Shear Wave Splitting in South Carolina, USA
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Date
2011-02-16
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Abstract
Seismic anisotropy uses earthquake data to measure the presence of deformation in the Earth s mantle. Upon travel through an anisotropic solid, a shear wave splits into two orthogonal components that have different velocities and vibration directions. A delay time accumulates between the arrival at the Earth s surface of the fast and slow components of the shear wave. Seismometers record both components, the delay time between them (δt) and the orientation of the fast component (ϕ). Evidence of shear wave splitting (SWS) indicates anisotropy, and thus deformation, somewhere along the raypath. Null measurements (δt = 0) are evidence for un-split seismic waves, which indicate either a lack of anisotropy beneath that station, anisotropy that is vertically aligned, or shear waves that were already oriented parallel or perpendicular to the fast axis and so were not split upon passing through the mantle.
A small array of seismometers in the southeastern United States recorded an intriguing pattern of splitting and null measurements (Long 2009). Several of the stations closest to the edge of the continent exhibit null measurements in a variety of directions and almost no splitting. These measurements indicate an unusual pattern of mantle deformation in this transition region between thick continental crust and thinner oceanic crust.
In this project, I further investigate the SWS pattern in South Carolina, using a novel data set from the South Carolina Earth Physics Project (SCEPP) seismograph network operated by the University of South Carolina. Between 2001 and 2004, the network of 25 seismometers deployed at high schools across South Carolina collected seismic data. I analyzed these data and found the stations in South Carolina to be dominated by null measurements, with only a few splitting results. At some stations, the null measurements have a clearly defined orientation, while at other stations there are a variety of orientations. The direction of the splitting results match the trends of absolute plate motion and/or fossil anisotropy, and no splitting results were found for the southern-most stations. I conclude that there is a complex pattern of anisotropy occurring beneath South Carolina, and that the two most probable explanations for this pattern are either active mantle flow in a subvertical direction around the edge of the continent or fossil anisotropy.
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geology, seismology, shear wave splitting, south carolina