic lithospheric fabrics formed during past tectonic events including passage of a hot spot in mid-Cretaceous time. The anisotropy may also be linked to the presence of a southwest dipping region of low P and S wave velocities below the ME or to deeper flow in the asthenosphere.