HR: 0800h
AN: T41F-1277    [Abstracts]
TI: Detecting Faults and Shear Zones With GPS and Other Survey Data
AU: * Griffiths, J
EM: griffiths@purdue.edu
AF: Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47906 United States
AB: I have found a method that seems to let us determine the orientation and nature of shear zones and, in some cases, faults with GPS and other survey data. I will describe the idea in two dimensions. 1) Let us define an ideal fault trace as a line at the ground surface across which there is a discontinuity in displacement. The component of the deformation gradient, $\frac{\partial s}{\partial N}$, is infinite. In this abstract, {\it s} is current position of points parallel to the fault or shear zone and {\it N} is original position of points normal to the fault or shear zone. 2) Let us define an ideal shear zone at the ground surface as a belt across which either: (a) the first derivative, $\frac{\partial }{\partial N}$, normal to the shear zone, in the deformation-gradient component $\frac{\partial s}{\partial N}$, that is, $\frac{\partial^2 s}{\partial N^2}$, is infinite or (b) the second derivative of the component, $\frac{\partial^3 s}{\partial N^3}$, is infinite. 3) A very narrow shear zone can be considered a fault if the component $\frac{\partial s}{\partial N}$ is very large. In practice, our survey is in terms of arbitrary coordinate systems---the initial $(X,Y)$ and current $(x,y)$ systems---rather than in terms of natural coordinate systems of the fault or shear zone---$(S,N)$, $(s,n)$ and $\alpha$, where $\alpha$ is the clockwise angle between {\it X} and {\it S}. One computes the deformation gradient tensor, $F_X_Y$, with the survey data and then determines the natural coordinates by maximizing the quantity $\frac{\partial s}{\partial N}$ as a function of $\alpha$. Then one uses quantities such as $\frac{\partial^2 s}{\partial N^2}$ and $\frac{\partial^2 s}{\partial \alpha \partial N}$ to recognize shear and displacement discontinuities.
DE: 8010 Fractures and faults
DE: 8110 Continental tectonics--general (0905)
DE: 1208 Crustal movements--intraplate (8110)
DE: 1294 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting