Elastic force dipole
= Elastic force dipole
{title2=$f_i=-D e_i e_j\partial_j\delta(\mathbf x-\mathbf y)$}
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.