Principal Axes
Principal Axes : In general, the principal axes are the eigenvectors of a symmetric tensor, i.e., the three orthogonal axes at which the rotated tensor has only diagonal elements. They can, for example, characterize the orientation of the stress and the strain at a point, and need not be tied to the geographic coordinate system. For example, in the case of the seismic moment tensor corresponding to a double couple source, the principal axes are called the compression axis, tension axis and null axis. With respect to seismic signals, as proposed by Penzien and Watabe (1975), the principal axes are a system of Cartesian coordinates on the ground surface for which three seismic signals, viewed as stochastic processes along respective axes ux(t), uy(t), and uz(t), are least correlated. The system of principal axes is expected to be situated in such a way that one of the three axes is perpendicular to ground surface and one is directed toward the epicenter (Fig.1, Zembaty, 2009a)
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