Elastic force dipole ($f_i=-D{e}_i{e}_j{\partial }_j\delta (\mathbf{x}-\mathbf{y})$)

Opposite forces separated along the unit direction $\mathbf{e}$, with fixed force times separation $D$, have this distributional body force limit. Its net force is zero. A symmetric seismic moment tensor is a sum of such dipoles along its principal directions.