Double-couple seismic source

A double-couple seismic source has a moment tensor proportional to $\mathbf{b}\otimes \mathbf{n}+\mathbf{n}\otimes \mathbf{b}$, with the slip direction $\mathbf{b}$ perpendicular to the fault plane's normal $\mathbf{n}$. Its eigenvalues are proportional to $(1,0,-1)$. A general symmetric seismic moment tensor need not have this restricted form.