= Seismic moment tensor
{title2=$f_i=-\partial_j[M_{ij}(t)\delta(\mathbf x-\mathbf y)]$}
A localized zero-resultant <seismic moment tensor> represents the leading force-dipole part of an <elastic wave> source through the displayed equivalent <body force>. For a prescribed displacement jump $b_i$ across a surface with normal $n_j$, its moment density is $c_{ijkl}b_kn_l$, where $c$ is the <elastic stiffness tensor>. Spatial integration gives the point-source moment when the source is small compared with the wavelengths.
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