= Solution
Let the <seismic moment tensor> of a <double-couple seismic source> be $M=M_0(\mathbf s\otimes\mathbf n+\mathbf n\otimes\mathbf s)$ for perpendicular unit slip and normal <vectors>. Its T axis is $\mathbf T=(\mathbf s+\mathbf n)/\sqrt2$, with <eigenvalue> $+M_0$, and its P axis is $\mathbf P=(\mathbf s-\mathbf n)/\sqrt2$, with <eigenvalue> $-M_0$. They bisect the fault-plane and auxiliary-plane quadrants.
These <earthquake pressure and tension axes> describe the source deformation. \b[They are not generally the <principal axes> of the ambient <stress tensor>.] <Friction>, inherited fault orientation and local <stress> changes can rotate their relationship to that <tensor>; a <stress> inversion requires further assumptions and multiple mechanisms.
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