Mantle Anisotropy beneath the New Madrid Seismic Zone and Adjacent Areas
A total of 93 well-defined PKS, 54 SKKS, and 126 SKS shear-wave splitting parameters are determined at 25 broadband seismic stations in an approximately 1000 by 1000 km2 area centered at the New Madrid seismic zone (NMSZ) in order to map the direction and strength of mantle fabrics and to explore the origin of seismic anisotropy. Unlike most previous shear-wave splitting studies which suggested uniform station-averaged splitting parameters (fast direction and splitting time) in the same area, our results, when presented in the form of individual (instead of station-averaged) splitting parameters and used the PKS, SKKS in addition to SKS phases, suggest significant and systematic spatial and azimuthal variations in the splitting parameters. The azimuthal variations at most stations can be best explained as the combinedeffects of present SW-NE-ward asthenospheric flow related to the motion of the North American plate in a hot-spot reference frame, and the NNE trending lithospheric fabrics formed during past orogenic events. In the NMSZ, anticipated rift-parallel fast directions associated with vertical magmatic dikes or along-rift flow, rift-orthogonal fast directions from small-scale convection, or reduction in splitting times as a result of vertical asthenospheric flow are not observed, suggesting that the NMSZ is a shallow feature which is consistent with results from other geophysical studies such as seismic reflection surveys. Due to the large spacing between the stations, the boundaries between the areas with different characteristics of splitting parameters cannot be decided accurately, and consequently the correlation between the splitting parameters and results from other geological and geophysical studies cannot be uniquely determined. The upcoming Transportable Array stations will greatly improve the spatial resolution of the anisotropy measurements, and thus will most likely lead to more insights to the structure and dynamics of the NMSZ.
M. Moidaki et al., "Mantle Anisotropy beneath the New Madrid Seismic Zone and Adjacent Areas,", vol. 42, no. 2 Geological Society of America, Apr 2010.
North-Central Section (44th Annual) and South-Central Section (44th Annual) Joint Meeting (2010: Apr. 11-13, Branson, MO)
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